Highway Engineering

5.4.3 Traffic capacity at roundabouts

The procedure for predicting the capacity of roundabouts is contained in TA

23/81 (DoT, 1981), based on research done at the Transport Research Labora-

tory (Kimber, 1980). The design reference flows are derived from forecast traffic

levels as explained above. The capacity itself depends mainly on the capacities

of the individual entry arms. This parameter is defined as entry capacity and

itself depends on geometric features such as the entry width, approach half-

width, entry angle and flare length. The main geometric parameters relevant to

each entry arm of a roundabout are illustrated in Fig. 5.24.

The predictive equation for entry capacity (QE) used with all forms of mini-

roundabouts or normal at-grade roundabouts is:

QE=entry capacity into circulatory area (vehicles per hour)

=k(F-fcQc) (5.22)

where

Qc=flow in the circulatory area in conflict with the entry (vehicles per hour)

k = 1 - 0.00347(f-30) -0.978 [(1/r) -0.05]

F = 303 X 2

fc =0.21tD(1 +0.2X 2 )

tD= 1 +0.5/(1 +M)

M=exp [(D -60)/10]

X 2 =v+(e-v)/(1 + 2 S)

S =1.6(e -v)/l¢

The Design of Highway Intersections 125

Example 5.3 Contd

Using these splits, and taking the upper flow ranges from Fig. 5.21, a set

of design reference flows is derived which can then be used to analyse the

roundabout intersection (Fig. 5.23).

100

200

100

300

900

300

600

200

200

300

(^150150) Figure 5.23Final
upper range flows
(vehicles per hour).