The whys and wherefores of roundabouts

K.P. Subramanian, retired professor of urban engineering, on the salient features of this traffic system

A roundabout, also known as a rotary, is a facility created at a junction to regulate traffic. It does away with the need for a traffic signal or traffic police personnel. In other words, at a rotary, motorists will be left with no other option but to move in one direction around a central island and weave out in their respective directions. This way, an automated regulatory system is created at a junction.

The shape

A rotary should be circular in shape. All the roads intersecting at the junction should carry approximately an equal volume of traffic. Interestingly, a rotary could also be square in shape, depending on the alignment of roads and volume and movement of traffic.

Traffic volume

A roundabout is required at a junction where five or more roads meet and when the volume of rush-hour traffic is between 500 Passenger Car Units (PCUs) and 3,000 PCUs and the right-turn vehicular movement is heavy. However, it is not suitable at junctions where the traffic volume exceeds 3,000 PCUs.

In such situations, the traffic comes to a standstill at a roundabout. According to the guidelines prescribed by the Indian Roads Congress, a traffic signal will be required if rush-hour traffic is between 3,000 PCUs and 10,000 PCUs. And if the rush-hour traffic exceeds 10,000 PCUs, the junction requires a flyover.

At most of the junctions in Chennai, the volume of rush-hour traffic is over 3,000 PCUs. With flyovers and grade-separators on the increase, a rotary is not as relevant as before. It has to be noted that a rotary cannot coexist with any other traffic regulatory system such as a traffic signal or a flyover or a grade-separator. Another factor that discourages urban planners from choosing a roundabout is that it requires a significant amount of space.

Minimum radius

A minimum radius of 13 to 15 metres is required at a junction where movement of small vehicles is predominant. At junctions that mainly experience the movement of heavy vehicles, a 30-metre radius is necessary. In Chennai, the minimum radius is getting reduced for various reasons, and this leads to difficulties for motorists while turning their vehicles at the junction, which defeats the whole objective of having a roundabout. The roundabout on Pantheon Road is a classic example.

Two systems

Another observation is that there are many roundabouts in the city where a traffic signal is also is in place. At such junctions, the roundabout becomes redundant. At these junctions – the clock tower in Royapettah, at the clock tower in Mint and the one near DGP office on Kamaraj Salai — only the traffic signals do the job of traffic regulation and not the roundabouts.