16-08-2012, 02:36 PM
Traffic rotaries
[attachment=31681]
Overview
Rotary intersections or round abouts are special form of at-grade intersections laid out for the movement of
traffic in one direction around a central traffic island. Essentially all the major conflicts at an intersection
namely the collision between through and right-turn movements are converted into milder conflicts namely
merging and diverging. The vehicles entering the rotary are gently forced to move in a clockwise direction in
orderly fashion. They then weave out of the rotary to the desired direction. The benefits, design principles,
capacity of rotary etc. will be discussed in this chapter.
Advantages and disadvantages of rotary
The key advantages of a rotary intersection are listed below:
1. Traffic flow is regulated to only one direction of movement, thus eliminating severe conflicts between
crossing movements.
2. All the vehicles entering the rotary are gently forced to reduce the speed and continue to move at slower
speed. Thus, none of the vehicles need to be stopped,unlike in a signalized intersection.
3. Because of lower speed of negotiation and elimination of severe conflicts, accidents and their severity are
much less in rotaries.
4. Rotaries are self governing and do not need practically any control by police or traffic signals.
5. They are ideally suited for moderate traffic, especially with irregular geometry, or intersections with more
than three or four approaches.
Although rotaries offer some distinct advantages, there are few specific limitations for rotaries which are listed
below.
1. All the vehicles are forced to slow down and negotiate the intersection. Therefore, the cumulative delay
will be much higher than channelized intersection.
2. Even when there is relatively low traffic, the vehicles are forced to reduce their speed.
3. Rotaries require large area of relatively flat land making them costly at urban areas.
4. The vehicles do not usually stop at a rotary. They accelerate and exit the rotary at relatively high speed.
Therefore, they are not suitable when there is high pedestrian movements.
Guidelines for the selection of rotaries
Because of the above limitation, rotaries are not suitable for every location. There are few guidelines that help
in deciding the suitability of a rotary. They are listed below.
1. Rotaries are suitable when the traffic entering from all the four approaches are relatively equal.
2. A total volume of about 3000 vehicles per hour can be considered as the upper limiting case and a volume
of 500 vehicles per hour is the lower limit.
3. A rotary is very beneficial when the proportion of the right-turn traffic is very high; typically if it is more
than 30 percent.
4. Rotaries are suitable when there are more than four approaches or if there is no separate lanes available
for right-turn traffic. Rotaries are ideally suited if the intersection geometry is complex.
Traffic operations in a rotary
As noted earlier, the traffic operations at a rotary are three; diverging, merging and weaving. All the other
conflicts are converted into these three less severe conflicts.
1. Diverging: It is a traffic operation when the vehicles moving in one direction is separated into different
streams according to their destinations.
2. Merging: Merging is the opposite of diverging. Merging is referred to as the process of joining the traffic
coming from different approaches and going to a common destination into a single stream.
3. Weaving: Weaving is the combined movement of both merging and diverging movements in the same
direction.
[attachment=31681]
Overview
Rotary intersections or round abouts are special form of at-grade intersections laid out for the movement of
traffic in one direction around a central traffic island. Essentially all the major conflicts at an intersection
namely the collision between through and right-turn movements are converted into milder conflicts namely
merging and diverging. The vehicles entering the rotary are gently forced to move in a clockwise direction in
orderly fashion. They then weave out of the rotary to the desired direction. The benefits, design principles,
capacity of rotary etc. will be discussed in this chapter.
Advantages and disadvantages of rotary
The key advantages of a rotary intersection are listed below:
1. Traffic flow is regulated to only one direction of movement, thus eliminating severe conflicts between
crossing movements.
2. All the vehicles entering the rotary are gently forced to reduce the speed and continue to move at slower
speed. Thus, none of the vehicles need to be stopped,unlike in a signalized intersection.
3. Because of lower speed of negotiation and elimination of severe conflicts, accidents and their severity are
much less in rotaries.
4. Rotaries are self governing and do not need practically any control by police or traffic signals.
5. They are ideally suited for moderate traffic, especially with irregular geometry, or intersections with more
than three or four approaches.
Although rotaries offer some distinct advantages, there are few specific limitations for rotaries which are listed
below.
1. All the vehicles are forced to slow down and negotiate the intersection. Therefore, the cumulative delay
will be much higher than channelized intersection.
2. Even when there is relatively low traffic, the vehicles are forced to reduce their speed.
3. Rotaries require large area of relatively flat land making them costly at urban areas.
4. The vehicles do not usually stop at a rotary. They accelerate and exit the rotary at relatively high speed.
Therefore, they are not suitable when there is high pedestrian movements.
Guidelines for the selection of rotaries
Because of the above limitation, rotaries are not suitable for every location. There are few guidelines that help
in deciding the suitability of a rotary. They are listed below.
1. Rotaries are suitable when the traffic entering from all the four approaches are relatively equal.
2. A total volume of about 3000 vehicles per hour can be considered as the upper limiting case and a volume
of 500 vehicles per hour is the lower limit.
3. A rotary is very beneficial when the proportion of the right-turn traffic is very high; typically if it is more
than 30 percent.
4. Rotaries are suitable when there are more than four approaches or if there is no separate lanes available
for right-turn traffic. Rotaries are ideally suited if the intersection geometry is complex.
Traffic operations in a rotary
As noted earlier, the traffic operations at a rotary are three; diverging, merging and weaving. All the other
conflicts are converted into these three less severe conflicts.
1. Diverging: It is a traffic operation when the vehicles moving in one direction is separated into different
streams according to their destinations.
2. Merging: Merging is the opposite of diverging. Merging is referred to as the process of joining the traffic
coming from different approaches and going to a common destination into a single stream.
3. Weaving: Weaving is the combined movement of both merging and diverging movements in the same
direction.