lec 12 capacity analysis ( transportation engineering dr.lina shbeeb )
TRANSCRIPT
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Transportation Engineering
Lina Shbeeb
Capacity Analysis
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Traffic flow theory in practice
Used in facility planning
The understanding of facility capacity
Used in design
The sizing and geometric features of facilities
Used in performance measure
The level of service offered by a specific facility
Delay
Travel speeds
Travel times
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Applications of Traffic Flow Theory
Determining turning lane lengths
Average delay at intersections
Average delay at freeway ramp merging areas
Change in the levels of freeway performance
Simulation (mathematical models (algorithms) to study
interrelationship among the different elements of traffic
itcdemo3d.ram
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Various Types of Traffic Volumes
Daily Volume
Average Annual Daily Traffic (AADT)
Average Annual Weekday Traffic (AAWT)
Average Daily Traffic (ADT)
Average Weekday Traffic (AWT)
Hourly Volume
Subhourly Volume
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AADT and AAWT
Average Annual Daily Traffic (AADT)
Average 24-hour traffic volume at a location over a full 365-day year, which is the total number of vehicles passing the location divided by 365
Average Annual Weekday Traffic (AAWT)
Average 24-hour traffic volume on weekdays over a full year, which is the total weekday traffic volume divided by 260
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ADT and AWT
Average Daily Traffic (ADT)
Average 24-hour traffic at a location for any period less than a
year (e.g. six months, a season, a month, a week or even two
days)
Average Weekday Traffic (AWT)
Average 24-hour traffic volume on weekdays for any period less
than a year
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DDHV/PHV
Directional Design Hourly volume (DDHV) ~ Peak
Hourly Volume
DDHV (veh/hour) = AADT x K x D
where AADT = Average Annual Daily Traffic (veh/day)
K = Proportion of daily traffic occurring in
peak hour (decimal)
D = Proportion of peak hour traffic traveling
in the peak direction (decimal)
Contd…
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DDHV/PHV
For design purposes, K factor is generally chosen as 30 HV and D
factor as the percentage of traffic in predominant direction during
the design hour
General ranges for K and D factors
Facility Type K Factor D Factor
Rural 0.15 – 0.25 0.65 – 0.80
Suburban 0.12 – 0.15 0.55 – 0.65
Urban 0.07 – 0.12 0.50 – 0.60
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Sub-Hourly Volume
Sub-hourly Volume ~ 15-min Volume
Suppose, the peak 15-min volume observed = 750 veh
So, the Hourly Volume =
15-min volume x 4
= 750 x 4
= 3000 veh/hour
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Peak Hour Factor
Relationship between hourly volume and maximum rate of flow within the hour
For intersection:
PHF = =
where HV = Hourly volume (veh/hour)
V15 = max. 15-min volume within the hour (veh)
Hourly Volume
Max. Rate of Flow
HV
4 x V15
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Example of PHF Calculation
Data collected are as follows:
Time Interval Volume (veh)
4:00-4:15 pm 950
4:15-4:30 pm 1100
4:30-4:45 pm 1200
4:45-5:00 pm 1050
Hourly Volume 4300
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Example of PHF Calculation
We find from the table,
HV = 4300 veh/hour
V15 = 1200 veh
Therefore,
PHF = = = 0.90
HV
4 x V15
4300
4 x 1200
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Peak Hour Factor
For freeway/expressway:
PHF =
where HV = Hourly volume (veh/hour)
V5 = max. 5-min volume within the hour (veh)
HV
12 x V5
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Traffic Volume
Number of vehicles passing a given point or a section of a
roadway during a specified time
Data collected by
Manual counting
Electro-mechanical devices
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Relationship of Highest Hourly Volume and
ADT on Rural Arterials
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Travel Time and Delay Surveys
Want to determine travel time between two points or causes of
delay along a route
Used for
Performance monitoring
Before and after evaluation of traffic management
schemes or new infrastructure
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Stream measurements: the moving
observer method Moving Observer
stopwatch, pen and paper
laptop PC
instrumented vehicle
=> Chase car versus floating car techniques
Stationary Observer
number plate survey Field staff or optical character recognition from video
Vehicle electronic tags
Mobile phone
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Travel Time
By travelling several times in a car from A to B, a
person notes down the time taken in each run and
then comes up with a statistical average of Travel
Time.
A B
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Delay
Delay = Actual travel time - Expected travel time
Stopped time delay
Travel time delay
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Calculation of Delay
Travel time delay Actual travel time data collected by traveling in a car through
the stretch of the road under study.
Expected travel time calculated from the distance and average speed.
Travel time delay calculated from the difference between actual travel time and expected travel time.
Stopped time data for the vehicular speed of 0 to 5 mph.
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Introduction
Shock wave - a rapid change in traffic conditions (speed,
density, and flow)
A moving boundary between two different traffic states
Forward, backward, and stationary shock waves
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Travel Time
By travelling several times in a car from A to B, a
person notes down the time taken in each run and
then comes up with a statistical average of Travel
Time.
A B
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Delay
Delay = Actual travel time - Expected travel time
Stopped time delay
Travel time delay
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Calculation of Delay
Travel time delay Actual travel time data collected by traveling in a car through
the stretch of the road under study.
Expected travel time calculated from the distance and average speed.
Travel time delay calculated from the difference between actual travel time and expected travel time.
Stopped time data for the vehicular speed of 0 to 5 mph.
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Level of Service (LOS)
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Concept – a qualitative measure describing operational conditions within a traffic stream and their perception by drivers and/or passengers
Levels represent range of operating conditions defined by measures of effectiveness (MOE)
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LOS A (Freeway)
Free flow conditions
Vehicles are unimpeded in
their ability to maneuver
within the traffic stream
Incidents and breakdowns
are easily absorbed
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LOS B
Flow reasonably free Ability to maneuver is
slightly restricted General level of physical and
psychological comfort provided to drivers is high
Effects of incidents and breakdowns are easily absorbed
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LOS C
Flow at or near FFS
Freedom to maneuver is noticeably restricted
Lane changes more difficult
Minor incidents will be absorbed, but will cause deterioration in service
Queues may form behind significant blockage
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LOS D
Speeds begin to decline with increasing flow
Freedom to maneuver is noticeably limited
Drivers experience physical and psychological discomfort
Even minor incidents cause queuing, traffic stream cannot absorb disruptions
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LOS E Capacity
Operations are volatile, virtually no usable gaps
Vehicles are closely spaced
Disruptions such as lane changes can cause a disruption wave that propagates throughout the upstream traffic flow
Cannot dissipate even minor disruptions, incidents will cause breakdown
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LOS F
Breakdown or forced flow
Occurs when: Traffic incidents cause a
temporary reduction in capacity
At points of recurring congestion, such as merge or weaving segments
In forecast situations, projected flow (demand) exceeds estimated capacity
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Design Level of Service
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This is the desired quality of traffic conditions from a driver’s perspective (used to determine number of lanes)
Design LOS is higher for higher functional classes Design LOS is higher for rural areas LOS is higher for level/rolling than mountainous terrain Other factors include: adjacent land use type and development
intensity, environmental factors, and aesthetic and historic values Design all elements to same LOS (use HCM to analyze)
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Design Level of Service (LOS)
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Capacity – Defined
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Capacity: Maximum hourly rate of vehicles or
persons that can reasonably be expected to pass a point, or
traverse a uniform section of lane or roadway,
during a specified time period under prevailing conditions
(traffic and roadway)
Different for different facilities (freeway, multilane, 2-
lane rural, signals)
Why would it be different?
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Ideal Capacity
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Freeways: Capacity (Free-
Flow Speed)
2,400 pcphpl (70 mph)
2,350 pcphpl (65 mph)
2,300 pcphpl (60 mph)
2,250 pcphpl (55 mph)
Multilane Suburban/Rural
2,200 pcphpl (60 mph)
2,100 (55 mph)
2,000 (50 mph)
1,900 (45 mph)
2-lane rural – 2,800 pcph
Signal – 1,900 pcphgpl
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Principles for Acceptable Degree of
Congestion:
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1. Demand <= capacity, even for short time
2. 75-85% of capacity at signals
3. Dissipate from queue @ 1500-1800 vph
4. Afford some choice of speed, related to trip length
5. Freedom from tension, esp long trips, < 42 veh/mi.
6. Practical limits - users expect lower LOS in expensive
situations (urban, mountainous)
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Multilane Highways
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Chapter 21 of the Highway Capacity Manual
For rural and suburban multilane highways
Assumptions (Ideal Conditions, all other conditions
reduce capacity):
Only passenger cars
No direct access points
A divided highway
FFS > 60 mph
Represents highest level of multilane rural and suburban
highways
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Multilane Highways
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Intended for analysis of uninterrupted-flow highway
segments
Signal spacing > 2.0 miles
No on-street parking
No significant bus stops
No significant pedestrian activities
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Source: HCM, 2000
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Source: HCM, 2000
Step 1: Gather data
Step 2: Calculate capacity
(Supply)
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Source: HCM, 2000
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42 Source: HCM, 2000
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Lane Width
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Base Conditions: 12 foot lanes
Source: HCM, 2000
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Lane Width (Example)
44 Source: HCM, 2000
How much does use of 10-foot lanes decrease
free flow speed?
Flw = 6.6 mph
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Lateral Clearance
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Distance to fixed objects
Assumes
>= 6 feet from right edge of travel lanes to obstruction
>= 6 feet from left edge of travel lane to object in median
Source: HCM, 2000
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Lateral Clearance
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TLC = LCR + LCL
TLC = total lateral clearance in feet
LCR = lateral clearance from right edge of travel lane
LCL= lateral clearance from left edge of travel lane
Source: HCM, 2000
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47 Source: HCM, 2000
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Example: Calculate lateral clearance adjustment for a 4-lane
divided highway with milepost markers located 4 feet to the
right of the travel lane.
TLC = LCR + LCL = 6 + 4 = 10
Flc = 0.4 mph
Source: HCM, 2000
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fm: Accounts for friction between opposing directions of
traffic in adjacent lanes for undivided
No adjustment for divided, fm = 1
Source: HCM, 2000
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Fa accounts for interruption due to access points along
the facility
Example: if there are 20 access points per mile, what is
the reduction in free flow speed?
Fa = 5.0 mph
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Estimate Free flow Speed
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BFFS = free flow under ideal conditions
FFS = free flow adjusted for actual conditions
From previous examples:
FFS = 60 mph – 6.6 mph - 0.4 mph – 0 – 5.0 mph =
48 mph ( reduction of 12 mph)
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52 Source: HCM, 2000
Step 3: Estimate
demand
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Calculate Flow Rate
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Heavy Vehicle Adjustment
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Heavy vehicles affect traffic Slower, larger fhv increases number of passenger vehicles to account for presence of heavy trucks
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f(hv) General Grade Definitions:
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Level: combination of alignment (horizontal and vertical) that
allows heavy vehicles to maintain same speed as pass. cars
(includes short grades 2% or less)
Rolling: combination that causes heavy vehicles to reduce
speed substantially below P.C. (but not crawl speed for any
length)
Mountainous: Heavy vehicles at crawl speed for significant
length or frequent intervals
Use specific grade approach if grade less than 3% is more than
½ mile or grade more than 3% is more than ¼ mile)
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Example: for 10% heavy trucks on rolling
terrain, what is Fhv?
For rolling terrain, ET = 2.5
Fhv = _________1_______ = 0.87
1 + 0.1 (2.5 – 1)
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Driver Population Factor (fp)
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Non-familiar users affect capacity
fp = 1, familiar users
1 > fp >=0.85, unfamiliar users
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58 Source: HCM, 2000
Step 4: Determine
LOS
Demand Vs.
Supply
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Calculate vp
Example: base volume is 2,500 veh/hour
PHF = 0.9, N = 2
fhv from previous, fhv = 0.87
Non-familiar users, fp = 0.85
vp = _____2,500 vph _____ = 1878 pc/ph/pl
0.9 x 2 x 0.87 x 0.85
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Calculate Density
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Example: for previous
D = _____1878 vph____ = 39.1 pc/mi/lane
48 mph
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LOS = E
Also, D = 39.1 pc/mi/ln, LOS E
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Design Decision
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What can we change in a design to provide an acceptable
LOS?
Lateral clearance (only 0.4 mph)
Lane width
Number of lanes
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Lane Width (Example)
63 Source: HCM, 2000
How much does use of 10 foot lanes decrease free
flow speed?
Flw = 6.6 mph
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Recalculate Density
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Example: for previous (but with wider lanes)
D = _____1878 vph____ = 34.1 pc/mi/lane
55 mph
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LOS = E
Now D = 34.1 pc/mi/ln, on border of LOS E
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Recalculate vp, while adding a lane
Example: base volume is 2,500 veh/hour
PHF = 0.9, N = 3
fhv from previous, fhv = 0.87
Non-familiar users, fp = 0.85
vp = _____2,500 vph _____ = 1252 pc/ph/pl
0.9 x 3 x 0.87 x 0.85
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Calculate Density
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Example: for previous
D = _____1252 vph____ = 26.1 pc/mi/lane
48 mph
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LOS = D
Now D = 26.1 pc/mi/ln, LOS D (almost C)