INTRODUCTION TO TRANSPORT

Lecture 3

Introduction to Transport

Lecture 3: Traffic Signal

INTRODUCTION TO TRANSPORT

Lecture 3

Basic Principles of Intersection Signalisation

• Four basic mechanisms

1. Discharge headways at signalised intersections

2. The critical lane and time budget concept

3. Effects of right turning vehicles

4. Delay

INTRODUCTION TO TRANSPORT

Lecture 3

Discharge Headways• Consider N vehicles discharging from the intersection

when a green indication is received.

• The first discharge headway is the time between the initiation of the green indication and the rear wheels of the first vehicle to cross over the stop line.

• The Nth discharge headway (N>1) is the time between the rear wheels of the N-1 th and N th vehicles crossing over the stop line.

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Lecture 3

Discharge Headways

• The headway begins to level off with 4 or 5th vehicle. • The level headway = saturation headway

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Lecture 3

Saturation flow rateIn a given lane, if every vehicle consumes an average of h seconds of green time, and if the signal continues to be uninterruptedly green, then S vph could enter the intersection where S is the saturation flow rate (vehicles per hour of green time per lane) given by

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3600S

h

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Lecture 3

Lost timeStart-up lost time: At the beginning of each green indication as the first few cars in a standing queue experience start-up delays,

e(i) = (actual headway-h) for vehicle I

Calculated for all vehicles with headway>h

green time necessary to clear N vehicles,

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Lecture 3

Lost timeThe change interval lost time: It is estimated by the amount of the change interval not used by vehicles; this is generally a portion of the yellow plus all-red intervals

• The 1994 Highway Capacity Manual (HCM) adds the two lost times together to form one lost time and put it at the beginning of an interval. Default value = 3.0 seconds per phase

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INTRODUCTION TO TRANSPORT

Lecture 3

Effective green time

• Actual green time• Yellow + all red time• The ratio of effective green time to cycle length is

‘green ratio’• Capacity of a lane,

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INTRODUCTION TO TRANSPORT

Lecture 3

Graphical representation

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INTRODUCTION TO TRANSPORT

Lecture 3

Notes on saturation flow

• Updated Greenshield’s Equation

• Ideal saturation headway and flow rate occurs under ideal conditions of 12-ft lanes, no grades, no parking zone, all passenger cars, no turning and location outside CBD

• Saturation flow rate in single lane approaches is less than multilane approaches

• Saturation flow rate and headway has a significant probabilistic component

1.1 2.1T N

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INTRODUCTION TO TRANSPORT

Lecture 3

Example

A given movement at a signalised intersectionreceives a 27-second green time, and 3seconds of yellow plus all red out of a 60-second cycle. If the saturation headway is2.14 seconds/vehicle, the start-up lost time is2 seconds/phase and the clearance lost timeis 1 second/phase, what is the capacity ofthe movement per lane?

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INTRODUCTION TO TRANSPORT

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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Lecture 3

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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INTRODUCTION TO TRANSPORT

Lecture 3

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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Lecture 3

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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Lecture 3

Adding consideration of v/c ratio and PHF

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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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Lecture 3

Adding consideration of v/c ratio and PHF

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Min. cycle length,

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

Desirable cycle length,

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INTRODUCTION TO TRANSPORT

Lecture 3

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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INTRODUCTION TO TRANSPORT

Lecture 3

Effects of right-turning vehicles

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INTRODUCTION TO TRANSPORT

Lecture 3

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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INTRODUCTION TO TRANSPORT

Lecture 3

Effects of right-turning vehicles• Through Car Equivalent

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

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INTRODUCTION TO TRANSPORT

Lecture 3

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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