At existing crossroads intersections where opposing right-turning move-
ments can be separated
Catering for junctions with more than four entries and overloaded single
roundabouts where overall capacity can be increased by reducing the circu-
lating flow travelling past critically important entry points.In situations where the double roundabout is composed of two mini-
roundabouts, the speed limit on the approaches must not exceed 48 km/h
(30 mph).Other forms
Other roundabout configurations include two-bridge roundabouts,dumbbell
roundabouts, ring junctions and signalised roundabouts.122 Highway Engineering
Figure 5.17Double
roundabout
configuration (with
central link road).Example 5.3 – Determination of flows at a roundabout intersection
A junction is to be constructed at the intersection of a single carriageway 2-
lane road (deemed the minor road) and a 2-lane dual carriageway (the major
road). Figure 5.18 shows the annual average daily traffic (AADT) flow ranges
predicted for each of the arms to the intersection in the design year, assumed
in this case to be 12 years after opening.
Taking the upper AADT flows of 20 000 for the major carriageway and
8000 for the minor carriageway, Fig. 5.5 indicates that the most appropriate
form of junction is a roundabout. Using the AADT values, the average 2-
way annual average hourly traffic (AAHT) on all four approach roads can be
computed by dividing each AADT value by 24, with the results shown in
Fig. 5.19.
Contd