Town Centre Topology and Traffic Flow

Abstract

Town centre redevelopment provides an opportunity to redesign the topology of the road network. Environmental and safety considerations will usually entail some streets being reserved for pedestrians only and others for access to car parks and service bays close to the pedestrian precinct. The effect of the new topology on traffic flow can and should be an input to the planning process. One way to appraise alternative schemes is to find the extent to which traffic could be routed to reduce the amount of conflict there will be between the different streams of traffic at the junctions; conflict at junctions is a simple measure indicating the potential for both accidents and congestion with its attendant pollution.

The problem of finding an assignment of flow such that conflict is minimised is a constrained optimisation problem. The main constraints are provision of routes for a demand specified by an origin-destination (OD) matrix. The objective function which measures the amount of conflict between the different streams is simple to formulate; it depends only on the topology of the network and the OD matrix of demand. But it is a quadratic function of a type which is not amenable to existing programming methods. However, the author has developed an iterative method for obtaining good, if not necessarily optimal, solutions.

The purpose of the paper is to demonstrate the appraisal of hypothetical networks involving four external zones. The OD matrix has been chosen to model some through as well as mostly inbound traffic flow. Various alternative schemes are appraised ranging from those which merely allow one access point to a car park within a ring of streets surrounding the pedestrian precinct, to those which retain one link across the ring and up to two car parks. This is a pilot study for a larger one involving the systematic analysis of hypothetical town centre topologies. So far it has shown the advantage of two access points over one, and the greater advantage of two separate car parks each with a single access point which allows each car park to be in a separate cell of the road network. The relative positions of the access points and the link across the ring also affect the amount of conflict.

Citation

Wackrill, P.A. (1998), "Town Centre Topology and Traffic Flow", Griffiths, J.D. (Ed.) Mathematics in Transport Planning and Control, Emerald Group Publishing Limited, Leeds, pp. 119-129. https://doi.org/10.1108/9780585474182-012