On the impact of obstructions on the capacity of nearby signalised intersections

Various obstructions exist that can impede maximum vehicle flow through signalised intersections. Examples include buses or freight vehicles dwelling at loading areas near the intersection, stalled vehicles, pre-signals that temporarily block car traffic to provide bus priority, on-street parking manoeuvres and permanent road fixtures. If the effects of these obstructions are not recognised or accounted for, vehicle discharge capacities at these critical locations can be overestimated, leading to ineffective traffic management strategies. This paper examines the capacity of an isolated signalised intersection when a nearby roadway obstruction is present in either the upstream or downstream direction. To quantify the loss of capacity caused by an obstruction, the paper applies the variational theory of kinematic waves in a moving-time coordinate system, which simplifies the traditional variational theory by reducing the number of local path costs that must be considered. The result is a simple recipe that requires few calculations and can be used to gain insights into signal operations when obstructions are present. Capacity formulae for general cases are also developed from the recipe. The results, recipe and formulae can be used to guide policies on the location of obstructions that can be controlled, like bus stops, pre-signals or permanent road fixtures and to develop strategies to mitigate the effects of obstructions that can be identified in real time. As an example, a simple adaptive signal control scheme is created using this methodology to more efficiently allocate green time between competing directions when an obstruction is present.