5.4 Roundabout intersections,
5.4.1 Introduction
In order to control merging and conflicting traffic flows at an intersection, a
roundabout performs the following two major functions:
It defines the priority between traffic streams entering the junction, usually
on the basis that traffic wanting to join the circulatory flow must give way
to the traffic to their right already circulating in the roundabout. (In the UK
and Ireland traffic circulates in a clockwise direction.)
It causes the diversion of traffic from its preferred straight-line path, requir-
ing drivers to slow down as they enter the junction.
In order to work efficiently, sufficient gaps must appear in the circulating flows
on the roundabout that drivers then accept. Traffic on the entry arms can thus
enter, circulate and then leave at their desired exit arm. Its operation has, there-
fore, certain similarities to that of a priority intersection and, as we shall see
later in the chapter, the design procedure in both has certain similarities. The
situation is more complex in the case of a roundabout intersection as there is
no clear identifiable major road traffic flow that can be used as a basis for design-
ing the junction, with the circulating flow depending on the operation of all
entry arms to it.
If properly designed, the angles at which traffic merges/diverges will be small.
This, combined with the relatively slow traffic speeds on the roundabout, will
help reduce accident rates.
As seen from Fig. 5.5, when traffic flows at an intersection are relatively low,
adequate control can be attained using the priority option. As flow levels
increase, however, with this intersection type, delays/queue lengths become
excessive and some alternative form is required. While grade-separated junctions
may be the preferred option at high flow levels, the expense involved may be
prohibitive. For this reason, particularly in an urban setting, at-grade round-
abouts or signalised intersections become viable junction options at levels of
The Design of Highway Intersections 119
a b
c
y
x
Figure 5.14Illustration of turning, deceleration and taper lengths for a ghost island
junction.
