Rotary intersections or roundabouts are special forms 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 an 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.

The key advantages of the 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 speed and continue to move at a slower speed. Thus, fewer of the vehicles need to be stopped.
3. Because of the slower 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 virtually 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.

However, there are a 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 intersections.
2. Even when there is relatively low traffic, the vehicles are forced to reduce their speed.
3. Rotaries require a large area of relatively flat land, making them costly in urban areas.
4. Since the vehicles are not stopping, and they accelerate at rotary exits, they are not suitable when there is high pedestrian movement.

Rotaries are not suitable for every location. There are a few guidelines that help in deciding the suitability of a rotary:
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 the upper limit, and a volume of 500 vehicles per hour is the lower limit.
3. A rotary is very beneficial when the proportion of right-turn traffic is very high; typically, it is suitable for more than 30 percent.
4. Rotaries are suitable when there are more than four approaches, or if there are no suitable lanes available for right-turn traffic.

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 are separated into different streams according to their destinations.
2. Merging: Merging is the opposite of diverging, when traffic streams coming from various places and going to a common destination are joined into a single stream.
3. Weaving: Weaving is the combined movement of both the merging and diverging movements in the same direction.

The design elements include design speed, radius at entry, exit, and the central island, weaving length and width, entry and exit widths. In addition, the capacity of the rotary can also be determined using some empirical formulas.

The capacity of a rotary is determined by the capacity of each weaving section. Transportation road research lab (TRL) proposed the following empirical formula to find the capacity of the weaving section:
Q = 280w[1+ e][1 - p] / (w - 3)
where e is the average entry and exit width, w is the weaving width, l is the length of weaving, and p is the proportion of weaving traffic to non-weaving traffic. This formula is valid only if certain conditions are satisfied.