Highway Engineering

exceeding 48 km/h (50 mph) (DoT, 1993). They are never used on highways with
high speed limits. In situations where physical deflection of vehicle paths to the
left may be difficult to achieve, road markings should be employed in order to
induce some vehicle deflection/speed reduction. If sufficient vehicle deflection
cannot be achieved, the speed of the traffic on the approach roads can be re-
duced using traffic calming techniques.
Because of the short distance between the entry points to the roundabout,
drivers arriving at the intersection must monitor very closely the movements of
other vehicles both within the junction and on the approaches in order to be in
a position to react very quickly when a gap occurs.

Normal roundabout (Fig. 5.16)

A normal roundabout is defined as a roundabout having a 1-way circulatory
carriageway around a kerbed central island at least 4 m in diameter, with an

The Design of Highway Intersections 121

Figure 5.16Normal

roundabout

configuration.

inscribed circle diameter (ICD) of at least 28 m and with flared approaches to
allow for multiple vehicle entry. The number of recommended entry arms is
either three or four. If the number is above four, the roundabout becomes larger
with the probability that higher circulatory speeds will be generated. In such sit-
uations double roundabouts may provide a solution (DoT, 1993).

Double roundabout (Fig. 5.17)

A double roundabout can be defined as an individual junction with two
normal/mini-roundabouts either contiguous or connected by a central link road
or kerbed island. It may be appropriate in the following circumstances:

For improving an existing staggered junction where it avoids the need to
realign one of the approach roads

In order to join two parallel routes separated by a watercourse, railway or
motorway