turns amber at the end of the phase. Some vehicles will stop; others may take
some time to do so. The flow returns to zero as the lights turn red.
From Fig. 5.31 it can been seen that the actual green time plus the amber
period is equal to the effective green time plus the two periods of lost time at
the beginning and end of the cycle. The effective green time is thus the length
of time during which saturation flow would have to be sustained in order to
obtain the same quantity of traffic through the lights as is achieved during an
actual green period. It is denoted by a rectangle in Fig. 5.31. This rectangle has
exactly the same area as that under the actual flow curve.The Design of Highway Intersections 139(^) Red Green Amber Red
Flow rate
(p
cu/hr
)
Lost time Lost time
Effective green time
Saturation flow
Figure 5.31Diagrammatic representation of actual green time, effective green time and lost time.
Normally, the lost time is assumed to be taken as equal to
2 seconds, with the amber time set at 3 seconds. Effective green time
is thus equal to actual green time plus 1 second.
5.5.5 Optimum cycle time
Assuming the signal system at an intersection is operating on fixed-
time control, the cycle length will directly affect the delay to vehicles
passing through the junction. There is always an element of lost time
every time the signal changes. If the cycle time is short, this lost time
becomes a significant proportion of it, leading to inefficiencies in the
working of the junction and consequent lengthy delays. Too long a