oregon tech civ475 lindgren1 civ 475 traffic engineering reference: 1. national institute for...

OREGON TECH CIV475 Lindgren

1

CIV 475 Traffic Engineering

Reference:1. National Institute for Advanced Transportation Technology NIATTUniversity of Idaho, Moscow, ID 83844-0901http://niatt.uidaho.edu2. Highway Capacity Manual, US Transportation Research Board


2

When streets meet at an “level” intersection, there is the potential for each vehicle to make up to three (or four) “movements” they are: go straight turn left turn right U-turn (many areas make this illegal)

So for an intersection of two, two-way streets, how many turning “movement” possibilities are there?


3

Conflicting traffic movements cannot share the same space at the same time.

Because of their ability to separate traffic movements in time, traffic signals are one of the most common regulatory fixtures found at intersections.

Other options include: sign control (stop, yield) “separation by space” (overpass / underpass)


4

Traffic signals consisting of manually operated flags were used in London, England as early as the 1860’s

The first American electric signals were installed in Cleveland, Ohio in 1914

By 1917, several signals were interconnected in Salt Lake City, Utah in the first American traffic control system


5

Signal Control Types

Pretimed signals assign right of way according to a

predetermined schedule (or timing plan) the length of each time interval is fixed, usually

based on periodic intersection traffic counts

Actuated signals have varying time intervals based on actual

traffic conditions traffic conditions are “read” by the signal

controller by the use of “vehicle detectors”


6

Inductive Loop Detectors - IDL

Most common form of traffic detectionA loop of metal wire is placed in a saw cut in

the pavement Unfortunately saw cuts have been found to

undermine the structural stability of the pavement in some cases.

Loop detectors operate on the principle of inductance, the property of a wire or circuit element to "induce" currents in isolated but adjacent conductive media.


7

IDL

An IDL detector consists of: an insulated electrical wire, placed on or below the

road surface, attached to a signal amplifier, a power source, and other electronics.

Driving an alternating current through the wire generates an electromagnetic field around the loop.

Any conductor, such as the engine of a car, which passes through the field will absorb electromagnetic energy and simultaneously decrease the inductance and frequency of the loop.


8

For most conventional installations, when the inductance or frequency changes a preset threshold in the actuate detector electronics, this indicates that a vehicle has been detected.

IDL’s are installed in a variety of shapes such as square, rectangle, diamond, circular and octagonal, though each configuration produces a different electromagnetic field. For instance, diamond loops reduce the

probability of detecting vehicles in adjacent lanes.


9

The controller electronics, usually housed in a rugged cabinet at the roadside, detect, amplify, and process loop signals.

The controller orchestrates loop operation and provides power.

A typical controller can handle up to forty loops, though in practice will probably oversee far fewer.


10


11

Detectors operate in either the pulse or presence mode. Presence operation, often used with traffic signals, implies that detector output will remain "on" while a vehicle is over the loop. Pulsed detection requires the detector to generate a short pulse (e.g. 100 to 150 ms) every time a vehicle enters the loop, regardless of the actual departure of the detected vehicle.


12

Rough component costs are given below. Loop with amplifier (purchase and installation) -

$700 per loop Controllers - $2500 per unit Controller Cabinet - $5,000 per unit Fiber optic cable (purchase and installation) -

$300,000 per mile Annual maintenance costs average around 10%

of the original installation and capital cost, adjusted for inflation.


13

Historically the inductive loop detector has been a principal element of freeway surveillance and incident detection systems.

However the information loops supply is limited, and they alone can not provide comprehensive freeway surveillance. Similarly, the precise nature of a loop detected incident can not be ascertained, and loops become less effective for incident detection in low volume conditions.


14

Advances in vehicle sensors and more sophisticated detection algorithms present transportation authorities with the opportunity to implement or enhance automatic incident detection systems. Promising technologies include video image processing, doppler radar detectors, cellular phones, and neural network algorithms.


15

Earlier preemption systems

The unique benefits of Priority One are best viewed in context of three earlier preemption systems, each with its own advantages and limitations.

1.Strobe-based systems

First introduced in the 1960's, these use a strobe lamp on the vehicle and at least one mastarm-mounted optical sensor for each approach to the intersection. The systems are relatively simple, but are limited to a clear line-of-sight path between the vehicle and the intersection. They will not handle curved approaches or work in all weather, in particular heavy fog. Light sources other than the strobes can cause preemptions. Periodic maintenance is required to keep the optical surfaces clean and to replace lamps. Range is based on received signal strength and cannot be controlled with precision. 2.Early radio-based systems

In these systems, one of four preempt directions (nominal north, east, south or west) is selected at the vehicle and is transmitted via one-way radio from the vehicle to the intersection. Radio works in all weather and overcomes the line-of-sight limitations of optical systems. However, the choice of direction at the vehicle can be a problem in case of curved approaches or road networks that do not follow a grid. All intersections within radio range will receive the same preempt direction, sometimes with undesired results. Turn-on and turn-off of preemption are based on received signal strength and cannot be controlled with precision. 3.Sound-based systems

In these systems, the siren of approaching vehicles is detected by directional microphones at the intersection. No special equipment is required for the vehicles. These systems are relatively short in range, and accuracy is affected by ambient noise, surrounding structures and the weather.


16

Priority One uses the satellite-based Global Positioning System (GPS) to determine exact vehicle position, direction of travel and speed, as well as the exact time of day. The same advanced 12-channel GPS receiver is installed at each intersection and in each vehicle. Each intersection serves as a GPS base station with a known location and transmits differential corrections via two-way radio to the GPS receiver in the approaching vehicle. The result is a Differential Global Positioning System (DGPS) with a typical position accuracy of 5 meters (16 feet), without the monthly costs that would normally be associated with a commercial DGPS service, if one is available. Priority One will provide this accuracy anywhere on the globe.

The DGPS approach allows preemption zones to be defined with unmatched accuracy with respect to absolute latitude and longitude, so that there is no possibility of false preemptions. Turn-on and turn-off of preemption calls to the traffic controller can be precisely timed to minimize disturbance of the city's traffic system. The position accuracy made possible by DGPS also makes Priority One suitable for transit applications. In particular, an intersection will know if a bus is transmitting from before or after a bus stop.

The decision of whether or not to issue a preemption call is made by the preemption module at the intersection based on preemption zone data that was previously entered during a setup run, when the module was in a special learn mode. The preempt decision will not only take into account vehicle position (latitude and longitude), but also vehicle speed, which determines the expected time of arrival (ETA). Priority One is an all-digital system based on absolute coordinates. There are no analog adjustments to be made or signal strength thresholds to be set. There is no need for directional antennas. There are no effects due to variations in ambient temperature or component aging, and hence no need for periodic recalibration.


17

Since traffic signals are so common, professional civil engineers (even non-transportation types!) are often expected to know the basics of Signal Timing Design.

Signal Timing Design, at its simplest level, involves finding the appropriate duration for all of the various signal indications.


18

The various movements allowed to move in turn, or in phases.

In some cases, a single movement is given a phase; or movements can be collected and that grouping given a phase

Collected movements are those that can proceed concurrently without any major conflict.


19

Campus Dr. & Alameda

For example, the straight-through and right-turn movements of Campus Drive are collected and permitted to use the intersection simultaneously without any serious danger to the motorists involved. This is one phase of a multi-phase cycle. What about danger to pedestrians, are they

“permitted” to make any conflicting movements during this phase?


20

The reality is that some movements are allowed to proceed during a phase even though they cause conflicts.

Is this ethical… pedestrian volumes are very low at most intersections

Those movements that are allowed but share the “space” with other movements are called permitted,

Those without any conflicts are called protected movements


21

The basic timing elements within each phase include: the green interval, the effective green time, the yellow or amber interval, the all-red interval, the intergreen interval, the pedestrian WALK interval, the pedestrian crossing interval.


22

The green interval is the period of the phase during which the green signal is illuminated.

The yellow or amber interval is the portion of the phase during which the yellow light is illuminated. 2-5 seconds

The effective green time is contained within the green interval and the amber interval. The effective green time, for a phase, is the

time during which vehicles are actually discharging at the design rate through the intersection.


23

Graph of Flow vs. time for one movement at a signalized intersection

Showing the need for use of Effective Green

Flow

(veh/hr)

Red Green (shown in intervals) Amber Red

Effective Green Time Saturation flow rate


24

The all-red interval is the period following the yellow interval in which all of the intersection's signals are red. Some suggest 1sec. min

The intergreen interval is simply the interval between the end of green for one phase and the beginning of green for another phase. It is the sum of the yellow and all-red intervals.


25

The pedestrian WALK interval is the portion of time during which the pedestrian signal says WALK. This period usually lasts around 4-7 seconds and is completely encompassed within the green interval for vehicular traffic. Some pedestrian movements in large cities are separate

phases unto themselves.

Finally, the pedestrian crossing time is the time required for a pedestrian to actually cross the intersection. This is used to calculate the intergreen interval and the

minimum green time for each phase.


26

Intergreen Time

The intergreen period of a phase consists of both the yellow (amber) indication and the all-red indication (if present).

This length of the intergreen phase is governed by three separate concepts: stopping distance, intersection clearance time, pedestrian crossing time, (if there are no

pedestrian signals)


27

The amber signal indication serves as a warning to drivers that another phase will soon be receiving the right-of-way. The intergreen interval, therefore, should be

long enough to allow cars that are greater than the stopping distance away from the stop-line to brake easily to a stop.

The intergreen interval should also allow vehicles that are already beyond the point-of-no-return to continue through the intersection safely.


28

If the intergreen time is too short, only those vehicles that are close to the intersection will be able to continue through the intersection safely.

In addition, only vehicles that are reasonably distant will have adequate time to react to the signal and stop.

This issue is called the"dilemma zone" concept. Those who are in between will be caught in a

"dilemma” and won’t have enough time to stop or safely cross the intersection.


29

The only responsible thing to do, it seems, is to eliminate the dilemma zone.

This would allow any vehicle, regardless of its location, to be able to safely stop or, alternatively, safely proceed during the intergreen period.

This is done by making sure that any vehicle closer to the intersection than its minimum braking distance can safely proceed through the intersection without speeding.


30

First, calculate safe stopping distance

(%)

)(2

2

gradeG

factorfrictionf

onacceleratig

timereactionperceptiont

speedv

Gfg

vvtS


31

Next, we calculate the time required for a vehicle to travel the minimum safe stopping distance and to clear the intersection. This is for the vehicle who has just passed the

safe stopping point (in his mind at least!)


32

velocityv

tionerofwidthW

vehicleoflengthL

cedisstoppingsafeS

timeclearanceT

v

WLST

secint

tan


33

Now that we’ve determined the first two elements of the intergreen period length—stopping distance and intersection clearance time— we may be done (on those intersections that have pedestrian lights)

The intergreen time for intersections that have signalized pedestrian movements is the same as the intersection clearance time.


34

If you have an intersection where the pedestrian movements are not regulated by a separate pedestrian signal, you need to protect these movements by providing enough intergreen time for a pedestrian to cross the intersection. In other words, if a pedestrian begins to cross the

street just as the signal turns yellow for the vehicular traffic, he/she must be able to cross the street safely before the next phase of the cycle begins.


35

)/3.1,/4(~

secint

sin

smsftvelocitypedestrianvp

tionerofwidthW

timegcrospedestrianPCT

v

WPCT

p


36

The intergreen time is equal to whichever is larger, the pedestrian crossing time or the intersection clearance time.

As you know, the intergreen period is composed of the yellow interval and the all-red interval.

The allocation of the intergreen time to these separate intervals is a question that is answered best by referring you to your local codes.


37

In some areas, the yellow time has been standardized for several speeds.

This would make the all-red time the difference between the standard yellow time and the intergreen time.

One other option is to allocate all of the intergreen period as calculated to the yellow interval.

You could then tack on an all-red period as a little extra safety. This, however, might increase delay at your intersection.


38

Pedestrian Crossing Time, Minimum Green Interval

The pedestrian crossing time serves as a constraint on the green time allocated to each phase of a cycle.

Pedestrians can safely cross an intersection as long as there are not any conflicting movements occurring at the same time. (Permitted left and right turns are common exceptions to this rule.)


39

This allows pedestrians to cross the intersection in both the green interval and the intergreen interval.

Thus, the sum of the green interval and the intergreen interval lengths, for each phase, must be large enough to accommodate the pedestrian movements that occur during that phase.


40

At this point, two separate conditions arise. If you have an intersection in which the

pedestrian movements are not assisted by a pedestrian signal, you need to make sure that the green interval that you provide for vehicles will service the pedestrians as well. In this case, the minimum green interval length is

somewhere between 4 and 7 seconds. You already took care of the pedestrian crossing time considerations when you calculated the intergreen period length.


41

If, on the other hand, you plan to provide a pedestrian signal, you need to calculate the pedestrian crossing time as described below.

This will not only give you the information you need to program the pedestrian signal, but it will also allow you to find the minimum green interval for your vehicular movements as well.


42

We need a few assumptions to calculate the pedestrian crossing time.

The WALK signal will be illuminated for approximately 7 seconds.

A pedestrian will begin to cross the street just as the DON'T WALK signal begins to flash.

Pedestrians walk at an average pace of 4 ft/s or 1.2m/s

The WALK interval must be completely encompassed by the green interval of the accompanying vehicle movements.


43

The total time required for the pedestrian movements (T) is the sum of the WALK allowance (Z) and the time required for a person to traverse the crosswalk (R).

R = width of intersection (in feet) ________________ 4 ft/sec

T = Z + R


44

The pedestrian crossing time governs the minimum green time for the accompanying phase in the following way.

If the time it takes the pedestrian to traverse the crosswalk (R) is greater than the intergreen time(I), the remainder of the time (Z+R-I) must be provided by the green interval.

Therefore, the minimum green interval length (gmin) for each phase can be calculated using the equation below.

gmin = T - intergreen time(I) or gmin = Z + R – I


45

If this equation results in a minimum green interval that is less than the WALK time (Z), the minimum green interval length is equal to the WALK time (Z).

gmin = Z

You now have the minimum length of the green interval for the vehicular movements, as governed by the pedestrian movements. The WALK interval for the pedestrians is whatever you assumed, and the DON'T WALK flashes for the remainder of the green and intergreen intervals.


46

Saturation Flow Rate and Capacity

Saturation Flow Rate can be defined with the following scenario:

Assume that an intersection’s approach signal were to stay green for an entire hour, and the traffic was as dense as could reasonably be expected. The number of vehicles that would pass through the intersection during that hour is the saturation flow rate.


47

Obviously, certain aspects of the traffic and the roadway will effect the saturation flow rate of your approach.

If your approach has very narrow lanes, traffic will naturally provide longer gaps between vehicles, which will reduce your saturation flow rate.

If you have large numbers of turning movements, or large numbers of trucks and busses, your saturation flow rate will be reduced.


48

The saturation flow rate is normally given in terms of straight-through passenger cars per hour of green.

Most design manuals and textbooks provide tables that give common values for trucks and turning movements in terms of passenger car units (pcu).


49

Determining the saturation flow rate can be a somewhat complicated matter.

The saturation flow rate depends on roadway and traffic conditions, which can vary substantially from one region to another.

It’s possible that someone in the area has already completed a measurement of the saturation flow rate for an approach similar to yours. If not, you'll need to measure it in the field.


50

One other possibility, which is used quite frequently, is to assume an ideal value for the saturation flow rate and adjust it for the prevailing conditions using adjustment factors.

A saturation flow rate of 1900 vehicles/hour/lane, which corresponds to a saturation headway of about 1.9 seconds, is a fairly common nominal value.

Design manuals usually provide adjustment factors that take parameters such as lane-width, pedestrian traffic, and traffic composition into account.


51

Capacity is an adjustment of the saturation flow rate that takes the real signal timing into account, since most signals are not allowed to permit the continuous movement of one phase for an hour!

If your approach has 30 minutes of green per hour, you could deduce that the actual capacity of your approach is about half of the saturation flow rate.


52

The capacity, therefore, is the maximum hourly flow of vehicles that can be discharged through the intersection from the lane group in question under the prevailing traffic, roadway, and signalization conditions.

c = (g/C) · s c = capacity (pcu/hour) g = Effective green time for the phase in question (sec) C = Cycle length (sec) s = Saturation flow rate (pcu/hour)


53

Capacity can be used as a reference to gauge the current operation of the intersection. For example, let us assume that you know the

current flow rate for a lane group and you also know the capacity of that lane group. If the current flow rate is 10% of the capacity, you would be inclined to think that too much green time has been allocated to that particular lane group. You'll see other uses for capacity as you explore the remaining signal timing design concepts.


54

Peak Hour Volume, Peak Hour Factor, Design Flow Rate

The peak hour volume is the volume of traffic that uses the approach, lane, or lane group in question during the hour of the day that observes the highest traffic volumes for that intersection.

For example, rush hour might be the peak hour for certain interstate acceleration ramps. The peak hour volume would be the volume of passenger car units that used the ramps during rush hour.


55

Notice the conversion to passenger car units.

The peak hour volume is normally given in terms of passenger car units, since changing turning all vehicles into passenger car units makes these volume calculations more representative of what is actually going on.


56

The peak hour factor (PHF) is derived from the peak hour volume.

It is simply the ratio of the peak hour volume to four times the peak fifteen-minute volume.

For example, during the peak hour, there will probably be a fifteen-minute period in which the traffic volume is more dense than during the remainder of the hour. That is the peak fifteen minutes, and the volume of traffic that uses the approach, lane, or lane group during those fifteen minutes is the peak fifteen-minute volume.


57

)min15(*4

)(

volumepeak

volumehourpeakPHF


58

The design flow rate or the actual flow rate, for an approach, lane, or lane group is the peak hour volume (flow rate) for that entity divided by the peak hour factor.

A simpler way to arrive at the design flow rate is to multiply the peak fifteen-minute volume by 4.

However you derive the value, most calculations, such as those that measure the current use of intersection capacity, require the actual flow rate (design flow rate).


59

Critical Movement or Lane

While each phase of a cycle can service several movements or lanes, some of these lanes will inevitably require more time than others to discharge their queue.

For example, the right-turn movement of an approach may service two cars while the straight-through movement is required to service 30 cars. The net effect is that the right-turn movement will be finished long before the straight-through movement.


60

What might seem to be an added complexity is really an opening for simplicity.

If each phase is long enough to discharge the vehicles in the most demanding lane or movement, then all of the vehicles in the movements or lanes with lower time requirements will be discharged as well.

This allows the engineer to focus on one movement per phase instead of all the movements in each phase.


61

The movement or lane for a given phase that requires the most green time is known as the critical movement or critical lane. The critical movement or lane for each phase can be determined using flow ratios.

The flow ratio is the design (or actual) flow rate divided by the saturation flow rate.

The movement or lane with the highest flow ratio is the critical movement or critical lane.


62

Cycle Length Determination

Once you know the total cycle length, you can subtract the length of the amber and all-red periods from the total cycle length and end up with the total time available for green signal indications.

Efficiency dictates that the cycle length should be long enough to serve all of the critical movements, but no longer.

If the cycle is too short, there will be so many phase changes during an hour that the time lost due to these changes will be high compared to the usable green time.

But if the cycle is too long, delays will be lengthened, as vehicles wait for their turn to discharge through the intersection.


63


64

Webster’s equation

Several methods for solving this optimization problem have already been developed, but Webster’s equation is the most prevalent.

Webster's equation, which minimizes intersection delay, gives the optimum cycle length as a function of the lost times and the critical flow ratios.

Many design manuals use Webster's equation as the basis for their design and only make minor adjustments to suit their purposes.


65

Co= 1.5L + 5 1 - S (V/s)

Co = Optimum cycle length (sec) L = Sum of the lost time for all phases, usually

taken as the sum of the intergreen periods (sec) V/s = Ratio of the design flow rate to the

saturation flow rate for the critical approach or lane in each phase


66

After you have calculated the optimum cycle length, you should increase it to the nearest multiple of 5.

For example, if you calculate a cycle length of 62 seconds, bump it up to 65 seconds.

Once you have done this, you are ready to go. If you know the intergreen times for all of the

phases, you can then calculate the total time and allocate it to the various phases based on their critical movements.


67

Green Split Calculations

Once you have the total cycle length, you can determine the length of time that is available for green signal indications by subtracting the intergreen periods from the total cycle length.

But, the result is useless unless you know how to allocate it to all of the phases of the cycle.


68

Green time is allocated using a ratio equation.

Each phase is given a portion of the available green time that is consistent with the ratio of its critical flow ratio to the sum of all the critical flow ratios.

The proportion of the available green time that should be allocated to phase "i" can be found using the following equation:


69

"")(

""int

*)(

)(

iphaseforrateflowcriticals

v

cyclethefortimegreenavilabletheG

iphaseforervalgreenoflengthg

G

svsv

g

i

T

i

T

i

i


70

Timing Adjustments

Once you have calculated the lengths of the minimum green intervals, green intervals, and intergreen intervals, as well as the design flow rates and capacities for each of your phases; it is time to ask yourself whether or not your results actually work.


71

The first and most obvious check involves the green intervals.

Check the length of the green interval for each phase.

If it is not greater than the length of the phase's minimum green interval, you need to bump up the cycle length and add green time to that phase until the green interval is equal to or greater than the minimum.


72

The second check involves capacity. If the capacity of a particular phase is below

the design flow rate for that phase, you should back-calculate the effective green time that would allow the phase to run at the design flow rate.

Once again, simply increasing the cycle length and allocating more time to the green interval of the troubled phase will solve the problem.


73

Webster noted that the cycle length can vary between 0.75Co and 1.5Co without adding much delay, so don't worry too much about adding a few seconds to the nominal cycle length.


74

Computing Delay and LOS

One way to check an existing or planned signal timing scheme is to calculate the delay experienced by those who are using, or who will use, the intersection.

The delay experienced by the average vehicle can be directly related to a level of service (LOS).

The LOS categories, contain information about the progression of traffic under the delay conditions that they represent.


75

LOS based on Highway Capacity Manual 2000 edition (HCM 2000)

Level of Service A - Operations with low delay, or delays of less than

10 seconds per vehicle. This LOS is reached when most of the oncoming

vehicles enter the signal during the green phase, and the driving conditions are ideal in all other respects as well.

Level of Service B - Operations with delays between 10 and 20

seconds per vehicle. This LOS implies good progression, with some vehicles arriving during the red phase.


76

Level of Service C - Operations with delays between 20 and 35

seconds per vehicle. This LOS witnesses longer cycle lengths and fair progression.

Level of Service D - Operations with delays between 35 and 55

seconds per vehicle. At this LOS, congestion is noticeable and longer delays may result from a combination of unfavorable progression, long cycle lengths, and high V/c ratios.


77

Level of Service E - Operations with delay between 55 and 80

seconds per vehicle. This LOS is considered acceptable by most drivers. This occurs under over-saturated intersection

conditions (V/c ratios over 1.0), and can also be attributed to long cycle lengths and poor progression.

Level of Service F – delay >80 seconds/vehicle


78

The first step in the LOS analysis is to calculate the average delay per vehicle for various portions of the intersection.

You might be interested in the LOS of an entire intersection, an approach (say northbound), or you might be interested in the LOS of each individual lane within an approach.


79

Average Stopped Delay Per Vehicle:

d=d1*PF + d2 + d3

d1= uniform control delay

d2= incremental delay

d3= initial queue delay

PF = progression factor

See HCM handout for details

oregon tech civ475 lindgren1 civ 475 traffic engineering reference: 1. national institute for...

Documents

traffic engineering

space overpass underpass

american electric signals

signal controller

electrical wire

use of vehicle detectors

signal amplifier

zloop detectors