Highway Engineering

The Design of Highway Intersections 141

Example 5.6 – Calculation of optimum cycle time
The actual and saturation flows for a three-phase signal system are detailed
in Table 5.8. The phasing details are given in Fig. 5.32. The intergreen period
is set at 5 seconds, the amber time at 3 seconds and the lost time due to start-
ing delays at 2 seconds.
Calculate the optimum cycle time for this intersection.

Stage Movement Flow (pcu/h) Saturation flow (pcu/h) y

11 A 460 1850 0.25*

1B 120 600 0.20

1C 395 1800 0.22

1D 100 550 0.18

22 A 475 1900 0.25*

2B 435 1900 0.23

33 A 405 1850 0.22*

3B 315 1850 0.17

Table 5.8Actual flows, saturation flows and yvaluations

Stage 1 Stage 2 Stage 3

Figure 5.32Staging diagram.

Solution

The three phases for the intersection in question, together with the move-
ments for each of the individual approaches, are:

Stage 1

Approach 1A – north/south movement (straight ahead and left-turning)
Approach 1B – north/south movement (right-turning)
Approach 1C – south/north movement (straight ahead and left-turning)
Approach 1D – south/north movement (right-turning)

Contd

*Maximum values