Highway Engineering

5.3.2 Equations for determining capacities and delays

Once the flows to be analysed have been determined and a generally appro-

priate geometric layout has been settled on, it is then necessary to establish the

capacity of each traffic movement through the priority junction. In each case,

it is mainly dependent on two factors:

 The quantity of traffic in the conflicting and merging traffic movements


 General geometric properties of the junction.

The traffic flows relevant to the determination of these capacities are shown in

Fig. 5.12.

The following equations for predicting the capacity of turning traffic streams

are contained in TA 23/81 (DoT, 1981), the advice note on the design of

major/minor intersections:

110 Highway Engineering

Example 5.1 Contd

Turning movements were assessed based on present day traffic flow data

from the junction, which indicated the proportions in Fig. 5.10.

0.80

0.20

0.60 0.40

0.80

0.20

Figure 5.10Turning movements.

Taking the upper flow ranges from Fig. 5.9, combined with the turning

proportions indicated in Fig. 5.10, a set of flows is derived which can then

be used to analyse the junction, as shown in Fig. 5.11.

307

77

158 105

718

179

Figure 5.11Final upper range flows (vehicles per hour).