Highway Engineering

flow above those suitable for priority control. If the cost of land is an impor-

tant factor, traffic signals will be preferred as land requirements for a standard

3 or 4-arm conventional roundabout would be greater. However, right-turning

vehicles can cause operational difficulties at signal-controlled junctions, partic-

ularly where volumes within this phase are large.

Roundabouts have difficulty dealing with unbalanced flows, in which case sig-

nalisation may be preferable. In situations where flows are relatively well bal-

anced, and where three or four entry arms exist, roundabouts cope efficiently

with the movement of traffic. Where the number of arms exceeds four, however,

efficiency may be affected by the failure of drivers to understand the junction

layout. It may also prove difficult to correct this even with comprehensive direc-

tion signing.

In addition to their ability to resolve conflicts in traffic as efficiently as pos-

sible, roundabouts are often used in situations where there is:


 A significant change in road classification/type

 A major alteration in the direction of the road


 A change from an urban to a rural environment.

5.4.2 Types of roundabout

Mini-roundabout (Fig. 5.15)

Mini-roundabouts can be extremely successful in improving existing urban junc-

tions where side road delay and safety are a concern. Drivers must be made

aware in good time that they are approaching a roundabout. Mini-roundabouts

consist of a 1-way circulatory carriageway around a reflectorised, flush/slightly

raised circular island less than 4 m in diameter which can be overrun with ease

by the wheels of heavy vehicles. It should be domed to a maximum height

of 125 mm at the centre for a 4 m diameter island, with the height reduced pro

rata for smaller islands. The approach arms may or may not be flared. Mini-

roundabouts are used predominantly in urban areas with speed limits not

120 Highway Engineering

Figure 5.15Mini-

roundabout

configuration.