introduction to transport lecture 3 introduction to transport lecture 4: traffic signal

INTRODUCTION TO TRANSPORT

Lecture 3

Introduction to Transport

Lecture 4: Traffic Signal


Lecture 3

Critical Lane• This concept is used for the allocation of the 3600

seconds in the hour to lost time and to productive movement.

• The amount of time required for each signal phase is determined by the most intensely used lane which is permitted to move during the phase.

• All other lane movement in the phase require less time than the critical lane.

• The timings of any signal phase is based on the flow and lost times of the critical lane.

• Each signal phase has one and only one critical lane.

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Capacity (using critical Lane volume)• Capacity can be maximum sum of critical lane

volumes that a signal can accommodate.• the max. total volume that can be handled on all

critical lanes for a given time budget (within an hour),

• tL total lost time per phase

• N is total number of phases in a cycle • C is cycle length

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1 36003600 ( )( )( )c LV N t

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Capacity (using critical Lane volume)

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•the effect of number of phases and cycle time on Vc

•Lost time remains constant through out (h= 2.15s, lost time = 3s/phase)


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Adding consideration of v/c ratio and PHF

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(volume-to-capacity) V/C ratio:

• flow rate in a period expressed as an hourly equivalent over capacity (saturation flow rate)

• the proportion of capacity being utilized

• A measure of sufficiency of existing or proposed capacity

• V/C ratio = 1.00 is not desirable


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Peak Hour Factor (PHF) :

• To account for flow variation within an hour

• PHF

• For 15 min. aggregate volume, PHF =

• The lower the value, the greater degree of variation in flow during an hour.

hourly volume (vph)

maximum rate of flow (vph)

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Min. cycle length,

• Considering desired v/c ratio,• Considering peaking within hour,

Desirable cycle length,

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13600

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1PHF 3600

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Effects of right-turning vehicles• Right turns can be made from a

• Shared lane operation

• Exclusive lane operation

• Traffic signals may allow permitted or protected right turn

• Right-turning vehicles look for a gap in the opposing traffic on a permitted turning movement, which is made through a conflicting pedestrian or an opposing vehicle flow.

• Right-turning vehicles consume more effective green time than through vehicles.

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Effects of right-turning vehicles

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Effects of right-turning vehicles• Through Car Equivalent

• Example: with an opposing flow of 700 vph which has no platoon structure, it is observed that the right lane of the figure processes two RT vehicles and three TH vehicles in the same time that the left lane processes 17 TH vehicles. What is the “THcar” equivalent of one right-turning vehicle (RT equivalent) in this case?

3 + 2 ERT = 17 or ERT = 7In this situation, 1 RT vehicle is equivalent to 7 TH vehicles in terms of headway.

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Effects of right-turning vehicles• Through Car Equivalent

– depends on the opposing– flows, and the number of opposing lanes

• The right turn adjustment factor is related to the fraction of RT vehicles and the TH equivalency

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RTf


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ExampleExample: consider an approach with 10% RT, two

lanes, permitted RT phasing, a RT equivalency factor of 5, and an ideal saturation headway of 2 sec per veh. Determine

(1) the equivalent saturation headway for this case,(2) the saturation flow rate for approach, and (3) the adjustment factor for the sat. flow rate? (adj.

flow rate / sat flow rate of TH vehicles)

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Performance measures

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• Delay

• Queuing

• Stops

Delay most directly affects driver experience.


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Performance measures

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• Stopped Time Delay: time a vehicle stopped waiting to pass the intersection.

• Approach Delay:

stopped time + acceleration + deceleration

• Travel Time Delay: (actual travel time-desired travel time)

• Time-in-queue Delay: Total time from joining a queue to passing the stop line


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Delay

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Webster’s Delay Model

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• Webster’s uniform delay (UD) formula


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Webster’s Delay Model • Webster’s uniform delay (UD) formula

• Red time,

• Height of the triangle,

• Area of the triangle, (UD)

• Average delay per vehicle, Del

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21

UD=2 1

C g c

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Webster’s optimum cycle length

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developed based on minimization of overall delay at the intersection.


Lecture 3

Webster’s optimum cycle length

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Notes on cycle length

• Optimum cycle length is min. delay point of the suitable curve

• Unnecessarily long cycle lengths cause substantial delays; short cycle lengths may cause congestion or violate the pedestrian crossing times.

• Cycle lengths between 45 and 180 s are used in the field. All cycle lengths typically end in 0 or 5; thus, if the cycle length estimate is 52s, a cycle equal to 50 or 55s should be selected.D

elay


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ExampleExample: consider an approach volume of 1000 vph,

saturation flow rate of 2800vphg, cycle length 90 s and g/C ratio 0.55.What average approach delay per vehicle is expected?

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