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T*P*D.M.V*2.4
4.5 Roundabouts
4-5.1 Introduction
4*5.1*1 Roundabouts could be considered as a specialized forms of
Priority Junction. Unlike normal priority junctions, however,
where major road traffic receives continuous priority, with
roundabouts the flow entering the junction from all arms gives
priority to vehicles already in the junction* Roundabouts are
thus well suited to situations of balanced flow. Also, because
right turns, and all other movements, are broken down into a
left turn entry and a left turn exit, roundabouts are well
suited to flows containing a high proportion of right turning
vehicles.
4.5.1.2 There are three basic types of roundabout namely : Normal, Mini
and Double. Other forms of roundabouts which are variants of
these three basic types are Ring Junctions, Grade Separated
Roundabouts and Signalized Roundabouts.
4.5*1*3 A normal roundabout can be considered as a one-way circulatory
carriageway around a kerbed central island 4m or more in
diameter and usually with flared approaches to allow multiple
vehicle entry.
4.5*1.4 A mini roundabout consists of a one-way circulatory carriageway
around a flush or slightly raised circular marking less than 4m
in diameter and with or without flared approaches.
4.5.1*5 A double roundabout is a single junction with two normal or mini
roundabouts either contiguous or connected by a central link
road or kerbed island.
4.5.1*6 A grade separated roundabout junction is defined as one which
includes a roundabout which has at least one entry road, via an
interconnecting slip road, from a road at a different level.
4.5.1*7 A ring junction retains the large central island characteristic
of a roundabout but the usual one-way circulation of vehicles is
replaced by two-way circulation with three-arm mini roundabouts
and/or traffic signals controlling the junction with each
approach arm.
4.5.1.8 A signalized roundabout is a normal roundabout with traffic
signals installed on one or more of the approach arms.