1 - HKU Libraries

T.P.D*M*V.2*4
4*5.5 Other Types of Roundabout
4*5*5*1 Variations on the three basic forms of roundabout discussed
above include Grade Separated Roundabouts, Ring Junctions and
Signalized Roundabouts. The most common forms of roundabout
used at grade separated junctions are the two bridge type and
the dumbell type, both of which are shown in Diagram No*
4.5.5.1* The two bridge type suffers from its large size which
increases speeds within the circulatory section, thereby
reducing capacity and safety. Design of this type should
therefore be made as compact as possible. The dumbell
roundabout design has the advantage of compactness and low
construction costs*
4.5.5*2 The conversion to ring junction is an effective solution for
very large roundabouts which exhibit entry problems. Studies
have shown that ring junction layouts can eliminate congestion
without reducing safety* There are no existing roundabouts in
the Territory however, which are large enough for consideration
to converting to ring junction control and it is unlikely that
such layouts will ever form part of our road network*
4.5.5.3 Signalized roundabouts, in certain circumstances, may constitute
a most useful design tool. The need for signals at a roundabout
normally stems from problems associated with one or more
particularly dominant flows which tends to defeat the self
regulating property of the roundabout. By installing traffic
signals entry to the roundabout can be regulated thus creating
entry opportunities for the non dominant flows.