Traffic Engineering-Lecture 7 PDF

University of Duhok
College of Engineering
Civil Department
Fourth Year Students
2019-2020
Traffic Engineering
Lecture 7
Lecturer: Dr. Nasreen A. Hussein
1
Traffic Signals
Due to conflicting traffic movements, roadway intersections are a source of great concern to
traffic engineers. Intersections can be a major source of crashes and vehicle delays. Most
roadway intersections are not signalized due to low traffic volumes and adequate sight distances.
However, at some point, traffic volumes and accident frequency/severity (and other factors)
reach a level that warrants the installation of a traffic signal. The conflicts arising from
movements of traffic in different directions is addressed by time sharing principle.
The installation and operation of a traffic signal to control conflicting traffic and pedestrian
movements at an intersection has advantages and disadvantages.
The advantages include:
 A potential reduction in some types of crashes (particularly angle crashes)

 Provision for pedestrians to cross the street
 Provision for side-street vehicles to enter the traffic stream
 Provision for the progressive flow of traffic in a signal-system corridor
 Possible improvements in capacity
 Possible reductions in delays
However, signals are by no means the perfect solution for delay or accident problems at an
intersection. A poorly timed signal or one that is not justified can have a negative impact on the
operation of the intersection.
The disadvantages include:
 Increasing vehicle delay

 Increasing the rate of vehicle accidents (particularly rear-end accidents)
 Causing a disruption in traffic progression (adversely impacting the through movement of
traffic)
 Encouraging the use of routes not intended for through traffic (such as routes through
residential neighborhoods)
 Traffic signals are also costly to install
2
Basic types of intersections:
Typical signalized intersection elements
3
Flared intersections: Flaring involves extra lanes through the intersection
Flared intersections
Types of conflict at intersections: Conflicts at an intersection are different for different types of
intersection.
4
Consider a typical four-legged intersection as shown in Figure (1).
 The number of conflicts for competing through movements are 4,

 While competing left turn and through movements are 8.
 The conflicts between left turn traffics are 4,
 And between merging traffic is 8.
 Diverging traffic also produces about 8 conflicts.
Therefore, a typical four legged intersection has about 32 different types of conflicts.
Potential conflicts at intersections with three and four legs.
A number of definitions and notations need to be understood in signal design. They are discussed
below:
Approach: The roadway leading to an intersection.
Cycle: One complete sequence (for all approaches) of signal indications (greens, yellows, reds).
Cycle length: Cycle length is the time in seconds that it takes a signal to complete one full cycle
of indications. It indicates the time interval between the starting of green for one approach till
the next time the green starts. It is denoted by C.
Green time: The amount of time within a cycle for which a movement or combination of
movements receives a green indication (the illumination of a signal lens). This is expressed in
seconds and given the symbol G.
5
Yellow time: The amount of time within a cycle for which a movement or combination of
movements receives a yellow indication. This is expressed in seconds and given the symbol Y.
This time is referred to as the change interval, as it alerts drivers that the signal indication is about
to change from green to red.
Red time: The amount of time within a cycle for which a movement or combination of
movements receives a red indication. This is expressed in seconds and given the symbol R.
All-red time: The time within a cycle in which all approaches have a red indication (expressed in
seconds and given the symbol AR). This time is referred to as the clearance interval, because it
allows vehicles that might have entered at the end of the yellow interval to clear the intersection
before the green phase starts for the next conflicting movement(s).
Phase: The sum of the displayed green, yellow, and red times for a movement or combination of
movements that receive the right of way simultaneously during the cycle. The sum of the phase
lengths (in seconds) is the cycle length. eg a through phase or a through-and-left phase.
Inter-green: is the time period between green displays for conflicting movements, consisting of:
–Yellow time
–All-red time
Lost time: It indicates the time during which the intersection is not effectively utilized for any
movement, see Figure (2).
6
Basic traffic signal capacity model
Total lost time/phase = start-up lost time + end lost time
l =l1+l2 for one phase, consider 2 sec if not given

Actual green time +amber= effective green +total lost time/phase
G+a=g+ l
Amber time = a= 3 sec. Lost time in I= I -a
Phase1 Red Green amber Red

Phase2 Red Green
Inter-green=I= Amber +All red
I=a+allred
In certain circumstances, there is another element of lost time (the signal on all phases showing
red)
Lost time in I= I - a
7
Total lost time/cycle = n l + n (I – a)
Where n= number of phases per cycle
Basic types of control:

Traffic signal controllers are designed to operate in one or more of the following modes:
 Pre-timed (Fixed time): Green Periods and Cycle lengths are pre-determined with fixed
duration and does not change in response to changes in traffic flow at the intersection.
 Semi-actuated: Flexible to adjust according to traffic demand and detectors are located only
on the side roads.
 Fully actuated: Green periods vary and are related to actual demands made by traffic.
Type Advantages
Pre-timed (Fixed time)  Simple construction
 inexpensive
Semi-actuated  Flexible to adjust accord to traffic demand
 Delay is minimized to attain maximum capacity
Fully actuated  Useful for junctions of a low traffic volume
Type Disadvantages
Pre-timed (Fixed time)  Inflexible

 Cause avoidable delay
Semi-actuated  High traffic accidents at times of light traffic
Fully actuated  Require costly equipment’s such as detectors
Pre-timed (fixed time) Signal

Signal faces we have are:
 Red
 Amber (Yellow)
 Green
8
The Determination of cycle time is one of most important steps in fixed-signal designing. Least
delay should be caused to the traffic passing through the intersection. Therefore for each traffic
flow there is an optimum cycle time which results in the minimum delay to the vehicles.
Optimum cycle time, Co
1.5𝐿 + 5
𝐶𝑜 =
1−𝑌
Where
Co = Optimum cycle time in sec
L= Total lost time per cycle in sec
Y= Sum of y values for each phase
Y = ∑ 𝑦𝑖
Y = y1+y2+………. +yn
 y1+y2+……….+yn are the maximum ratios of flow to saturation flow for phases1,2,…..n ( y=
q/s where q is the flow and s is the saturation flow).
 The Y value according to Webster is taken as the highest ratio of q/s.
 CMax = 120 sec
 CMin = 40 sec
The Co depends on traffic conditions. The cycle time is longer when the intersection is heavily
trafficked.
Example 1: Consider a four legged intersection controlled by two phase traffic signals carrying
the traffic data shown in the figure below. If there are to be 10 sec each cycle when red shown
simultaneously on both phases, determine the optimum cycle time and actual green time of
signal setting.
9
Solution:
1.5L + 5
Co =
1−Y
n (I – a)= 10 sec, Given
L= Total lost time/cycle = n l + n (I – a)
L=2*2+10= 14 sec
Phase 1 Phase 2
N S E W
q 500 400 300 250
S 1500 1200 1200 1250
q/S 0.33 0.33 0.25 0.2
y 0.33 0.25
Y 0.58
10
1.5∗14+5
Co = = 62𝑠𝑒𝑐
1−0.58
Total effective green time (gT)= Co – L

gT= 62- 14= 48 sec
The effective green times for each phase can be calculated as:
𝑦(𝑁𝑆)
𝑔(𝑁𝑆) = ∗ 𝑔𝑇
𝑌
0.33
𝑔(𝑁𝑆) = ∗ 48 = 27.4 ≈ 27 sec
0.58
𝑦(𝐸𝑊)
𝑔(𝐸𝑊) = ∗ 𝑔𝑇
𝑌
0.25
𝑔(𝐸𝑊) = ∗ 48 = 20.6 ≈ 21 sec
0.58
Actual green time=G= g+ l- a
G(NS) = 27+2-3= 26 sec

G(EW) = 21+2-3= 20 sec
11

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University of Duhok

Lecturer: Dr. Nasreen A. Hussein

 A potential reduction in some types of crashes (particularly angle crashes)

 Increasing vehicle delay

Typical signalized intersection elements

 The number of conflicts for competing through movements are 4,

Potential conflicts at intersections with three and four legs.

Total lost time/phase = start-up lost time + end lost time

l =l1+l2 for one phase, consider 2 sec if not given

Amber time = a= 3 sec. Lost time in I= I -a

Phase1 Red Green amber Red

Inter-green=I= Amber +All red

Where n= number of phases per cycle

Basic types of control:

Pre-timed (Fixed time)  Inflexible

Fully actuated  Require costly equipment’s such as detectors

Pre-timed (fixed time) Signal

Optimum cycle time, Co

Total effective green time (gT)= Co – L

G(NS) = 27+2-3= 26 sec

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