6.9.9 Coordination Timing Criteria
The principal objective of coordination timing is to optimise a selected performance measure. Usually, delay, number of stops or a combination of delay and stops are used as the performance measure. It is generally recognised that the following factors favour minimising stops:
- crash risk – this is greatest at the change of signal phases, and is reduced if fewer vehicles are stopped
- fuel consumption, exhaust pollution and operating cost – these are increased by stop‑start driving cycles, therefore reduced if fewer vehicles are stopped
- driver expectation – drivers relate coordination more to the number of stops than to overall delay.
Minimising the number of stops, fuel consumption, emissions or operating cost does not yield the same signal timing plans as the minimum delay criterion due to the different offset and cycle time requirements. However, as shown in Figure 6.20, such longer cycle times do not involve a significant delay penalty.
On the other hand, long cycle times result in longer delays to side-road traffic and pedestrians, and result in longer queue lengths. In areas such as CBD networks where queue storage spaces are limited, long cycle times may lead to the blockage of upstream signal stop lines (queue spill back), and the resulting loss in the network capacity leads to increased delays and stops.