Rotary and Roundabouts Conundrum

 Roundabouts

Roundabouts are special form of at-grade intersections laid out for the movement of traffic in one direction around a central traffic island. Essentially roundabouts work to reduce the number of traffic conflict points as compared to other type of signalized or non-signalized intersection.  The vehicles entering the roundabout are gently forced to move in a clockwise direction in orderly fashion.

Operation of a Roundabout in a right side driving country 

But there are many myths associated with it. People generally think it as a western name for Rotary. But there are many things which differentiate them.

The Indian code is for Rotary, published in 1976, by IRC-65-1976( Indian Road Congress). There has be no revision for it, nor has any other code been introduced for Roundabout.

While, researching for my seminar in the course work I could find this confusion present in the minds of students, academia, and in the minds of working professionals. So I decided to post my findings after the completion of my research.


The table 1 lists some of the differences in the roundabout as well as in a rotary

                                               Table 1: Comparing Roundabout and Rotary

Feature	Roundabout	Rotary
Control at entry	Yield sign for entering vehicles	STOP, signal or give priority to entering vehicles
Operational Characteristics	Vehicles in roundabout will be given priority over entering vehicles	Allow weaving areas to resolve the conflict movement
Deflection	Use deflection to control low speed operation throughout roundabout	Some large traffic circles provide straight path for major movement
Parking	No parking is allowed on the circulatory roadway	Some larger traffic circles permit parking within roadway.
Pedestrian Crossing	No Pedestrian activities take place on the centre island	Some larger traffic circles provide for  pedestrian crossing
Turning movement	All vehicles circulate around central island	Mini traffic circles, left turning vehicles are expected to turn to the left of central island
Splitter island	Required	Optional

Roundabouts differ with rotary in the traffic operations in the following ways:

a)    Indian code IRC 65-1976 specifies that flow of traffic occurs by entering traffic yielding to the circulating traffic which is not always followed in rotary codes of other countries, but in roundabout it is mandatory to follow it.

b)      IRC 65-1976 identifies weaving as an operation inside a rotary but inside a roundabout it is avoided by clear pavement markings. The vehicles which wants to take a immediate left or straight keeps on the outer lane and the inner lane is taken by vehicles which want to move straight or right, thus avoiding any lane changing and thus avoiding weaving.

c)      Thus the non lane changing movement along with yielding to the circulating traffic rules out the weaving of traffic in a roundabout which is lacking in a rotary.

d)     Also, IRC 65-1976 states that the capacity of a rotary is synonymous with the capacity of the weaving section, where as HCM method of capacity for roundabout is based on the headway of the stream thus having lower capacity in a rotary as compared to a roundabout.

CONFLICT POINTS IN ROUNDABOUTS vs RIGHT ANGLED INTERSECTION

A roundabout eliminates vehicle-vehicle crossing conflicts by converting all movements to right turns.  Separate turn lanes and  traffic  control (stop signs or signalization) can often reduce but not eliminate the number of crossing conflicts  at a traditional  intersection by separating conflicts  in space and/or  time. However, the most severe crashes at signalized intersections occur when there is a violation of the traffic control device designed to separate conflicts by time (e.g., a right-angle collision due  to a motorist running  a red  light, or vehicle-pedestrian collisions). The ability of roundabouts to reduce conflicts through physical, geometric features has been demonstrated to be more effective than the reliance on driver obedience to traffic control devices. At intersections with more than four legs,  a roundabout or pair of roundabouts may sometimes be the  most practical  alternative to minimize the  number of conflicts.

Drivers approaching a single-lane roundabout have five basic decisions regarding other users. First, drivers must be  mindful  of any bicyclists  merging into motor vehicle traffic from the right side of the road or a bicycle lane or shoulder. Then they must yield to any pedestrians crossing at the entry. Third, they must choose an acceptable gap in which to enter the roundabout. Then they must choose the correct  exit, and finally, they  must yield to any pedestrians crossing the  exit lane.

By contrast, a driver  making  a left  turn  from  the  minor  leg  of a two-way stop- controlled intersection has to yield to pedestrians and bicyclists,  and judge gaps  in both  of the  major street through movements from both  directions, as well as the major street left and right turns  and opposing minor through and right turns.

Signalized intersections have simplified  the  decision-making process for drivers, especially at locations where protected left-turn phasing is provided, by separating conflicts  in time and space. However, the rules and driver decisions for negotiating signalized intersections are still quite complex when all the possible signal phasing schemes are accounted for. For signals  with permitted left-turn phasing, the driver.