A decomposition approach for signal optimisation in road networks

The optimisation of signal timings plays an important role in the management of urban traffic, and in the full usage of existing and planned road networks. In recent years, considerable advances have been made in techniques for the optimisation of signal timings at isolated junctions operating under fixed-time control. This paper shows how these techniques can be applied in the optimisation of signal timings in coordinated networks by using a decomposition approach. The signal timings at a junction in a network can be specified fully by the sequence of stages, interstage structures, cycle time, stage durations and offset. Of these variables, the third, fourth and last are endogenous to network optimisation methods, the first and second being exogenous. Techniques have been developed recently to optimise all but the last variable (which is not there defined) at individual junctions, and these have been found to give considerable improvements in operational performance. The computational requirements of these methods is such that their direct extension to networks is not yet a practical proposition. This paper shows how the differences inherent between individual junction and network optimisation methods can be reconciled within a decomposition approach so that the latter can benefit from some of the advantages of the former. A simple example is used to illustrate the substantial benefits that can arise from this approach.