The computer program OSCADY (Burrow, 1987; Binning, 1999) can be used
to analyse new and existing signalised junctions, producing optimum cycle times,
delays and queue lengths as well as information on predicted accident frequen-
cies at the junctions under examination.5.5.8 Signal linkage
Within an urban setting, where signalised intersections are relatively closely
spaced, it is possible to maximise the efficiency of flow through these junctions
through signal co-ordination. This can result in the avoidance of excessive
queuing with consequent tailbacks from one stop line to the preceding signals,
and in the ability of a significant platoon of traffic passing through the entire
set of intersections without having to stop. Lack of co-ordination can result in
some vehicles having to stop at each of the junctions.
One method for achieving this co-ordination is through use of a time-and-
distance diagram, with time plotted on the horizontal axis and distance on the
vertical axis. The rate of progression of any given vehicle through the network
of signals is denoted by the slope of any line charted on it.
In order to construct the diagram, each intersection is examined individually
and its optimum cycle time computed. The one with the largest required cycle
time (called the ‘key intersection’) is identified and taken up as the cycle time
for the entire network C 1. Knowing the lost time per cycle, the effective green
time and hence the actual green time for the key intersection can be computed.
The actual green time along the main axis of progression in this instance will
determine the minimum actual green time along this axis at the other junctions
within the network. With regard to the other non-key intersections, the maxi-
mum actual green time in each case is derived through determination of the
smallest acceptable green time for the minor road phases:(5.40)
The minimum actual green time for the side/minor roads can then be calculated
by addition of the lost time per phase and subtraction of amber time. (As stated
earlier, actual time is 1 second less than effective time.) By subtracting this value
from the cycle time plus the intergreen period, the maximum actual green time
permitted for the junction in question along the main axis of movement can be
calculated.
When these minima and maxima have been determined, the time-and-distance
diagram can be plotted once the distance between the individual intersections
in the network is known and an average speed of progression in both directions
along the major axis is assumed.Min. effective greenminor route=¥()yCside 1 ∏09.146 Highway Engineering