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
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.