highway capacity manual 2000 contents - unesp

Highway Capacity Manual 2000

June 1999 3-i Chapter 13 - Freeway Concepts6020.209.1116.02

CONTENTS

I. INTRODUCTION ........................................................................................................ 13-1II. BASIC FREEWAY SEGMENTS ................................................................................ 13-1

Freeway Capacity Terms ................................................................................. 13-2Flow Characteristics ......................................................................................... 13-2

Speed/Flow/Density Relationships .................................................... 13-3Queue Discharge and Congested Flow ............................................. 13-4

Factors Affecting Free-Flow Speed.................................................................. 13-5Lane Width and Lateral Clearance .................................................... 13-5Number of Lanes ............................................................................... 13-6Interchange Density ........................................................................... 13-6Other Factors ..................................................................................... 13-6

Passenger-Car Equivalents ............................................................................. 13-6Driver Population.............................................................................................. 13-7Levels of Service .............................................................................................. 13-7Required Input Data and Estimated Values ................................................... 13-11

Segment Length............................................................................... 13-11Lane Width and Lateral Clearance .................................................. 13-12Interchange Density ......................................................................... 13-12Specific Grade or General Terrain ................................................... 13-12Base Free-Flow Speed and Free-Flow Speed................................. 13-12Analysis Period Demand ................................................................. 13-12Peak-Hour Factor ............................................................................. 13-13Heavy Vehicles ................................................................................ 13-13Driver Population ............................................................................. 13-13

Service Volume Table .................................................................................... 13-14III. FREEWAY WEAVING ............................................................................................ 13-15

Weaving Configurations ................................................................................. 13-15Type A Weaving Configurations ...................................................... 13-16Type B Weaving Configurations ...................................................... 13-17Type C Weaving Configurations ...................................................... 13-18Impacts of Weaving Configuration ................................................... 13-18

Weaving Length ............................................................................................. 13-19Weaving Width ............................................................................................... 13-19Type of Operation........................................................................................... 13-20Service Volumes ............................................................................................ 13-21

IV. RAMPS AND RAMP JUNCTIONS ......................................................................... 13-27Ramp Components ........................................................................................ 13-27Operational Characteristics ............................................................................ 13-27Important Parameters..................................................................................... 13-28Capacity of Merge and Diverge Areas ........................................................... 13-29Levels of Service ............................................................................................ 13-30Required Input Data and Estimated Values ................................................... 13-30

Ramp Lanes..................................................................................... 13-30Length of Acceleration/Deceleration Lane ....................................... 13-30Ramp Free-Flow Speed ................................................................... 13-31Analysis Period Demand ................................................................. 13-31Peak-Hour Factor ............................................................................. 13-31Heavy Vehicles ................................................................................ 13-31Driver Population Factor .................................................................. 13-31

Service Volume Tables .................................................................................. 13-31V. FREEWAY FACILITIES .......................................................................................... 13-33

Traffic Management Strategies ...................................................................... 13-33


Chapter 13 - Freeway Concepts Page 13-ii June 19996020.209.116.02

The Freeway Traffic Management Process..................................... 13-33Freeway Management Strategies .................................................... 13-34

Capacity Management Strategies ..................................... 13-34Demand Management Strategies...................................... 13-35

Performance Measures .................................................................................. 13-35VII. REFERENCES...................................................................................................... 13-36

EXHIBITS

Exhibit 13-1. Example of Basic Freeway Segment................................................... 13-1Exhibit 13-2. Speed-Flow Relationships for Basic Freeway Segments .................... 13-3Exhibit 13-3. Density-Flow Relationships on Basic Freeway Segments .................. 13-3Exhibit 13-4. Queue Discharge and Congested Flow............................................... 13-4Exhibit 13-5. Required Input Data and Default Values for Basic Freeway

Segments .......................................................................................... 13-11Exhibit 13-6. Default Ramp Lengths for Segment Length Estimation .................... 13-11Exhibit 13-7. Default Ramp Lengths ....................................................................... 13-12Exhibit 13-8. Default Percent Heavy Vehicles by Functional Class ........................ 13-13Exhibit 13-9. Freeway Driver Population Factor ..................................................... 13-14Exhibit 13-10. Maximum Service Volumes for Basic Freeway Segments ................ 13-14Exhibit 13-11. Formation of a Weaving Area ............................................................ 13-15Exhibit 13-12. Type A Weaving Areas ...................................................................... 13-16Exhibit 13-13. Type B Weaving Areas ...................................................................... 13-17Exhibit 13-14. Type C Weaving Areas ...................................................................... 13-18Exhibit 13-15. Measuring the Length of a Weaving Area ......................................... 13-19Exhibit 13-16. Maximum Use of Lanes by Weaving Vehicles .................................. 13-21Exhibit 13-17. Type A Weaving Area Service Volumes ............................................ 13-22Exhibit 13-18. Type B Weaving Area Service Volumes ............................................ 13-23Exhibit 13-19. Type C Weaving Area Service Volumes............................................ 13-26Exhibit 13-20. On- and Off-Ramp Influence Areas ................................................... 13-28Exhibit 13-21. Capacity of Merge Areas ................................................................... 13-29Exhibit 13-22. Capacity of Diverge Areas ................................................................. 13-29Exhibit 13-23. Required Input Data and Default Values ........................................... 13-30Exhibit 13-24. Default Acceleration/Deceleration Lane Lengths .............................. 13-31Exhibit 13-25. Service Volumes for Single-Lane on Ramps (veh/h) ......................... 13-32Exhibit 13-26. Service Volumes for Single-Lane Off-Ramps (veh/h) ........................ 13-32


June 1999 Page 13-1 Chapter 13 - Freeway Concepts6020.209.116.02 Introduction

I. INTRODUCTION

This chapter introduces capacity and level of service concepts for freeways. Thischapter can be used in conjunction with the methodologies of Chapter 22 (FreewayFacilities), Chapter 23 (Basic Freeway Segments), Chapter 24 (Freeway Weaving), andChapter 25 (Ramps and Ramp Junctions).

Freeways provideuninterrupted flowA freeway is defined as a divided highway with full control of access and having two

or more lanes for the exclusive use of traffic in each direction. Freeways provideuninterrupted flow. There are no signalized or stop-controlled at-grade intersections, anddirect access to and from adjacent property is not permitted. Access to and from thefreeway is limited to ramp locations. Opposing directions of flow are continuouslyseparated by either a raised barrier, an at-grade median, or a continuous raised median.

Operating conditions on a freeway primarily result from interactions among vehiclesand drivers in the traffic stream, and between vehicles and their drivers and the geometriccharacteristics of the freeway. Operations can also be affected by environmentalconditions, such as weather or lighting, by pavement conditions, and by the occurrence oftraffic incidents.

Toll road as freewayA tollway or toll road is similar to a freeway, except that tolls are collected atdesignated points along the facility. While the collection of tolls usually involvesinterruptions to traffic, these facilities may generally be treated as freeways. Specialattention should, however, be given to the unique characteristics, constraints, and delayscaused by toll collection facilities.

II. BASIC FREEWAY SEGMENTS

Basic Freeway Segments are outside of the influence area of ramps or weaving areasof the freeway. Exhibit 13-1 illustrates a basic freeway segment.

EXHIBIT 13-1. EXAMPLE OF BASIC FREEWAY SEGMENT

Basic Freeway Segment

Basic Freeway Segment

Not to scale


Chapter 13 - Freeway Concepts Page 13-2 June 1999Basic Freeway Segments 6020.209.116.021

FREEWAY CAPACITY TERMS

• Freeway Capacity: the maximum sustained 15-minute rate of flow, expressed inpassenger-cars per hour per lane, which can be accommodated by a uniform freewaysegment under prevailing traffic and roadway conditions in one direction of flow.

• Traffic Characteristics: any characteristic of the traffic stream which may affectcapacity, free-flow speed, or operations, including the percentage composition of thetraffic stream by vehicle type and the familiarity of drivers with the freeway.

• Roadway Characteristics: the geometric characteristics of the freeway segmentunder study; these include the number and width of lanes, right-shoulder lateral clearance,interchange spacing, vertical alignment, and lane configurations.

• Free-flow Speed: the mean speed of passenger cars under low to moderate flowrates which can be accommodated on a uniform freeway segment under prevailingroadway and traffic conditions.

• Base Conditions: an assumed set of geometric and traffic conditions which areused as a starting point for computations of capacity and level of service.

It should be noted that capacity analysis is based upon freeway segments withuniform traffic and roadway conditions. If any of these prevailing conditions changesignificantly, the capacity of the segment, and its operating conditions change as well.Therefore, each uniform segment should be analyzed separately.

FLOW CHARACTERISTICS

Traffic flow within basic freeway segments can be highly varied depending upon theconditions which constrict flow at upstream and downstream bottleneck locations.Bottlenecks can be created by ramp merge and weaving areas, lane drops, maintenanceand construction activities, accidents, and objects in the road. An incident does not haveto block a travel lane in order to create a bottleneck. For example, disabled vehicles inthe median or on the shoulder can influence traffic flow within the freeway lanes.

The three flow types onbasic freeway segmentsare: free flow, queuedischarge flow, andcongested flow

Freeway research has resulted in a better understanding of the characteristics offreeway flow relative to the influence of upstream and downstream bottlenecks. Trafficflow within a basic freeway segment can generally be categorized into three flow types:free flow, queue discharge flow, and congested flow. Each flow type can be definedwithin general speed-flow-density ranges and each represents different conditions on thefreeway.

• Free flow represents traffic flow which is unaffected by upstream or downstreamconditions. This flow regime is generally defined within a speed range of 90 to 120 km/hat low to moderate flows and between 70 and 100 km/h at high flow rates.

• Queue discharge flow represents traffic flow which has just passed through abottleneck and is accelerating back up to the free-flow speed of the freeway. Queuedischarge flow is characterized by relatively stable flow as long as the effects of anotherbottleneck downstream are not present. This flow type is generally defined within anarrow range of flows, 2000 to 2300 pc/h/ln, with speeds typically ranging from 55 km/hup to the free-flow speed of the freeway segment. Lower speeds are typically observedjust downstream of the bottleneck. Depending upon horizontal and vertical alignments,queue discharge flow usually accelerates back up to the free-flow speed of the facilitywithin 1 to 2 kilometers downstream from the bottleneck. Studies suggest that the queuedischarge flow rate from the bottleneck is lower than the maximum flows observed priorto breakdown. A typical value for this drop in flow rate is approximately 5 percent.

• Congested flow represents traffic flow which is influenced by the effects of adownstream bottleneck. Traffic flow in the congested regime can vary over a broadrange of flows and speeds depending upon the severity of the bottleneck. Queues mayextend several kilometers upstream from the bottleneck. Freeway queues differ fromqueues at intersections in that they are not static, or standing. On freeways, vehiclesmove slowly through a queue, with periods of both stopping and movement.


June 1999 Page 13-3 Chapter 13 - Freeway Concepts6020.209.116.02 Basic Freeway Segments

Speed/Flow/Density Relationships

Speed-flow and density-flow relationships for a typical basic freeway segment undereither base or non-base conditions in which free-flow speed is know, are shown inExhibits 13-2 and 13-3 (1).

All recent freeway studies indicate that speed on freeways is insensitive to flow inthe low to moderate range. This is reflected in Exhibit 13-2, which shows speed to beconstant for flows up to 1300 pc/h/ln for a 120 km/h free-flow speed. For lower free-flowspeeds, the region over which speed is insensitive to flow extends to even higher flowrates. Free-flow speed is measured in the field as the average speed of passenger carswhen flow rates are less than 1300 pc/h/ln. Field determination of free-flow speed isaccomplished by performing travel time or spot speed studies during periods of low flowsand low densities.

EXHIBIT 13-2. SPEED-FLOW RELATIONSHIPS FOR BASIC FREEWAY SEGMENTS

130

120

110

100

90

80

70

60

50

40

30

20

10

00 400 800 1200 1600 2000 2400

Aver

age

Pass

enge

r-Ca

r Spe

ed (k

m/h

)

Flow Rate (pc/h/ln)

Free-Flow Speed = 120 km/h

110 km/h

100 km/h

90 km/h

1300

1450

1600

1750

EXHIBIT 13-3. DENSITY-FLOW RELATIONSHIPS ON BASIC FREEWAY SEGMENTS

0 400 800 1200 1600 2000 2400

Flow Rate (pc/h/ln)

Dens

ity (p

c/km

/ln)

35

30

25

20

15

10

5

0

Free-Flow Speed = 110 km/h

120 km/h90 km/h

100 km/h



Although Exhibit 13-2 shows curves only for free-flow speeds of 120, 110, 100, and90 km/h, a curve representing any free-flow speed between 120 km/h and 90 km/h can bedefined by interpolation.

The research leading to these curves found that a number of factors affect free-flowspeed (1). These factors include number of lanes, lane width, lateral clearance, andinterchange density or spacing. Other factors which are believed to influence free-flowspeed, but for which little is known quantitatively, include horizontal and verticalalignment, speed limit, level of enforcement, lighting conditions, and weather.

Under base traffic and geometric conditions, freeways will operate with capacities ashigh as 2400 pc/h/ln. This capacity is typically achieved on freeways with free-flowspeeds of 120 km/h or greater. As the free-flow speed decreases, there is a slightdecrease in capacity. For example, capacity of a basic freeway segment with a free-flowspeed of 90 km/h is expected to be approximately 2250 pc/h/ln.

The average speed of passenger cars at flow rates which represent capacity areexpected to range from 86 km/h (free-flow speeds of 120 km/h or greater) to 80 km/h fora segment with a 90 km/h free-flow speed. Note that the higher the free-flow speed, thegreater the drop in speed as flow rates move toward capacity. Thus, for a 120 km/h free-flow speed, there is a 34 km/h drop from low volume conditions to capacity conditions.The drop is only 10 km/h for a freeway with a 90 km/h free-flow speed.

As indicated in Exhibit 13-2, the point at which an increase in flow rate begins toimpact the average passenger car speed varies from 1300 to 1750 pc/h/ln. Speed willbegin to be reduced at 1300 pc/h/ln for freeway segments with free-flow speeds of 120km/h. For lower free-flow speed facilities, the average speed begins to diminish at higherflow rates.

Queue Discharge and Congested Flow

Unlike free flow, queue discharge and congested flow have not been extensivelystudied, and these traffic flow types can be highly variable. However, freeway researchperformed since 1990 has provided valuable insight into possible speed-flowrelationships which describe these two flow regimes. Exhibit 13-4 presents onerelationship which has been suggested and is displayed here for informational purposesonly.

EXHIBIT 13-4. QUEUE DISCHARGE AND CONGESTED FLOW

110100

9080

70

60

50

40

30

20

10

00 400 800 1200 1600 2000 2400

Flow Rate (pc/h/ln)

Aver

age

Pass

enge

r-Ca

r Spe

ed (k

m/h

)

Regime 1 (Free Flow) Regime 2 (Queue Discharge) Regime 3 (Congested Flow)



Users are cautioned that although the relationship in Exhibit 13-4 may provide ageneral predictive model for speed under queue discharge and congested flows, it shouldbe considered conceptual at best and further research is needed in order to better defineflow in these two regimes. Chapter 22 describes other methods to determine speed underqueue discharge and congested flow.

FACTORS AFFECTING FREE-FLOW SPEED

The free-flow speed of a freeway is dependent upon traffic and roadway conditions.These conditions are described below.

Lane Width and Lateral Clearance

When lane widths are less than 3.6 m, drivers are forced to travel laterally closer toone another than they would normally desire. Drivers tend to compensate for this byreducing their travel speed.

Lateral clearance is measuredfrom edge of travel lane tocurb, guardrail, or otherphysical obstruction

The effect of restricted lateral clearance is similar. When objects are located tooclose to the edge of the median and roadside lanes, drivers in these lanes will shy awayfrom them, positioning themselves further from the lane edge. This has the same effectas narrow lanes which force drivers closer together laterally. Drivers have been found tocompensate by reducing their speed. The closeness of objects has been found to have agreater effect on drivers in the right most travel lane than in the median lane.

Drivers in the median lane appear to be unaffected by lateral clearance, based on aminimum clearance of 0.6 m, while drivers in the right (shoulder) lane are affected whenlateral clearance is less than 1.8 m. Illustration 13-1 shows the impacts of lane width andlateral clearance on lateral placement of vehicles. Illustration 13-2 shows a freewaysegment which is considered to meet or exceed base conditions with respect to lane widthand lateral clearance.

Illustration 13-1Note how vehicles shy awayfrom both roadside andmedian barriers, driving asclose to the lane marking aspossible. The existence ofnarrow lanes compounds theproblem, making it difficult fortwo vehicles to travelalongside each other.



Illustration 13-2This cross sectionillustrates baseconditions of lane widthand lateral clearance.The concrete medianbarrier does not causevehicles to shift their laneposition, and therefore,would not be consideredan obstruction.

Number of Lanes

The number of lanes on a freeway segment influences free-flow speed. As thenumber of lanes increases, so does the opportunity for drivers to position themselves inorder to avoid slower moving traffic. In typical freeway driving, traffic tends to distributeacross lanes according to speed. Traffic in the median lane typically moves faster than inthe lane adjacent to the right shoulder. Thus, a four-lane freeway (two lanes in eachdirection) provides less opportunity for drivers to move around slower traffic than does afreeway with six, eight, or ten lanes. Decreased maneuverability tends to reduce theaverage speed of vehicles.

Interchange Density

Freeway segments with closely spaced interchanges, such as those in heavilydeveloped urban areas, operate at lower free-flow speeds than suburban or rural freewayswhere interchanges are less frequent. The merging and weaving associated withinterchanges affects the speed of traffic. Speeds generally decrease with increasingfrequency of interchanges. The ideal average interchange spacing over a reasonably longsection of freeway (8 to 10 km) is 3 km or greater. The minimum average interchangespacing considered possible over a substantial length of freeway is 1 km.

Other FactorsThe horizontal andvertical geometry mayinfluence free-flow speed

The design speed of the primary physical elements of a freeway can impact travelspeed. In particular, the horizontal and vertical alignments may contribute to the free-flow speed of a given freeway segment. If a freeway has significant horizontal or verticalconditions, the user is encouraged to determine free-flow speed from field observationand field study.

PASSENGER-CAR EQUIVALENTS

The concept of vehicle equivalence is based on observations of freeway conditionswhere the presence of heavy vehicles, including trucks, buses, and recreational vehicles(RVs) creates less than base conditions. These less than base conditions include longerand more frequent gaps of excessive length both in front of and behind heavy vehicles.Also, the speed of vehicles in adjacent lanes and their spacing may be impacted by thesegenerally slower moving large vehicles. Finally, physical space taken up by a largevehicle is typically two to three times greater in terms of length than that of a typicalpassenger car. To allow the analysis method for freeway capacity to be based on aconsistent measure of flow, each heavy vehicle is converted into an equivalent number ofpassenger cars. This conversion results in a single value for flow rate in terms ofpassenger cars per hour per lane. The conversion factor used depends on the proportion



of heavy vehicles in the traffic stream as well as the length and severity of the upgrade ordowngrade.

Illustrations 13-3 and 13-4 show the impact of trucks and other heavy vehicles onfreeway traffic.

DRIVER POPULATION

Studies have noted that non-commuter driver populations do not display the samecharacteristics as regular commuters. For recreational traffic, capacities have beenobserved to be as much as 15 to 20 percent lower than for commuter traffic traveling onthe same segment, but free-flow speed does not seem to be similarly impacted. If theanalyst elects to take this possible effect into account, locally derived data should beobtained and used for the analysis.

Illustration 13-3Note the formation of largegaps in front of slow-movingtrucks climbing the grade

Illustration 13-4Even on relatively levelterrain, the development oflarge gaps in front of trucks orother heavy vehicles iscommon

LEVELS OF SERVICE

Although speed is a major concern of drivers as related to service quality, freedom tomaneuver within the traffic stream and proximity to other vehicles are equally noticeableconcerns. These qualities are related to the density of the traffic stream. Unlike speed,density increases as flow increases up to capacity, resulting in a measure of effectivenesswhich is sensitive to a broad range of flows.

Operating characteristics for the six levels of service are depicted in Illustrations 13-5through 13-10. The levels of service are defined to represent reasonable ranges in thethree critical flow variables: speed, density, and flow rate.

Level of Service A describes free-flow operations. Free-flow speeds prevail.Vehicles are almost completely unimpeded in their ability to maneuver within the traffic



stream. Even at the maximum density for LOS A, the average spacing between vehiclesis about 167 m, or 27 car lengths, which affords the motorist a high level of physical andpsychological comfort. The effects of incidents or point breakdowns are easily absorbedat this level.

Level of Service B represents reasonably free flow, and free-flow speeds aremaintained. The lowest average spacing between vehicles is about 100 m, or 16 carlengths. The ability to maneuver within the traffic stream is only slightly restricted, andthe general level of physical and psychological comfort provided to drivers is still high.The effects of minor incidents and point breakdowns are still easily absorbed.

Illustration 13-5. Level of Service A

Illustration 13-6. Level of Service B



Illustration 13-7. Level of Service C

Illustration 13-8. Level of Service D

Illustration 13-9. Level of Service E



Illustration 13-10. Level of Service F

Level of Service C provides for flow with speeds at or near the free-flow speed of thefreeway. Freedom to maneuver within the traffic stream is noticeably restricted and lanechanges require more care and vigilance on the part of the driver. Minimum averagespacings are in the range of 67 m, or 11 car lengths. Minor incidents may still beabsorbed, but the local deterioration in service will be substantial. Queues may beexpected to form behind any significant blockage.

Level of Service D is the level in which speeds can begin to decline slightly withincreasing flows and density begins to increase somewhat more quickly. Freedom tomaneuver within the traffic stream is more noticeably limited, and the driver experiencesreduced physical and psychological comfort levels. Even minor incidents can beexpected to create queuing, as the traffic stream has little space to absorb disruptions. Atthe limit, vehicles are spaced at about 50 m, or eight car lengths.

At its highest density value, Level of Service E describes operation at capacity.Operations in this level are volatile, as there are virtually no usable gaps in the trafficstream. Vehicles are spaced at approximately six car lengths, leaving little room tomaneuver within the traffic stream at speeds which are still over 80 km/h. Any disruptionto the traffic stream, such as vehicles entering from a ramp, or a vehicle changing lanes,can establish a disruption wave which propagates throughout the upstream traffic flow.At capacity, the traffic stream has no ability to dissipate even the most minor disruptions,and any incident can be expected to produce a serious breakdown with extensive queuing.Maneuverability within the traffic stream is extremely limited, and the level of physicaland psychological comfort afforded the driver is poor.

Level of Service F describes breakdowns in vehicular flow. Such conditionsgenerally exist within queues forming behind breakdown points. Breakdowns occur for anumber of reasons.

• Traffic incidents can cause a temporary reduction in the capacity of a shortsegment, such that the number of vehicles arriving at the point is greater than the numberof vehicles that can move through it.

• Points of recurring congestion, such as merge or weaving areas and lane drops,experience very high demand in which the number of vehicles arriving is greater than thenumber of vehicles discharged.

• In forecasting situations, the projected peak hour (or other) flow rate can exceedthe estimated capacity of the location.

Note that in all cases, breakdown occurs when the ratio of existing demand to actualcapacity or of forecasted demand to estimated capacity exceeds 1.00. Operationsimmediately downstream of such a point, however, are generally at or near capacity, anddownstream operations improve (assuming that there are no additional downstreambottlenecks) as discharging vehicles move away from the bottleneck.


June 1999 -11 Chapter 13 - Freeway Concepts6020.209.116.02 Basic Freeway Segments

It should be noted that the LOS F operations within a queue are the result of abreakdown or bottleneck at a downstream point. LOS F is also used to describeconditions at the point of the breakdown or bottleneck, as well as the operations withinthe queue which forms upstream. Whenever LOS F conditions exist, there is the potentialfor these conditions to extend upstream for significant distances.

REQUIRED INPUT DATA AND ESTIMATED VALUES

Exhibit 13-5 illustrates default values of parameters for use by the analyst when morespecific locally-generated values are not available.

Segment Length

The actual distance between ramps is most accurately determined from as-built plansor from scale maps or aerial photos of the facility. In the absence of such data, thedistance between ramps can be estimated using the average or actual spacing betweeninterchanges and Exhibit 13-6 and 13-7. Analysts may wish to construct their owndiagram for estimating ramp lengths based upon discussions with local highwayoperating agencies.

EXHIBIT 13-5. REQUIRED INPUT DATA AND DEFAULT VALUES FOR BASIC FREEWAY SEGMENTS

Required Data Defaults

Geometric Data

No. of through lanes 2 to 5 (one direction)Lane width 3.6 mLateral clearance 3.0 mInterchange density -Specific grade or general terrain LevelBase free-flow speed -

Demand Data

Analysis period demand -PHF 0.95Heavy vehicles % 5%Driver population factor 1.00

EXHIBIT 13-6. DEFAULT RAMP LENGTHS FOR SEGMENT LENGTH ESTIMATION

A A M = L - A - B B B

L



EXHIBIT 13-7. DEFAULT RAMP LENGTHS

Interchange Type Typical Value for A or B

Freeway to Freeway 500 mFreeway to Surface Street 250 m

Lane Width and Lateral Clearance

The standard lane width for new freeway construction in the United States is 3.6 m.The standard shoulder width is 3.0 m, but can be increased to 3.6 m for high speedhighways carrying large numbers of trucks (3). These standards may be reduced toaccommodate special historical or environmental constraints.

Lane width data are needed only if it is known that the lanes are significantlynarrower than 3.6 m. Shoulder widths are significant only when narrower than 1.8 m.The analyst should assume that all future facilities will have standard lane widths of 3.6m and shoulders of at least 1.8 m unless the analyst is aware of any over-ridingcircumstances (such as mountainous topography, historic structures, or a physicalobstruction) that might restrict the facility width.

In the case of varying lane widths within a segment, the analyst should compute theaverage of the lane widths and use this average for computing the effects on free-flowspeed.

Interchange Density

The mean number of interchanges per kilometer should be computed for at least a 10km length of freeway in which the segment is located. The interchange density becomessignificant for speed estimation purposes only when the density exceeds 0.5 interchangesper km (an average spacing of 2 km or less).

Specific Grade or General Terrain

The general terrain type of analysis can be used instead of specific grades whereverthere is no single grade on the segment that extends for more than 0.8 km or exceeds 3percent for more than 0.4 km.

The maximum extended grade for freeways is usually 6 percent (4). If fieldmeasurement is not possible and construction plans are not available, extended gradescan be approximated using the analyst’s general knowledge of the local terrain. Use 2percent grade for an extended grade on interstate freeways in otherwise flat terrain. Use 4percent for an extended grade in rolling terrain, and 6 percent for an extended grade inmountainous terrain.

Base Free-Flow Speed and Free-Flow Speed

The base free-flow speed is usually not measurable in the field. It should be set at120 km/h for freeways located in rural areas. For urban areas, the base free-flow speedshould be set at 110 km/h. The segment free-flow speed can be measured in the field. Ifthe estimated free-flow speed is lower than the measured free-flow speed, the analystshould use the measured free-flow speed.

Analysts should be careful not to assume that the free-flow speed for a freeway isequal to its posted speed limit, or the field measured 85th percentile speed. The free-flowspeed is the mean speed measured in the field when volumes are less than 1,300 pc/h/ln.Note that mean speed is not the same as the 50th percentile speed, although they are oftenclose in value.

Analysis Period Demand

The agency's planning, design, analysis policies, and available resources determinethe selection of the analysis period. The duration of the analysis period is typically 1



hour but can be as short as 15 minutes, or can extend for the entire peak period. Theanalysis period should be long enough so that all of the predicted vehicular demandoccurring during that period can be serviced.

The duration of the analysis period should be at least twice as long as the estimatedtravel time to traverse the length of the facility being analyzed, so as to assure computedoperating conditions are applicable to the entire length of the facility.

The analyst may choose to evaluate facility operations over several sequentialanalysis periods. Chapter 8 describes a procedure to compute peak direction, peak-hourdemands from average daily traffic.

Peak-Hour Factor

If all four 15-minute periods within the peak hour have the same volume (whichcould occur under congested conditions), the PHF will be 1.0. If all of the peak-hourtraffic occurs in a single 15-minute period (which is unlikely to occur), the PHF will be0.25. These are the extremes for values of PHF. In the absence of field measurements ofPHF, approximations can be used. For congested conditions, 0.95 is a reasonableapproximation. For conditions in which there is fairly uniform flow throughout the peakhour, but a recognizable peak does occur within the hour, 0.85 could be used for PHF.For conditions in which the peak activity within the peak hour occurs during a relativelyshort period of time, with significantly lower flows during the remainder of the hour, 0.70is a reasonable approximation for PHF.

Heavy Vehicles

The percent of heavy vehicles in rolling and mountainous terrain should be obtainedfrom locally available data for similar facilities and demand conditions. If the breakdownbetween RVs, trucks, and buses is not known, the heavy vehicles can be considered to beall trucks for the purposes of selecting passenger car-equivalents and computing theheavy vehicle adjustment factor.

The percent heavy vehicles tends to be lower in urban areas and higher in rural areas.The percent of trucks on urban freeways will typically range from 3 to 10 percent of totaldaily traffic. Values higher than 10 percent can be observed particularly on urban bypassroutes. These values will be lower during the peak hours. The percentage of trucks canbe quite high on intercity freeways passing through agricultural areas, ranging from 10 to40 percent. Exhibit 13-8 defaults can be used in the absence of local data.

EXHIBIT 13-8. DEFAULT PERCENT HEAVY VEHICLES BY FUNCTIONAL CLASS

Functional Class Caltrans(7) FHWA(9) Default

Rural Interstate 10%-40% 18% 15%Rural Other 10%-40% 12% 10%Urban Interstate 3%-10% 12% 5%Urban Other 3%-10% 10% 5%

Driver Population

The reciprocal of the driver population factor is used to increase the effective baseflow rate to account for a driver population not familiar with the freeway facility. Thefactor should normally be 1.00 but can be reduced to 0.85 for the analysis of weekendconditions in a recreational area.

Research (8) has developed a regression equation relating the driver populationadjustment factor to the seasonal and daily variability in traffic demand levels. Equation13-6 is used to compute the non-local driver index for the facility.

NDI = -2.6 + MF + 1.5DF (13-6)



whereNDI = non-local driver indexMF = ratio of current month mean average daily traffic to average annual

daily traffic (AADT).DF = ratio of hourly volume measured from 1 to 2 pm, divided by the

morning peak-hour volume (7 to 8 am) measured from Monday throughThursday, and averaged over a month.

The resulting NDI is then used to select an appropriate driver population adjustmentfactor (fp) from Exhibit 13-9 .

EXHIBIT 13-9. FREEWAY DRIVER POPULATION FACTOR

NDI fp0.0 or less 1.00

1.0 0.972.0 0.94

3.0 or greater 0.91

SERVICE VOLUME TABLE

Exhibit 13-10 can be used to estimate the number of through lanes required to obtaina desired level of service for basic freeway segments under default conditions. The tablecan be used to test the impact of different interchange densities and is sensitive to thedifferent operating characteristics of urban and rural freeways.

EXHIBIT 13-10. MAXIMUM SERVICE VOLUMES FOR BASIC FREEWAY SEGMENTS(SEE FOOTNOTE FOR ASSUMED VALUES)

Maximum Service Volumes (veh/h)

InterchangeDensity(I/km)

No. ofLanes

Free-FlowSpeed(km/h)

A B C D E

0.63 2 100 1230 1930 2810 3630 40403 102 1880 2960 4300 5480 60904 104 2510 3940 4730 7310 8110

Urban 5 107 3290 5170 7470 9290 10,250

1.25 2 91 1120 1760 2560 3460 39603 93 1710 2700 3920 5260 59704 96 2360 3710 5390 7130 80105 98 3020 4730 6880 8990 10,060

0.31 2 120 1390 2190 3050 3640 39803 120 2090 3290 4570 5470 59704 120 2790 4380 6090 7290 7960

Rural 5 120 3480 5470 7610 9110 9950

0.63 2 117 1360 2140 3000 3620 39603 117 2040 3200 4500 5420 59304 117 2720 4270 6010 7230 79105 117 3400 5340 7510 9040 9890

Assumptions:Urban: 110 km/h base free-flow speed, 3.6 m wide lanes, 1.8 m wide shoulders, level terrain, 5% heavy vehicles, no driverpopulation adjustment, 0.90 PHF.Rural: 120 km/h base free-flow speed, 3.6 m wide lanes, 1.8 m wide shoulders, level terrain, 5% heavy vehicles, no driverpopulation adjustment, 0.85 PHF.


June 1999 Page 13-15 Chapter 13 - Freeway Concepts6020.209.116.02 Freeway Weaving

III. FREEWAY WEAVING

Weaving is defined as the crossing of two or more traffic streams traveling in thesame general direction along a significant length of highway without the aid of trafficcontrol devices (with the exception of guide signs). Weaving areas are formed when amerge area is closely followed by a diverge area, or when an on-ramp is closely followedby an off-ramp and the two are joined by an auxiliary lane. It should be noted that whena one-lane on-ramp is closely followed by a one-lane off-ramp, and the two are notconnected by an auxiliary lane, the merge and diverge movements are consideredseparately using procedures for analysis of ramp terminals.

Weaving areas require intense lane-changing maneuvers as drivers must access lanesappropriate to their desired exit points. Thus, traffic in a weaving area is subject toturbulence in excess of that normally present on basic freeway segments. This turbulencepresents special operational problems and design requirements that are addressed by theprocedures described in Chapter 24.

Exhibit 13-11 shows the formation of a weaving area. If entry and exit roadways arereferred to as legs, vehicles traveling from leg A to leg D must cross the path of vehiclestraveling from leg B to leg C. Flows A-D and B-C are, therefore, referred to as weavingflows. Flows A-C and B-D may also exist in the section, but these need not cross thepath of other flows, and are referred to as non-weaving flows.

EXHIBIT 13-11. FORMATION OF A WEAVING AREA

A

B

C

D

Exhibit 13-11 shows a simple weaving area, formed by a single merge pointfollowed by a single diverge point. Multiple weaving areas may be formed where onemerge is followed by two diverge points, or where two merge points are followed by onediverge point. The analysis of weaving areas is detailed in Chapter 24.

Weaving areas may exist on any type of facility: freeways, multilane highways, two-lane highways (in an interchange area), arterials, or on collector-distributor roadways.While the methodology of Chapter 24 was developed for freeways, guidance is given onadapting the procedure to analyze weaving areas on multilane and/or collector-distributorroadways. No guidance is given for analysis of weaving on arterials, which isconsiderably more complex, and involves signalization issues. At present, there are nogenerally accepted procedures for the analysis of arterial weaving.

There are three geometric variables that influence weaving area operations:configuration, length, and width. These important variables are discussed in the sectionsthat follow.

WEAVING CONFIGURATIONS

The most critical aspect of operations within a weaving area is lane-changing.Weaving vehicles, which must cross a roadway to enter on the right and leave on the left,or vice-versa, accomplish these maneuvers by making the appropriate lane-changes. Theconfiguration of the weaving area (i.e., the relative placement of entry and exit lanes) hasa major impact on the number of lane-changes required of weaving vehicles tosuccessfully complete their maneuvers. There is also a distinction between those lane-changes which must be made to successfully weave, and additional lane-changes that are


Chapter 13 - Freeway Concepts Page 13-16 June 1999Freeway Weaving 6020.209.116.02

discretionary, i.e., are not necessary to complete the weaving maneuver. The former musttake place within the confined length of the weaving area, while the latter are notrestricted to the weaving area itself.

The methodology of Chapter 24 identifies three major categories of weavingconfigurations: Type A, Type B, and Type C. Each has unique characteristics that aredescribed below.

Type A Weaving Configurations

Exhibit 13-12 illustrates two subcategories of Type A weaving areas. Theidentifying characteristic of a Type A weaving area is that all weaving vehicles mustmake one lane change to successfully complete their maneuvers. All of these lanechanges occur across a lane line that connects from the entrance gore area directly to theexit gore area. Such a lane line is referred to as a crown line. Type A weaving areas arethe only such areas to have a crown line.

EXHIBIT 13-12. TYPE A WEAVING AREAS

A

A

B

B

C

C

D

D

(a) Ramp-Weave

(b) Major Weave

The most common form of Type A weaving area is illustrated in Exhibit 13-12 (a).It is formed by a one-lane on-ramp followed by a one-lane off-ramp, with the twoconnected by a continuous auxiliary lane. The lane line between the auxiliary lane andthe right-hand freeway lane is the crown line for the weaving area. All on-ramp vehiclesentering the freeway must make a lane change from the auxiliary lane to the shoulder laneof the freeway; all freeway vehicles exiting at the off-ramp must make a lane changefrom the shoulder lane of the freeway to the auxiliary lane. This type of configuration isalso referred to as a ramp-weave.

Exhibit 13-12 (b) illustrates a major weaving section that also has a crown line. Amajor weaving section is formed when three or four of the entry and exit legs havemultiple lanes. As in the case of a ramp-weave, all weaving vehicles, regardless of thedirection of the weave, must execute one lane change across the crown line of the section.

The two sections illustrated differ primarily in the impact of ramp geometrics onspeed. For most ramp-weave sections, the design speed of the ramps is significantlylower than that of the freeway. Thus, on- and off-ramp vehicles must accelerate ordecelerate as they traverse the weaving area. For major weaving sections, entry and exitlegs often have design speeds that are similar to that of the mainline freeway, and suchacceleration and deceleration is not required. It should be noted that the methodology ofChapter 24 does not reflect this difference, and that the model was calibrated for ramp-weave configurations. Thus, the use of these procedures to analyze the type ofconfiguration illustrated in Exhibit 13-12 (b) is an approximation.



Because all weaving vehicles in a Type A configuration must execute a lane-changeacross the crown line, weaving vehicles are generally confined to occupying the two lanesadjacent to the crown line. Some non-weaving vehicles will also share these lanes. Thiswill essentially limit the number of lanes that weaving vehicles can occupy.

Type B Weaving Configurations

Type B weaving areas are illustrated in Exhibit 13-13. All Type B weaving areas fallinto the general category of major weaving sections, in that such sections always have atleast three entry and exit legs with multiple lanes (except for some collector-distributorconfigurations).

EXHIBIT 13-13. TYPE B WEAVING AREAS

A

BD

C

(a) Major Weave with Lane Balanceat Exit Gore

A

B

C

D

(b) Major Weave with Merge atEntry Gore

A

B

C

D

(c) Major Weave with Merge at EntryGore and Lane Balance at Exit Gore

Once again, it is the lane-changing required of weaving vehicles that characterizesthe Type B configuration:

• One weaving movement may be accomplished without making any lane-changes.• The other weaving movement requires at most one lane change.Exhibit 13-13 (a) and 13-13 (b) show two such weaving areas. In both cases,

movement B-C (entry on the right, departure on the left) may be made without executingany lane changes, while movement A-D (entry on the left, departure on the right) requiresonly one lane-change. Essentially, there is a continuous lane that allows for entry on theright and departure on the left. In Exhibit 13-13 (a), this is accomplished by providing adiverging lane at the exit gore. From this lane, a vehicle may proceed down either exitleg without executing a lane-change. This type of design is also referred to as lanebalanced, that is, the number of lanes leaving the diverge is one more than the number oflanes approaching it.

In Exhibit 13-13 (b), the same lane-changing scenario is provided by having a lanefrom leg A merge with a lane from leg B at the entrance gore. This is slightly lessefficient than providing lane balance at the exit gore, but produces similar numbers oflane-changes by weaving vehicles.

The configuration shown in Exhibit 13-13 (c) is unique, and has both a merge of twolanes at the entrance gore and lane balance at the exit gore. In this case, both weaving



movements can take place without making a lane-change. Such configurations are mostoften found on collector-distributor roadways as part of an interchange.

Type B weaving areas are extremely efficient in carrying large weaving flows,primarily because of the provision of a through lane for at least one of the weavingmovements. Weaving movements can also be made with a single lane-change fromeither of the lanes adjacent to the through lane. Thus, weaving vehicles can occupy asubstantial number of lanes in the weaving section, and are not as restricted in this regardas in Type A sections.

Type C Weaving Configurations

Type C weaving areas are similar to Type B in that one or more through lanes areprovided for one of the weaving movements. The distinguishing characteristic of a TypeC weaving area is that the other weaving movement requires a minimum of two lane-changes for successful completion of a weaving maneuver. Thus, a Type C weaving areais characterized by:

• One weaving movement may be accomplished without making a lane-change.• The other weaving movement requires two or more lane-changes.Exhibit 13-14 illustrates two categories of Type C weaving areas. In Exhibit 13-14

(a), movement B-C does not require a lane-change, whereas movement A-D requires twolane-changes. This type of section is formed when there is neither merging of lanes at theentrance gore nor lane balance at the exit gore, and no crown line exists. Although such asection is relatively efficient for weaving movements in the direction of the through lane,it cannot efficiently handle large weaving flows in the other direction.

EXHIBIT 13-14. TYPE C WEAVING AREAS

A

B

C

D

(a) Major Weave without Lane Balance or Merging

A

B

C

D

(b) Two-Sided Weave

Exhibit 13-14 (b) shows a two-sided weaving area. It is formed by when a right-hand on-ramp is followed by a left-hand off-ramp, or vice-versa. In such cases, thethrough freeway flow becomes a functional weaving flow. Ramp-to-ramp vehicles mustcross all lanes of the freeway to execute their desired maneuver. Freeway lanes are, ineffect, through weaving lanes, and ramp-to-ramp vehicles must make multiple lane-changes as they cross from one side of the freeway to the other. Although it is technicallya Type C configuration, there is little information concerning the operation of suchsections. The methodology of Chapter 24 was calibrated for the type of section in Exhibit13-14 (a), and provides only the roughest of approximations when applied to two-sidedweaving sections.

Impacts of Weaving Configuration

The configuration of the weaving area has a marked affect on weaving areaoperations due to the impact on lane-changing behavior. A weaving area with 1000 vphweaving across 1000 vph in the other direction requires at least 2000 lane-changes in a



Type A section, as each vehicle makes one lane-change. In a Type B section, only onemovement must change lanes, reducing the number of required lane-changes to 1000. Ina Type C section, one weaving flow would not have to change lanes, while the otherwould have to make at least two lane-changes, for a total of 2000 lane-changes.

Because of this, the models and algorithms of Chapter 24 are keyed to the type ofconfiguration, with parameters that depend specifically on configuration. Thus, for agiven number of lanes and length of section, models will predict different operatingcharacteristics for different configurations.

Configuration has a further impact on the proportional use of lanes by weaving andnon-weaving vehicles. Given that weaving vehicles must occupy specific lanes toefficiently complete their maneuvers, the configuration can limit the ability of weavingvehicles to use outer lanes of the section. This effect is most pronounced for Type Asections, as weaving vehicles must primarily occupy the two lanes adjacent to the crownline. It is least severe for Type B sections, as these require the least lane-changing ofweaving vehicles, thus allowing more flexibility in lane use.

WEAVING LENGTH

Because weaving vehicles must execute all of the required lane-changes for theirmaneuver within the weaving area boundary from the entry gore to the exit gore, theparameter of weaving length becomes quite important. The length of the weaving areaconstrains the time and space in which the driver must make all required lane changes.Thus, as the length of a weaving area decreases (configuration and weaving flow beingconstant), the intensity of lane-changing, and the resulting level of turbulence, increases.

The measurement of weaving length is illustrated in Exhibit 13-15. Length ismeasured from a point at the merge gore where the right edge of the freeway shoulderlane and the left edge of the merging lane(s) are 0.6 m apart to a point at the diverge gorewhere the two edges are 3.7 m apart.

EXHIBIT 13-15. MEASURING THE LENGTH OF A WEAVING AREA

Length of Weaving Section

0.6 m 3.7 m

The origin of this measurement methodology relates to early data bases collected bythe then Bureau of Public Roads in the early 1960’s, which were in this format. Laterdata bases conformed to this method for consistency.

Procedures in Chapter 24 generally apply to weaving areas of up to 600 m in length.Weaving may exist in longer sections, but merging and diverging movements are oftenseparated, with lane-changing tending to concentrate near merge and diverge gore areas.For longer sections, merge and diverge areas may be separately analyzed as rampterminals using the procedures of Chapter 25, Ramps and Ramp Terminals. Weavingturbulence may exist to some degree throughout such a long section, but operations areapproximately the same as those for a basic freeway segment, except for the rampinfluence areas near the entry and exit gore areas.

WEAVING WIDTH

The third geometric variable which influences the operation of the weaving area is itswidth, which is defined as the total number of lanes between the entry and exit gore areas,including the auxiliary lane, if present. As the number of lanes increases, the throughputcapacity obviously increases. At the same time, the opportunity for lane-changing alsoincreases for those discretionary lane-changes that may also take place within theweaving area.



TYPE OF OPERATION

While the total number of lanes in the weaving area is of great importance, theproportional use of those lanes by weaving and non-weaving vehicles is even moreimportant. Under normal circumstances, weaving and non-weaving vehicles compete forspace, and operations across all lanes of the facility tend to reach an equilibrium in whichall drivers are experiencing relatively similar conditions. In a weaving area, there is somesegregation of weaving and non-weaving flows, as non-weaving vehicles tend to stay inoutside lanes and weaving vehicles tend to occupy those lanes involved in crossing theroadway. Nevertheless, there is substantial sharing of lanes by weaving and non-weavingvehicles.

Under normal circumstances, weaving vehicles and non-weaving vehicles will reachequilibrium operation in which weaving vehicles effectively occupy Nw lanes of the

section, with non-weaving vehicles occupying the remaining lanes.In a very real sense, however, the lane configuration limits the total number of lanes

that can be utilized by weaving vehicles because of the lane-changes which must bemade. The following general statements illustrate this effect:

• Weaving vehicles may occupy all of a lane in which weaving is accomplishedwithout a lane-change.

• Weaving vehicles may occupy most of a lane from which a weaving maneuvercan be accomplished with a single lane-change.

• Weaving vehicles may occupy a small portion of a lane from which a weavingmaneuver can be completed by making two lane-changes.

• Weaving vehicles cannot occupy a measurable portion of any lane from which aweaving maneuver would require three or more lane-changes.

This translates into limitations on the maximum number of lanes that weavingvehicles can occupy based upon the configuration of the section, as illustrated in Exhibit13-16.

In a typical Type A configuration, almost all ramp vehicles are weaving, i.e., there islittle ramp-to-ramp flow. Thus, the auxiliary lane is almost fully occupied by weavingvehicles. The shoulder lane of the freeway is shared, however, by weaving andnon-weaving vehicles. Studies have shown that weaving vehicles rarely occupy morethan 1.4 lanes of a Type A configuration.

Type B configurations are far more flexible. There is always one through lane forweaving vehicles that can be fully occupied by those vehicles. In addition, the two lanesadjacent to the through lane can also be substantially used by weaving vehicles. Therecan be some usage of the next adjacent lanes as well. Studies have shown that weavingvehicles can occupy up to 3.5 lanes in a Type B configuration.

Type C configurations are somewhat more restrictive than Type B sections,particularly for the movement requiring two or more lane-changes. Weaving vehicles canstill occupy all of the through lane, and substantial portions of the lanes adjacent to thethrough lane. Partial use of other lanes, however, is usually quite restricted. Studiesindicate that the practical limit on lane usage by weaving vehicles in a Type Cconfiguration is 3.0.

In this discussion, two important parameters have been defined:Nw = number of lanes weaving vehicles must occupy to achieve equilibrium

operation with non-weaving vehicles, andNw(max) = maximum number of lanes that can be occupied by weaving vehicles,

based upon geometric configuration.The Chapter 24 methodology includes models for determining values of Nw while

values of Nw(max) have been specified herein. The comparison of the two values

determines the type of operation that is present in the weaving area.



EXHIBIT 13-16. MAXIMUM USE OF LANES BY WEAVING VEHICLES

(a) Type A Weaving Areas

1.4 lanes

3.5 lanes

3.0 lanes

(b) Type B Weaving Areas

(c) Type C Weaving Areas

Where Nw ≤ Nw(max), equilibrium operation will be established. This is referred to

as unconstrained operation, because there are no constraints preventing the equilibriumfrom occurring. Where Nw > Nw(max), weaving vehicles can only occupy Nw(max)

lanes. Thus, they will occupy less space than is needed to establish equilibrium, whilenon-weaving vehicles occupy more space than they would normally. Operations forweaving vehicles become worse, while those for non-weaving vehicles get better. This isreferred to as constrained operation, because the configuration constrains weavingvehicles from establishing equilibrium with non-weaving vehicles.

Under unconstrained operation, weaving and non-weaving vehicles usuallyexperience very similar operational characteristics. In constrained operation, weavingvehicles often experience operating conditions that are markedly worse than for non-weaving vehicles in the same section. Thus, the determination of type of operation is akey step in the Chapter 24 analysis methodology.

SERVICE VOLUMES

The Chapter 24 methodology does not readily produce service volumes. Theprocedure is set up to determine levels of service, with flows and geometrics fullyspecified. Nevertheless, service volumes can be produced by trial-and-errorcomputations, finding volume levels that result in threshold densities for the variouslevels of service.

Service volumes depend upon the type of configuration, the length of the section, thevolume ratio (the proportion of total flow that is weaving), the number of lanes in thesection, and the free-flow speed of the freeway.

Exhibits 13-17 through 13-19 show service volumes for weaving areas for thefollowing standard conditions:

• free-flow speed is 120 km/h,• PHF is 0.90, and• 5 percent truck presence on level terrain (fHV = 0.976).



Service volumes are the maximum full-hour volumes that would result at theindicated levels of service during the worst 15 minutes period of the hour.

EXHIBIT 13-17. TYPE A WEAVING AREA SERVICE VOLUMES1

Service Volume for Level of Service (veh/h)Length (m) VR A B C D E

Three-Lane Sections150 0.10 1690 2940 3820 4600 5310150 0.20 1580 2700 3490 4180 4820150 0.30 1480 2500 3210 3840 4420150 0.40 1390 2330 2970 3560 4100150 0.452 1350 2250 2870 3440 3960300 0.10 1790 3220 4240 5150 5990300 0.20 1710 3010 3920 4740 5490300 0.30 1630 3820 3650 4390 5080300 0.40 1550 2640 3410 4090 4730300 0.452 1510 2560 3300 3820 4250450 0.10 1840 3370 4470 5470 63206

450 0.20 1770 3170 4170 5080 5900450 0.30 1700 2990 3910 4730 5480450 0.40 1630 2830 3670 4430 4860450 0.453 1600 2750 3270 3960 4600600 0.10 1870 3460 4630 5690 63206

600 0.20 1810 3280 4350 5310 6190600 0.30 1750 3110 4090 4970 5770600 0.40 1680 2960 3860 4380 5100

Four-Lane Sections150 0.10 2250 3920 5090 6130 7080150 0.20 2110 3610 4650 5580 6430150 0.30 1980 3330 4280 5120 5900150 0.354 1920 3210 3920 4730 5500300 0.10 2390 4290 5650 6870 7990300 0.20 2280 4010 5230 6320 7330300 0.30 2170 3750 4680 5670 6600300 0.354 2120 3410 4440 5390 6260450 0.10 2460 4490 5960 7290 84306

450 0.20 2360 4230 5570 6770 7870450 0.30 2270 3810 5010 6100 7110450 0.354 2090 3650 4770 5800 67506003 0.10 2500 4610 6170 7580 84306

600 0.20 2410 4380 5790 7080 8250600 0.30 2230 3980 5240 6400 7480600 0.354 2170 3820 5010 6110 7120

Five-Lane Sections150 0.10 2810 4900 6360 7660 8850150 0.205 2640 4510 5810 6970 8030300 0.10 2990 5370 7060 8590 9990300 0.205 2850 5010 6540 7930 9260450 0.10 3070 5610 7450 9120 10,5406

450 0.205 2950 5210 6910 8460 99006003 0.10 3120 5770 7710 9480 10,5406

600 0.205 2960 5390 7180 8830 10,360

Notes:1. Service volumes for a free-flow speed of 120 km/h, PHF = 0.90, and 5% trucks on level terrain; all values rounded to thenearest 10 veh/h.2. For three-lane Type A configurations, volume ratios in excess of 0.45 result in poor operations and likely formation of localqueues.3. The maximum length for Type A weaving sections is 600 m. For longer lengths, merge and diverge junctions are treatedseparately using Ramp procedures.4. For four-lane Type A configurations, volume ratios in excess of 0.35 result in poor operations and likely formation of localqueues.5. For five-lane Type A configurations, volume ratios in excess of 0.20 result in poor operations and likely formation of localqueues.6. Service volume or capacity limited to basic freeway segment capacity.



EXHIBIT 13-18. TYPE B WEAVING AREA SERVICE VOLUMES1

Service Volume for Level of Service (veh/h)

Length (m) VR A B C D E

Three-Lane Sections

150 0.10 1820 3400 4580 5700 63203

150 0.20 1710 3110 4150 5100 6000150 0.30 1600 2840 3760 4590 5380150 0.40 1490 2610 3420 4170 4870150 0.50 1390 2410 3150 3840 4480150 0.60 1310 2250 2940 3580 4180150 0.70 1240 2130 2780 3140 3680150 0.82 1060 1820 2390 2920 3420

300 0.10 1870 3450 4820 6030 63203

300 0.20 1780 3290 4430 5490 63203

300 0.30 1680 3050 4060 4990 5870300 0.40 1590 2830 3740 4570 5360300 0.50 1500 2640 3470 4230 4940300 0.60 1420 2480 3240 3950 4620300 0.70 1360 2350 3070 3750 4380300 0.82 1300 2260 2960 3610 4230

450 0.10 1900 3610 4950 6210 63203

450 0.20 1820 3390 4580 5700 63203

450 0.30 1730 3160 4230 5230 6160450 0.40 1640 2950 3920 4810 5650450 0.50 1560 2770 3650 4460 5230450 0.60 1480 2600 3430 4180 4890450 0.70 1420 2480 3260 3970 4650450 0.82 1370 2390 3140 3830 43904

600 0.10 1910 3660 5020 6320 63203

600 0.20 1840 3450 4690 5840 63203

600 0.30 1760 3240 4350 5390 63203

600 0.40 1680 3040 4050 4980 5850600 0.50 1600 2850 3780 4630 5430600 0.60 1530 2700 3560 4350 5090600 0.70 1460 2570 3390 4140 4850600 0.82 1420 2480 3260 3990 43904

750 0.10 1920 3690 5090 63203 63203

750 0.20 1860 3500 4760 5950 63203

750 0.30 1780 3300 4440 5510 63203

750 0.40 1700 3100 4140 5110 6010750 0.50 1630 2920 3880 4760 5590750 0.60 1560 2770 3660 4480 5250750 0.70 1500 2640 3490 4270 5000750 0.82 1450 2550 3370 4120 43904



EXHIBIT 13-18. TYPE B WEAVING AREA SERVICE VOLUMES (CONT.)



Four-Lane Sections

150 0.10 2430 4530 6110 7590 84303

150 0.20 2280 4140 5530 6800 8000150 0.30 2130 3790 5010 6130 7170150 0.40 1990 3480 4570 5560 6500150 0.50 1860 3100 4080 5000 5860150 0.60 1640 2840 3720 4540 5320150 0.70 1520 2610 3430 4180 4900150 0.82 1420 2430 3190 3890 43904

300 0.10 2500 4720 6430 8040 84303

300 0.20 2380 4390 5900 7320 84303

300 0.30 2250 4070 5410 6660 7830300 0.40 2120 3770 4980 6100 7150300 0.50 2000 3510 4620 5640 6590300 0.60 1890 3300 4330 5270 58604

300 0.70 1810 2890 3800 4630 58604

300 0.82 1560 2700 3540 4320 43904

450 0.10 2530 4820 6590 8280 84303

450 0.20 2420 4520 6110 7600 84303

450 0.30 2310 4220 5650 6970 8210450 0.40 2190 3940 5220 6420 7530450 0.50 2080 3690 4860 5950 6970450 0.60 1980 3470 4570 5580 58604

450 0.70 1890 3300 4340 4920 50604

450 0.82 1650 2870 3760 43904 43904

600 0.10 2550 4880 6700 8430 84303

600 0.20 2450 4600 6250 7790 84303

600 0.30 2350 4320 5800 7180 84303

600 0.40 2240 4050 5390 6640 7800600 0.50 2130 3800 5040 6170 70304

600 0.60 2030 3590 4740 5800 58604

600 0.70 1950 3430 4510 50604 50604

600 0.82 1710 2990 3920 43904 43904

7505 0.10 2560 4920 6780 84303 84303

750 0.20 2480 4670 6350 7930 84303

750 0.30 2380 4400 5920 7340 84303

750 0.40 2270 4140 5520 6810 8020750 0.50 2170 3900 5170 6350 70304

750 0.60 2080 3690 4880 58604 58604

750 0.70 2000 3520 4650 50604 50604

750 0.82 1930 3080 4060 43904 43904



EXHIBIT 13-18. TYPE B WEAVING AREA SERVICE VOLUMES (CONT.)



Five-Lane Sections

150 0.10 3040 5660 7640 9490 10,5403

150 0.20 2850 5180 6910 8510 10,010150 0.30 2660 4740 6260 7660 8970150 0.40 2410 4270 5650 6930 8140150 0.50 2220 3880 5100 6240 7320150 0.60 2050 3550 4650 5680 6650150 0.70 1900 3270 4280 5230 6120150 0.82 1770 3040 3980 43904 43904

300 0.10 3120 5900 8040 10,050 10,5403

300 0.20 2970 5490 7380 9140 10,5403

300 0.30 2810 5080 6770 8330 9790300 0.40 2650 4710 6230 7620 87804

300 0.50 2500 4390 5770 70304 70304

300 0.60 2230 3900 5130 58604 58604

300 0.70 2080 3610 4750 50604 50604

300 0.82 1950 3370 4390 43904 43904

450 0.10 3160 6020 8240 10,340 10,5403

450 0.20 3030 5650 7640 9500 10,5403

450 0.30 2880 5270 7060 8710 10,260450 0.40 2740 4920 6530 8020 87804

450 0.50 2600 4610 6080 70304 70304

450 0.60 2470 4120 5430 58604 58604

450 0.70 2190 3830 5030 50604 50604

450 0.82 2060 3580 4390 43904 43904

600 0.10 3180 6100 8380 10,540 10,5403

600 0.20 3070 5750 7810 9740 10,5403

600 0.30 2930 5400 7250 8980 10,5403

600 0.40 2800 5060 6740 8300 87804

600 0.50 2660 4760 6300 70304 70304

600 0.60 2540 4490 5850 58604 58604

600 0.70 2270 3980 5060 50604 50604

600 0.82 2140 3730 4390 43904 43904

7505 0.10 3200 6160 8470 10,680 10,5403

750 0.20 3090 5830 7930 9920 10,5403

750 0.30 2970 5490 7400 9180 10,5403

750 0.40 2840 5170 6900 8510 10,020750 0.50 2710 4870 6460 70304 70304

750 0.60 2600 4610 5860 58604 58604

750 0.70 2330 4100 5060 50604 50604

750 0.82 2200 3850 4390 43904 43904

Notes:1. Service volumes for a free-flow speed of 120 km/h, PHF = 0.90, and 5% trucks on level terrain; all values rounded to thenearest 10 veh/h.2. For Type B configurations, volume ratios in excess of 0.80 result in poor operations and likely formation of local queues.3. Service volume or capacity limited to basic freeway segment capacity.4. Service volume or capacity limited by maximum practical weaving flows.5. The maximum length for Type B and Type C weaving sections is 750 m. For longer lengths, merge and diverge areas aretreated separately using the Ramp procedures.



EXHIBIT 13-19. TYPE C WEAVING AREA SERVICE VOLUMES1

Service Volume for Level of Service (veh/h)Length (m) VR A B C D E

Three-Lane Sections150 0.10 1820 3350 4490 5540 63204

150 0.20 1700 3060 4040 4950 5790150 0.30 1590 2790 3650 4440 5170150 0.40 1480 2560 3330 4030 4680150 0.503 1380 2360 3060 3700 4300300 0.10 1880 3530 4780 5950 63204

300 0.20 1790 3280 4380 5400 63204

300 0.30 1690 3030 4010 4910 5750300 0.40 1600 2810 3690 4490 5240300 0.503 1510 2620 3420 4150 4830450 0.10 1900 3610 4930 6170 63204

450 0.20 1830 3390 4570 5660 63204

450 0.30 1750 3170 4220 5180 6080450 0.40 1660 2960 3900 4770 5580450 0.503 1580 2770 3640 4420 5160600 0.10 1920 3670 5030 6310 63204

600 0.20 1850 3470 4690 5830 63204

600 0.30 1780 3260 4360 5370 632600 0.40 1700 3060 4050 4970 05820600 0.503 1620 2880 3790 4620 5400750 0.10 1930 3710 5100 6410 63204

750 0.20 1870 3520 4780 5950 63204

750 0.30 1800 3330 4460 5510 63204

750 0.40 1730 3130 4170 5120 6000750 0.503 1660 2960 3910 4770 5580

Four-Lane Section150 0.10 2420 4460 5990 7390 84304

150 0.20 2270 4070 5390 6590 7710150 0.30 2120 3720 4870 5920 6900150 0.40 1970 3410 4440 5370 6240150 0.503 1850 3150 4080 4930 5730300 0.10 2500 4700 6370 7930 84304

300 0.20 2380 4370 5840 7200 84304

300 0.30 2260 4050 5350 6550 7660300 0.40 2130 3750 4920 5990 6990300 0.503 2010 3490 4560 5530 61505

450 0.10 2540 4820 6570 8220 84304

450 0.20 2440 4520 6090 7540 84304

450 0.30 2330 4220 5620 6910 8110450 0.40 2210 3950 5210 6360 7440450 0.503 2100 3700 4850 5900 61505

600 0.10 2560 4890 6700 8410 84304

600 0.20 2470 4620 6250 7770 84304

600 0.30 2370 4350 5810 7160 8430600 0.40 2270 4080 5410 6620 76905

600 0.503 2160 3840 5050 61505 61505

750 0.10 2580 4940 6790 8550 84304

750 0.20 2500 4700 6370 7940 84304

750 0.30 2400 4430 5950 4350 84304

750 0.40 2310 4180 5560 4820 76905

750 0.503 2210 3940 5210 61505 61505

Notes:1. Service volumes for a free-flow speed of 120 km/h, PHF = 0.90, and 5% trucks on level terrain; all values rounded to thenearest 10 veh/h.2. The maximum length for Type B and Type C weaving sections is 750 m. For longer lengths, merge and diverge areas aretreated separately using the Ramp procedures.3. For Type C configurations, volume ratios in excess of 0.50 result in poor operations and likely formation of local queues.4. Service volume or capacity limited to basic freeway segment capacity.5. Service volume or capacity limited by maximum practical weaving flows.


June 1999 Page 13-27 Chapter 13 - Freeway Concepts6020.209.116.02 Ramps and Ramp Junctions

IV. RAMPS AND RAMP JUNCTIONS

A ramp is a length of roadway providing an exclusive connection between twohighway facilities. On freeways, all entering and exiting maneuvers take place on rampsthat are designed to facilitate smooth merging of on-ramp vehicles into the freeway trafficstream and smooth diverging of off-ramp vehicles from the freeway traffic stream ontothe ramp. Computational procedures for the analysis of ramps are contained in Chapter25 of this manual.

RAMP COMPONENTS

A ramp may consist of three geometric elements of interest: the ramp-freewayjunction, the ramp roadway, and the ramp-street junction.

A ramp-freeway junction is generally designed to permit high-speed merging ordiverging to take place with a minimum of disruption to the adjacent freeway trafficstream. The geometric characteristics of ramp-freeway junctions vary. Elements such asthe length and type (taper, parallel) of acceleration or deceleration lanes, free-flow speedof the ramp in the immediate vicinity of the junction, sight distances, and other aspectsmay all influence ramp operations.

Geometric characteristics of ramp roadways also vary from location to location.Ramps may vary in terms of number of lanes (usually one or two), design speed, grades,and horizontal curvature. The design of ramp roadways is seldom a source of operationaldifficulty, unless a traffic incident causes disruption along their length. Ramp-streetterminal problems can cause queuing along the length of a ramp, but this is generally notrelated the design of the ramp roadway.

Freeway-to-freeway ramps have two ramp-freeway terminals, and do not have aramp-street terminal. Many ramps, however, connect limited access facilities to localarterials and collectors. For such ramps, the ramp-street terminal is often a criticalelement in the overall design. Ramp-street junctions can permit uncontrolled mergingand diverging movements, or can take the form of an at-grade intersection. Queuesforming at a ramp-street junction can, under extreme conditions, back up into the ramp-freeway junction, and indeed onto the freeway mainline itself.

OPERATIONAL CHARACTERISTICS

A ramp-freeway junction is an area of competing traffic demands for space.Upstream freeway traffic competes for space with entering on-ramp vehicles in mergeareas. On-ramp demand is usually generated locally, although arterials and collectorsmay bring some drivers to the ramp from more distant origins.

In a merge area, individual on-ramp vehicles attempt to find gaps in the adjacentfreeway lane traffic stream. As most ramps are on the right side of the freeway, thefreeway lane in which on-ramp vehicles seek gaps is designated as lane 1 in this manual.By convention, freeway lanes are numbered from 1 to N, from the right shoulder to themedian.

The action of individual merging vehicles entering the lane 1 traffic stream createsturbulence in the vicinity of the ramp. Approaching freeway vehicles move towards theleft to avoid this turbulence. Studies (2) have shown that the operational impact ofmerging vehicles is heaviest in freeway lanes 1 and 2 and the acceleration lane for adistance extending from the physical merge point to 450 m downstream. Exhibit 13-20shows this influence area for on-ramp and off-ramp junctions.

Interactions are dynamic in ramp influence areas. Approaching freeway vehicleswill move left as long as there is capacity to do so. While the intensity of ramp flowgenerally influences the behavior of freeway vehicles, general freeway congestion canalso act to limit ramp flow, causing diversion to other interchanges or routes.


Chapter 13 - Freeway Concepts Page 13-28 June 1999Ramps and Ramp Junctions 6020.209.113.02

EXHIBIT 13-20. ON- AND OFF-RAMP INFLUENCE AREAS

450 m

Diverge Influence Area

450 m

Merge Influence Area

At off-ramps, the basic maneuver is a diverge, that is a single traffic streamseparating into two separate streams. Exiting vehicles must occupy the lane adjacent tothe off-ramp, lane 1 for a right-hand off-ramp. Thus, as the off-ramp is approached,diverge vehicles move right. This effects a redistribution of other freeway vehicles, asthey move left to avoid the turbulence of the immediate diverge area. Studies (2) showthat the area of most intense turbulence is the deceleration lane plus lanes 1 and 2 in a 450m length extending upstream from the physical diverge point, as shown in Exhibit 13-20.

IMPORTANT PARAMETERS

A number of variables influence the operation of ramp-freeway junctions. Theseinclude all of the variables affecting basic freeway segment operation, including lanewidths, lateral clearances, terrain, driver population, and the presence of heavy vehicles.In addition, there are additional parameters of particular importance to the operation oframp-freeway junctions, including length of acceleration/deceleration lane, ramp free-flow speed, and lane distribution of upstream traffic.

The length of the acceleration or deceleration lane provided has a significant impacton merging and diverging operations. Short lanes provide on-ramp vehicles withrestricted opportunity to accelerate before merging, and off-ramp vehicles with littleopportunity to decelerate off-line. The result is that most acceleration and decelerationmust take place on the mainline, which disrupts through vehicles. Short accelerationlanes also force many vehicles to slow significantly and even stop while seeking anappropriate gap in the lane 1 traffic stream.

Many characteristics influence the free- flow speed of the ramp. These includedegree of curvature, number of lanes, grades, sight distances and others. It is a veryinfluential factor, as it determines the speed at which merging vehicles will enter theacceleration lane and the speed at which diverging vehicles must enter the ramp. This, inturn, determines the amount of acceleration or deceleration which must take place. Rampfree-flow speeds generally vary between 30 km/h and 80 km/h. While best determined inthe field, a default value of 55 km/h is recommended where specific measurements orpredictions are unavailable.

Several factors influence the lane distribution of traffic immediately upstream of anon- or off-ramp. These include number of lanes on the facility, proximity of adjacentupstream and downstream ramps, and the activity on those ramps. As conditions forcemore approaching freeway flow into lanes 1 and 2, merging and diverging maneuversbecome more difficult. Therefore, estimation of the upstream freeway flow approachingin lanes 1 and 2 of the freeway (which are the freeway lanes included in the merge anddiverge influence areas) is important.



CAPACITY OF MERGE AND DIVERGE AREAS

There is no evidence that merging or diverging maneuvers additionally restrict thetotal capacity of the upstream or downstream basic freeway segments. Their influence isprimarily to add or subtract demand at the ramp-freeway junction.

Thus, the capacity of a downstream basic freeway segment is not influenced byturbulence in a merge area. The capacity will be the same as if the segment were a basicfreeway segment. However, as on-ramp vehicles enter the freeway at a merge area, thetotal number of ramp and approaching freeway vehicles that can be accommodated is thecapacity of the downstream basic freeway segment, as shown in Exhibit 13-21.

EXHIBIT 13-21. CAPACITY OF MERGE AREAS

c1

c2

c1 = capacity of merge area, controlled by the capacity of the downstream basic freeway segment.c2 = maximum flow into the merge influence area is 4,600 pc/h.

Similarly, the capacity of an upstream basic freeway segment is not influenced by theturbulence in a diverge area. The total capacity that may be handled by the divergejunction is limited by either the capacity of the approaching (upstream) basic freewaysegment, or the capacity of the downstream basic freeway segment and/or the ramp itself,as shown in Exhibit 13-22. Most breakdowns at diverge areas occur because the capacityof the exiting ramp is insufficient to handle the ramp demand flow. This results inqueuing that backs up into the freeway mainline.

EXHIBIT 13-22. CAPACITY OF DIVERGE AREAS

c4

c3

c2c1

Total diverge capacity cannot be more than the upstreambasic freeway capacity (c1) or the total downstreamcapacity of the basic freeway (c2) plus the ramp (c3).

c4 = maximum freeway flow in lanes 1 and 2 that mayenter the diverge influence area is 4,400 pc/h.

Another capacity value that affects ramp-freeway junction operation is an effectivemaximum number of freeway vehicles that can enter the ramp junction influence areawithout causing local congestion and local queuing. For on-ramps, the total entering flowin lanes 1 and 2 of the freeway plus the on-ramp flow cannot exceed 4,600 pc/h. For off-ramps, the total entering flow in lanes 1 and 2 of the freeway (which includes the off-ramp flow) cannot exceed 4,400 pc/h. Demands exceeding these values will cause localcongestion and local queuing which is designated as level of service F.

It is important to note that when breakdowns occur because too many vehicles areattempting to enter the diverge (c4 in Exhibit 13-22) or merge (c2 in Exhibit 13-21)



influence areas, that total capacity of the merge or diverge area is unaffected. Localcongestion and local queuing is expected to occur without causing a decrease in totalcapacity of the section.

LEVELS OF SERVICE

Levels of service in merge (and diverge) influence areas are defined in terms ofdensity for all cases of stable operation, levels of service A - E. Level of service F existswhen any of the capacity values are exceeded by demand.

LOS A represents unrestricted operations. Density is low enough to permit mergingand diverging to occur smoothly with virtually no turbulence in the traffic stream. AtLOS B, merging and diverging maneuvers become noticeable to through drivers, andminimal turbulence occurs. Merging drivers must adjust speeds to accomplish smoothtransitions from the acceleration lane to the freeway. At LOS C, speed within theinfluence area begins to decline as turbulence levels become noticeable. Both ramp andfreeway vehicles begin to adjust their speeds to accomplish smooth transitions. At LOSD, turbulence levels in the influence area become intrusive, and virtually all vehicles slowto accommodate merging and diverging. Some ramp queues may form at heavily usedon-ramps, but freeway operation remains stable. LOS E represents conditionsapproaching capacity. Speeds reduce significantly, and turbulence is felt by virtually alldrivers. Flow levels approach capacity, and small changes in demand or disruptionswithin the traffic stream can cause both ramp and freeway queues to form.

REQUIRED INPUT DATA AND ESTIMATED VALUES

Exhibit 13-23 illustrates default values of parameters for use by the analyst whenmore specific locally-generated values are not available.

EXHIBIT 13-23. REQUIRED INPUT DATA AND DEFAULT VALUES

Item Default

Geometric Data

Ramp lanes 1 or 2Acceleration lane length 300 mDeceleration lane length 50 mRamp free-flow speed 55 km/h

Demand Data

Demand volume -PHF 0.90Heavy vehicles % 5%Driver population factor 1.0

Ramp Lanes

The analyst should assume single lane ramps unless there is an indication ofparticularly heavy ramp demand. Ramp demands in excess of 1500 veh/h generallywarrant a second lane (5). A metered on-ramp may have two approach lanes toaccommodate demand levels that could otherwise be accommodated by a single lane.One lane may be an HOV bypass lane.

Length of Acceleration/Deceleration Lane

The typical length of acceleration lanes and deceleration lanes for ramps should beobtained from the design standards used by the highway operating agency. In theabsence of this information, values described in Exhibit 13-24 can be used (6).



EXHIBIT 13-24. DEFAULT ACCELERATION/DECELERATION LANE LENGTHS

Item CBD Urban Suburban Rural

Length of acceleration lane (m) 200 200 225 250Length of deceleration lane (m) 50 50 75 100

Assumes 3.6 m wide single lane, between a 2˚- 5˚ angle for exit ramp, and between a 50:1 and 60:1 taper for entry ramp.

Ramp Free-Flow Speed

Ramp free-flow speeds usually range between 30 to 55 km/h depending upon thegrade, alignment, and control. In absence of field observed or locally developed values,55 km/h can be assumed.

Analysis Period Demand

Refer to basic freeway segment description of analysis period demand.

Peak-Hour Factor

Refer to basic freeway segment description of peak-hour factor.

Heavy Vehicles

Refer to basic freeway segment description of heavy vehicles.

Driver Population Factor

Refer to basic freeway segment description of driver population factor.

SERVICE VOLUME TABLES

Service volumes for ramps are difficult to describe due to the number of variableswhich affect operations. The following default conditions have been established for thepresentation of service volume tables:

• ramps consist of a single lane, and are isolated from the influence of adjacentramps,

• the free-flow speed of the ramp is 55 km/h,• the length of the acceleration lane for on-ramps is 300 m; the length of the

deceleration lane for off-ramps is 50 m,• ramps and freeway mainline are in level terrain,• there are 5 percent trucks in all ramp and freeway volumes,• peak hour factor is 0.90, and• lane widths are 3.6 m with standard shoulders.For these conditions, it is possible to establish maximum ramp volumes that can be

accommodated at various levels of service, given an assumed freeway mainline flow.These service volumes are shown in Exhibit 13-25 for on-ramps, and in Exhibit 13-26 foroff-ramps.

Service volumes for levels of service A through D are based on conditions producingthe limiting densities for these levels of service. Service volumes for level of service Eare based on the minimum of three limiting criteria: the capacity of the freeway, themaximum volume that can enter the ramp influence area, and the capacity of the ramp. Insome cases, capacity constraints are more severe than are density constraints. In suchcases, a number of levels of service may not exist in practical terms for combinations oframp and freeway volumes.



EXHIBIT 13-25. SERVICE VOLUMES FOR SINGLE-LANE ON RAMPS (veh/h)

EApproach Mainline Mainline Free-Flow Speed (km/h)

Volume (veh/h) A B C D 120 110 100Four - Lane Freeways

1000 190 1340 17601 17601 17601 17601 17601

2000 N/A 290 1250 17601 17601 17601 17601

3000 N/A N/A 200 10402 10402 10402 10402

4000 N/A N/A N/A 402 402 402 402

Six - Lane Freeways1000 620 17601 17601 17601 17601 17601 17601

2000 5 1660 17601 17601 17601 17601 17601

3000 N/A 550 1510 17601 17601 17601 17601

4000 N/A N/A 890 17002 17002 16952 17002

5000 N/A N/A 280 11102 11102 11102 10603

6000 N/A N/A N/A N/A 11102 11102 10603

Eight-Lane Freeways1000 950 17601 17601 17601 17601 17601 17601

2000 650 17601 17601 17601 17601 17601 17601

3000 360 1520 17601 17601 17601 17601 17601

4000 70 1220 17601 17601 17601 17601 17601

5000 N/A 930 17601 17601 17601 17601 17601

6000 N/A 640 1600 17601 17601 17601 17601

7000 N/A 340 1310 N/A 10502 10502 10502

8000 N/A 50 N/A N/A 4303 2503 803

Notes:1. Service volume limited by capacity of single-lane ramp with free-flow speed of 55 km/h.2. Service volume limited by maximum flow able to enter the ramp influence area.3. Service volume limited by the capacity of the freeway.

EXHIBIT 13-26. SERVICE VOLUMES FOR SINGLE-LANE OFF-RAMPS (veh/h)E

Downstream Mainline Mainline Free-Flow Speed (km/h)Volume (veh/h) A B C D 120 110 100

Four-Lane Freeways500 210 1200 17601 17601 17601 17601 17601

1000 N/A 700 1530 17601 17601 17601 17601

2000 N/A N/A 530 1360 17601 17601 17601

3000 N/A N/A N/A 360 8402 8402 8402

Six-Lane Freeways500 350 1390 17601 17601 17601 17601 17601

1000 N/A 1060 17601 17601 17601 17601 17601

2000 N/A 350 1340 17601 17601 17601 17601

3000 N/A N/A 670 17601 17601 17601 17601

4000 N/A N/A N/A 1140 17601 17601 17601

5000 N/A N/A N/A 450 13002 11903 10603

Eight - Lane Freeways500 490 1480 17601 17601 17601 17601 17601

1000 270 1270 17601 17601 17601 17601 17601

2000 N/A 830 1660 17601 17601 17601 17601

3000 N/A 390 1220 17601 17601 17601 17601

4000 N/A N/A 790 1620 17601 17601 17601

5000 N/A N/A 350 1180 17601 17601 17601

6000 N/A N/A N/A 740 12602 12602 12602

7000 N/A N/A N/A 310 8002 8002 8002

Notes:1. Service volume limited by capacity of single-lane ramp with free-flow speed of 55 km/h.2. Service volume limited by maximum flow able to enter the ramp influence area.3. Service volume limited by the capacity of the freeway.


June 1999 Page 13-33 Chapter 13 - Freeway Concepts6020.209.116.02 Freeway Facilities

V. FREEWAY FACILITIESBasic freeway segments areoutside the influence of rampsor weaving

A freeway facility is composed of four component parts. Weaving Areas aresegments of the freeway where two or more vehicle flows must cross each other’s path.These are usually formed when merge areas are followed by diverge areas. They are alsoformed when an on-ramp is followed by an off-ramp and the two are connected by anauxiliary lane. Ramp Junctions are points at which on- and off-ramps join the freeway.The junction formed at this point is an area of turbulence due to concentrations ofmerging or diverging vehicles. Basic Freeway Segments are outside of the influence areaof ramps or weaving areas of the freeway.

HOV facility with two or morelanes and limited access canbe analyzed as a basicfreeway segment

High Occupancy Vehicle (HOV) lanes are lanes designated for use by vehicles withtwo or more persons and buses, adjacent to general freeway lanes. If an HOV facility hastwo or more lanes in each direction all or part of the day and where access to the HOVfacility is limited from adjacent freeway lanes (e.g., 1.6 km or greater access pointspacing), these procedures may be used.

TRAFFIC MANAGEMENT STRATEGIES

Freeway traffic management is the implementation of strategies to improve freewayperformance when the number of vehicles desiring to use a portion of the freeway at aparticular time exceeds its capacity. There are two approaches to improving systemoperation. Supply management strategies work on improving the efficiency andeffectiveness of the existing freeway or adding additional freeway capacity. Demandmanagement strategies work on controlling, reducing, changing time of travel, oreliminating vehicle trips on the freeway while providing a wider variety of mobilityoptions to those who wish to travel. However, in actual application, some strategies mayaddress both sides of the supply/demand equation. The important point is that there aretwo basic ways to improve system performance.

Supply management strategies are intended to increase capacity. Capacity may beincreased by building new pavement or by managing existing pavement. Supplymanagement has been the traditional form of freeway system management for manyyears. Increasingly, focus is turning to demand management as a tool to address freewayproblems. Demand management programs include alternatives to reduce freeway vehicledemand by increasing the number of persons in a vehicle, by diverting traffic to alternateroutes, by influencing the time of travel, or by reducing the need to travel. To accomplishthese types of changes, demand management programs must rely on incentives ordisincentives to make these shifts in behavior attractive. The application of these twotypes of freeway traffic management strategies is intended to change the relationshipbetween freeway demand and capacity so as to improve overall system performance.

Freeway traffic demand management strategies include the use of priority for highoccupancy vehicles, congestion pricing, and traveler information systems. Somealternative strategies such as ramp metering may restrict demand and possibly increasethe existing capacity. In some cases, spot capacity improvements such as the addition ofauxiliary lanes or minor geometric improvements may be implemented to better utilizeoverall freeway system capacity. The remainder of this section will present the processfor evaluating freeway management strategies, and the most common freeway trafficmanagement techniques. The freeway traffic management process is used to assess theimpact of performance that these strategies might produce.

The Freeway Traffic Management Process

Freeway traffic management is the application of strategies that are intended toreduce the traffic demand using the facility or increase the capacity of the facility. Persondemand can be shifted in time or space, vehicle demand can be reduced by a shift inmode, or total demand can be reduced by a variety of factors. Factors affecting totaldemand include changes in land use, trips being eliminated due to telecommuting,


Chapter 13 - Freeway Concepts Page 13-34 June 1999Freeway Facilities 6020.209.116.02

reduced work-week, or a decision to forego travel. By shifting demand in time (e.g.,leave earlier), shifting demand in space (e.g., take an alternative route), shifting in mode,or by changes in total demand, traffic on a freeway segment may be reduced. Likewise,if freeway capacity has been reduced (e.g., as the result of an incident which has closed alane or adverse weather conditions), improved traffic management can return the freewayto normal capacity sooner, reducing the total delay to travelers.

The basic approach used to evaluate traffic management is to compare alternativestrategies. The base case would be the facility operating without any freeway trafficmanagement. The alternative case would be with the freeway traffic managementstrategy or strategies being evaluated. The alternative case could have different demandsand capacities based on the conditions being evaluated. The evaluations could also bemade for existing or future traffic demands. Combinations of strategies are also possible,but some combinations may be difficult to evaluate due to limited quantifiable data.

Freeway Management Strategies

Freeway traffic management strategies are implemented to make the most effectiveand efficient use of the freeway system. Activities that reduce capacity include incidents(including traffic accidents, disabled or stalled vehicles, spilled cargo, emergency orunscheduled maintenance, traffic diversions, or adverse weather), construction activities,scheduled maintenance activities, and major emergencies (such as earthquakes).Activities that increase demand include special events. Freeway traffic managementstrategies which mitigate capacity reductions include incident management, traffic controlplans for construction, maintenance activities, special events and emergencies, and minordesign improvements (e.g., auxiliary lanes, emergency pull-outs, and accidentinvestigation sites). Freeway traffic management strategies to reduce demand includeplans for incidents, special events, construction, and maintenance activities; entrycontrol/ramp metering; on-freeway HOV lanes; HOV bypass lanes on ramps; travelerinformation systems; and road pricing.

Capacity Management Strategies

Incident management is the most significant freeway strategy generally used byoperating agencies. Incidents can cause significant delays even on facilities that do notroutinely experience congestion. It is generally believed that more than 50 percent ofcongestion is the result of incidents. Strategies to mitigate the effects of incidents includeearly detection and quick response with the appropriate resources. During an incident,effective deployment of management resources can result in a significant reduction in theeffects of incidents. Effective incident management includes proper application of trafficcontrol devices including signage and channelization. It includes quick removal ofvehicles and debris. Incident management may also include the use of accidentinvestigation sites on conventional streets near freeways for follow-up activities.

Lane control signals are a means of conveying the status of individual freeway lanesto motorists. By providing positive guidance through incident sites and maintenanceactivities, the capacity of the location may be maintained, as well as safety.

Geometric adjustments can be made to a freeway to enhance overall freewaycapacity. Examples include the use of auxiliary lanes, the addition of capacity throughthe use of narrow lanes and/or shoulders to eliminate isolated bottlenecks, and thereconfiguration of ramps or ramp geometry.

Construction and reconstruction activities also reduce freeway capacity. Through theuse of sound traffic control plans these effects can be minimized. Maintenance activitiesare similar to construction and reconstruction activities, except that the duration tends tobe shorter. Scheduling this capacity reduction during periods of lower demand canmitigate the effects of these capacity reducing activities.


June 1999 Page 13-35 Chapter 13 - Freeway Concepts6020.209.116.02 Freeway Facilities

Demand Management Strategies

The number of vehicles entering the freeway system is the primary determinant offreeway system performance. Entry control is the most straightforward means to limitingfreeway demand. Entry control can be in the form of temporary or permanent rampclosure. Ramp metering is a more dynamic form of entry control which can limit demandbased on a variety of factors which can be either pre-programmed or implemented inresponse to measured freeway conditions. Freeway demand can be either delayed(changed in time) or can be diverted (changed in space to an alternative route), changedin mode (such as HOV), or eliminated (the trip not being made). The difficult issue inassessing ramp-metering strategies is estimating how demand will shift as a result ofmetering.

High occupancy vehicle alternatives such as mainline HOV lanes or ramp meterbypass lanes are intended to reduce the vehicle demand on the facility without changingthe total number of person trips. Assessing these types of alternatives also requires theability to estimate the number of persons who make a change of mode to HOV (orpossibly an increase in vehicle occupancy). In addition, it is necessary to know the originand destination of the HOV travelers in order to determine what portions of the HOVfacility they can use, since many HOV facilities have some form of restricted access.

Special events result in traffic demands that are based on the particular event. Theseoccasional activities are amenable to the same types of freeway traffic management asused for more routine activities such as daily commuting. In the case of special events,more planning and promotion are required than typically needed for more routineactivities.

Traveler information can affect all aspects of travel demand. It can delay oraccelerate a trip, it can influence trip routing, it can influence mode choice, and it canresult in the elimination of trips.

Road pricing is a complex and evolving freeway traffic management alternative.Initially, road pricing involved a fee for use type operation to provide a means to financehighways. More recently, toll roads have been built as alternatives to congestion. Now,congestion-pricing schemes are being implemented to manage demand on variousfacilities or in some cases to sell excess capacity on HOV facilities. The congestionpricing approach to demand management is to price the facility such that demand atcritical points in time and space along the freeway are kept below capacity byencouraging some users during peak traffic periods to consider alternatives. Non-traditional road pricing schemes are still in their infancy, so little information is currentlyavailable on their effects compared to more traditional toll roads, which only view tolls asa means to recover facility costs.

PERFORMANCE MEASURES

Performance measures for freeway facilities can be summarized by the user in theform of time-space domain contour maps. The most common contour maps are based onspeed and density. The contours on the maps join points of similar traffic performancevalues. For example, the valleys in the speed contour maps indicate time-space regionsof lower speed operations while the ridges in the density contour maps indicate time-space regions of higher density operations. Careful selection of speed and densitycontour threshold values associated with capacity operations will clearly indicateboundaries between undersaturated and oversaturated flow conditions. Other contourthreshold values can be selected to further identify different levels of undersaturated andoversaturated flow conditions.

Contour maps can also be constructed for volume-to-capacity ratios and congestionstatus. The volume-to-capacity ratio contour map is helpful in identifying bottlenecks(v/c values of 1.00) and segments operating close to capacity (v/c values >0.90).Congested portions of the freeway would be identified by negative v/c ratios. Thecongestion status contour map concentrates on providing the shapes and locations of


Chapter 13 - Freeway Concepts Page 13-36 June 1999Freeway Facilities 6020.209.116.02

congested regions. The vertical projection of the congested region denotes the durationof the congestion while the horizontal projection of the congested region denotes thegeographic extent of the congestion. An interesting means of summarizing thecongestion status map is to calculate the area of the congested region on the contour map,which results in units of distance-hours of congestion.

Aggregating the estimated traffic performance measures over the entire length of thefreeway facility provides facility-wide estimates for each 15-minute time interval.Average and cumulative distributions of speed and density for each time interval can bedetermined and patterns of their variation over the connected 15-minute time intervalscan be assessed. Trip times, vehicle-kilometers (or person-kilometers) of travel, andvehicle-hours (or person-hours) of travel can be computed and patterns of their variationover the connected 15-minute time intervals can be assessed.

Aggregating the estimated segment traffic performance measures over the study timeduration provides an assessment of the performance of each segment along the freewayfacility. Average and cumulative distributions of speed and density for each segment canbe determined and patterns of their variation over connected freeway segments can becompared. Trip times, vehicle-kilometers (or person-kilometers) of travel, andvehicle-hours (or person-hours) of travel can be assessed for each segment and compared.

The user can aggregate the estimated traffic performance measures over the entiretime-space domain to provide an overall assessment of the entire freeway facility over thestudy time period. Average speeds, average trip times, vehicle-kilometers (orperson-kilometers) of travel, and vehicle-hours (or person-hours) of travel can be used toassess the overall traffic performance.

VII. REFERENCES

1. SCHOEN, J., MAY, A., REILLY, W., and URBANIK, T. "Speed-FlowRelationships for Basic Freeway Sections." NCHRP Project 3-45 Final Report,JHK & Associates, Tucson, AZ, May 1995.

2. ROESS, R., and ULERIO, J. "Capacity of Ramp-Freeway Junctions." NCHRPProject 3-37 Final Report, Polytechnic University, Brooklyn, NY (November1993).

3. American Association of State Highway and Transportation Officials, A Policyon Geometric Design of Highways and Streets, Washington, D.C. (1990), p. 338.

4. Op. cit., AASHTO, p. 585.5. Op. cit., AASHTO, p. 87.6. Op. cit., AASHTO, p. 985, 989.7. California Department of Transportation, 1995 Annual Average Daily Truck

Traffic on California State Highway System, Sacramento, CA (October 1996).8. LU, J.J., HUANG, W., MIERZEJEWSKI, E., "Driver Population Factors in

Freeway Capacity." Center for Urban Transportation Research, University ofSouth Florida, Tampa, FL, May 1997.

9. Federal Highway Administration, Quick Response Freight Manual, FinalReport, Washington, D.C. (September 1996).

highway capacity manual 2000 contents - unesp

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