16-08-2012, 02:21 PM
Traffic intersections
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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.
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 39:1.
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 that has to be adopted depends on the traffic volume, road
geometry, cost involved, importance of the road etc.
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.
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.
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 all-way stop control are some examples.
The GIVE WAY control requires the driver in the minor road to slow down to a minimum speed and
allow the vehicle on the major road to proceed. Two way stop control requires the vehicle drivers on the
minor streets should see that the conflicts are avoided. Finally an all-way stop control is usually used
when it is difficult to differentiate between the major and minor roads in an intersection. In such a case,
STOP sign is placed on all the approaches to the intersection and the driver on all the approaches are
required to stop the vehicle. The vehicle at the right side will get priority over the left approach. The
traffic control at ’at-grade’ intersection may be uncontrolled in cases of low traffic. 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.