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Traffic Intersections - Traffic Engineering and Management - Lecture Notes, Study notes of Software Project Management

Alagappa UniversitySoftware Project Management

Some concept of Traffic Engineering and Management are Non-Intrusive Technologies, Non-Transportation Designers, Parametric Description, Pedestrian Crossing. Main points of this lecture are: Traffic Intersections, Intersection, Traffic Engineers, Conflicts, Time Sharing, Space Sharing, Intersection Control, Passive Control, Explicit Control, Traffic Signs

Typology: Study notes

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Traffic Engineering And Management 27. Principles of Traffic Control
Chapter 27
Principles of Traffic Control
27.1 Overview
Intersection is an area shared by two or more roads. This area is designated for the vehicles
to turn to different directions to reach their desired destinations. Its main function is to
guide vehicles to their respective directions. Traffic intersections are complex locations on any
highway. This is because vehicles moving in different direction wan to occupy same space at the
same time. In addition, the pedestrians also seek same space for crossing. Drivers have to make
split second decision at an intersection by considering his route, intersection geometry, speed
and direction of other vehicles etc. A small error in judgment can cause severe accidents. It also
causes delay and it depends on type, geometry, and type of control. Overall traffic flow depends
on the performance of the intersections. It also affects the capacity of the road. Therefore,
both from the accident perspective and the capacity perspective, the study of intersections very
important for the traffic engineers especially in the case of urban scenario.
27.2 Conflicts at an intersection
Conflicts at an intersection are different for different types of intersection. Consider a typical
four-legged intersection as shown in figure. The number of conflicts for competing through
movements are 4, while competing right turn and through movements are 8. The conflicts
between right turn traffics are 4, and between left turn and merging traffic is 4. The conflicts
created by pedestrians will be 8 taking into account all the four approaches. Diverging traffic
also produces about 4 conflicts. Therefore, a typical four legged intersection has about 32
different types of conflicts. This is shown in figure 27:1.
The essence of the intersection control is to resolve these conflicts at the intersection for
the safe and efficient movement of both vehicular traffic and pedestrians. Two methods of
intersection controls are there: time sharing and space sharing. The type of intersection control
Dr. Tom V. Mathew, IIT Bombay 1 April 2, 2012
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Chapter 27

Principles of Traffic Control

27.1 Overview

Intersection is an area shared by two or more roads. This area is designated for the vehicles to turn to different directions to reach their desired destinations. Its main function is to guide vehicles to their respective directions. Traffic intersections are complex locations on any highway. This is because vehicles moving in different direction wan to occupy same space at the same time. In addition, the pedestrians also seek same space for crossing. Drivers have to make split second decision at an intersection by considering his route, intersection geometry, speed and direction of other vehicles etc. A small error in judgment can cause severe accidents. It also causes delay and it depends on type, geometry, and type of control. Overall traffic flow depends on the performance of the intersections. It also affects the capacity of the road. Therefore, both from the accident perspective and the capacity perspective, the study of intersections very important for the traffic engineers especially in the case of urban scenario.

27.2 Conflicts at an intersection

Conflicts at an intersection are different for different types of intersection. Consider a typical four-legged intersection as shown in figure. The number of conflicts for competing through movements are 4, while competing right turn and through movements are 8. The conflicts between right turn traffics are 4, and between left turn and merging traffic is 4. The conflicts created by pedestrians will be 8 taking into account all the four approaches. Diverging traffic also produces about 4 conflicts. Therefore, a typical four legged intersection has about 32 different types of conflicts. This is shown in figure 27:1. The essence of the intersection control is to resolve these conflicts at the intersection for the safe and efficient movement of both vehicular traffic and pedestrians. Two methods of intersection controls are there: time sharing and space sharing. The type of intersection control

P

P

P P

P

P

P P

P 8 Pedestrian

Conflicts in a traffic signal

8 Right turn−Through

Total = 32 Conflicts

4 Merging

4 Right turn

4 Through traffic

4 Diverging

Figure 27:1: Conflicts at an intersection

that has to be adopted depends on the traffic volume, road geometry, cost involved, importance of the road etc.

27.3 Levels of intersection control

The control of an intersection can be exercised at different levels. They can be either passive control, semi control, or active control. In passive control, there is no explicit control on the driver. In semi control, some amount of control on the driver is there from the traffic agency. Active control means the movement of the traffic is fully controlled by the traffic agency and the drivers cannot simply maneuver the intersection according to his choice.

27.3.1 Passive control

When the volume of traffic is less, no explicit control is required. Here the road users are required to obey the basic rules of the road. Passive control like traffic signs, road markings etc. are used to complement the intersection control. Some of the intersection control that are classified under passive control are as follows:

  1. No control If the traffic coming to an intersection is low, then by applying the basic rules of the road like driver on the left side of the road must yield and that through movements will have priority than turning movements. The driver is expected to obey these basic rules of the road.
  2. Traffic signs: With the help of warning signs, guide signs etc. it is able to provide some level of control at an intersection. Give way control, two-way stop control, and

27.3.3 Active control

Active control implies that the road user will be forced to follow the path suggested by the traffic control agencies. He cannot maneuver according to his wish. Traffic signals and grade separated intersections come under this classification.

  1. Traffic signals: Control using traffic signal is based on time sharing approach. At a given time, with the help of appropriate signals, certain traffic movements are restricted where as certain other movements are permitted to pass through the intersection. Two or more phases may be provided depending upon the traffic conditions of the intersection. When the vehicles traversing the intersection is very large, then the control is done with the help of signals. The phases provided for the signal may be two or more. If more than two phases are provided, then it is called multiphase signal. The signals can operate in several modes. Most common are fixed time signals and vehicle actuated signals. In fixed time signals, the cycle time, phases and interval of each signal is fixed. Each cycle of the signal will be exactly like another. But they cannot cater to the needs of the fluctuating traffic. On the other hand, vehicle actuated signals can respond to dynamic traffic situations. Vehicle detectors will be placed on the streets approaching the intersection and the detector will sense the presence of the vehicle and pass the information to a controller. The controller then sets the cycle time and adjusts the phase lengths according to the prevailing traffic conditions.
  2. Grade separated intersections: The intersections are of two types. They are at-grade intersections and grade-separated intersections. In at-grade intersections, all roadways join or cross at the same vertical level. Grade separated intersections allows the traffic to cross at different vertical levels. Sometimes the topography itself may be helpful in con- structing such intersections. Otherwise, the initial construction cost required will be very high. Therefore, they are usually constructed on high speed facilities like expressways, freeways etc. These type of intersection increases the road capacity because vehicles can flow with high speed and accident potential is also reduced due to vertical separation of traffic.

27.4 Summary

Traffic intersections are problem spots on any highway, which contribute to a large share of accidents. For safe operation, these locations should be kept under some level of control de- pending upon the traffic quantity and behavior. Based on this, intersections and interchanges

are constructed, the different types of which were discussed in the chapter.

27.5 Problems

  1. The GIVE WAY control

(a) requires the driver in the minor road to slow down to a minimum speed and allow the vehicle on the major road to proceed. (b) requires the driver in the major road to slow down to a minimum speed and allow the vehicle on the minor road to proceed. (c) requires the drivers on both minor and major roads to stop. (d) is similar to one way control.

  1. Traffic signal is an example of

(a) Passive control (b) No control (c) Active control (d) none of these

27.6 Solutions

  1. The GIVE WAY control

(a) requires the driver in the minor road to slow down to a minimum speed and allow the vehicle on the major road to proceed.

(b) requires the driver in the major road to slow down to a minimum speed and allow the vehicle on the minor road to proceed. (c) requires the drivers on both minor and major roads to stop. (d) is similar to one way control.

  1. Traffic signal is an example of

(a) Passive control (b) No control