Test: Capacity of Rotary - Civil Engineering (CE) MCQ

Rotaries require large area hence they are a costly affair in urban areas. The advantages of rotary is that they are self-governing, eliminates severe conflicts and accidents.

Traffic rotary
Traffic Rotary at road intersections is a special form of grade change of lanes to channelize the movement of vehicles in one direction around a central traffic island. With the rapid growth of traffic, it is experienced the widening of roads, and providing flyovers have become imperative to overcome major conflicts at intersections such as the collision between through and right-turn movements. In this way, major conflicts are converted into milder conflicts like merging and diverging. The vehicles entering the rotary are gently forced to move in a clockwise direction.

Limitation of the traffic rotary

• The maximum traffic which a rotary can efficiently handle is 3000 vehicle/hour and a minimum of 500 vehicle/hour.
• Design speed is 40 km/hour for rural areas and 30 km/hr for urban areas.
• Entry to the rotary is never made straight, small curvature is always introduced to forcefully reduce the speed of the vehicle.
• Superelevation in a rotary is provided equally to the camber of the road.
• When the distance between intersections on an important highway is less, rotaries become troublesome.

There are 3 traffic operations in a rotary and they are diverging, merging and weaving. Diverging is a traffic operation when the vehicles moving in one direction parts into different directions according to their destinations. Merging is the opposite of diverging and weaving is a combination of both.

Rotary intersections:

• Rotary intersections or roundabouts are a 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 an orderly fashion. They then weave out of the rotary to the desired direction.

Figure: Component of a rotary intersection
The capacity of the rotary(Q_p):
The capacity of the rotary is determined by the capacity of each weaving section.
The transportation road research lab (TRL) proposed the following empirical formula to find the capacity of the weaving section.

Where e = the average entry and exit width, w = the weaving width, l = the length of weaving, p = the proportion of weaving traffic to the non-weaving traffic 
The proportion of weaving traffic to the non-weaving traffic(p) is given by

where a and d are the non-weaving traffic and b and c are the weaving traffic.

Qw = (280w(1+e/w)(1-p/3))/(1+w/l) is the empirical formula to find the capacity of the weaving section. Here, Qw = capacity of weaving section, w = weaving width, l = length of weaving, p = proportion of weaving traffic to the non-weaving traffic.