6.9.2 Objectives of Signal Coordination
The design objective in determining a signal coordination plan (the system cycle time, durations of green displays, and offsets) is to optimise a selected performance measure (e.g. minimise delay or the number of stops or a combination of delay and stops). The performance measure can be applied for the area as a whole, or for selected routes in the area (e.g. major arterial roads).
Signal coordination is an important tool for achieving other key traffic management and environmental objectives, such as improving the level of service of arterial roads to reduce the pressure on residential streets and central business district (CBD) areas, and reducing fuel consumption and pollutant emissions.
The benefits of traffic signal coordination include:
- reduction in travel time and delay
- reduction in the number of stops
- improved capacity of closely-spaced signalised intersections
- reduction in intersection crashes
- reduction of noise levels, air pollution and energy (fuel) consumption
- achievement of other area or corridor traffic management goals
- increased capacity of the road network which may avoid or defer expensive road widening.
Reduction in intersection crashes is often cited as a benefit of signal coordination. However, research into the safety benefits of signal coordination is limited. Ma et al (2016) estimated crash modification factors for adaptive traffic signal control (ATSC) in Virginia USA. The study concluded that ATSC can have a statistically significant effect on reducing total crashes at urban signalised intersections. The estimated crash modification factor (CMF) was 0.83. However, fatal and injury crashes did not change by a statistically significant amount.
Turner et al (2012) conducted research into key geometric, traffic and operational features of traffic signals and their effect on specific crash type, including right angle, right turn against, rear-end and pedestrian–vehicle crashes. The research objectives were to develop:
- crash prediction models for traffic signals in New Zealand
- a safety toolkit that could be used by transport engineers to predict the expected number of crashes at new and upgraded traffic signal sites.
The study analysed crash data from five New Zealand cities and Melbourne, Australia. It found that the presence of signal coordination increased the risk of right angle crashes and pedestrians being struck by right turning vehicles. However, the effect was not uniform across all cities. In Melbourne and Auckland, signal coordination was found to decrease the risk of right angle crashes. The study did not include any conclusions regarding the net safety effect of traffic signal coordination.
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, 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.