Innovative techniques used in traffic impact assessments of developments in congested networks

Abstract

Paper presented at the 20th Annual South African Transport Conference 16 - 20 July 2001 "Meeting the transport challenges in Southern Africa", CSIR International Convention Centre, Pretoria, South Africa. ABSTRACT: For uncongested networks, peak hour trip generation rates are used to generate traffic for a development based on the land use budget. The assumption is made that the trips will be able to move to and from the site during the peak hour, and hence the generated trips are superimposed on the existing peak hour traffic flows, to produce the post development(future) flows. These future flows can be analysed and the impact of the development determined. However, in congested networks, the current peak hour demand for travel on certain routes within the network, cannot be satisfied. Capacity constraints (bottlenecks)on these routes restrict (meter)flows along these routes. Under these conditions, normal trip generation and superimposition of generated trips onto the existing flows, is unrealistic. In order to produce realistic future traffic scenarios for analysis purposes it was necessary to develop a method of unconstraining (releasing)the existing traffic flows on the network, by theoretically unlocking the existing bottlenecks in the network. Once unlocked, the development traffic (generated new trips)could be superimposed on the existing unconstrained flows. These future flows could be re-constrained to produce the realistic future traffic flow scenarios, based on predicted future bottleneck capacities. The analysis of congested networks required innovation in the methods of data survey and interpretation. The paper focusses on the innovative method used to unconstrain and then re-constrain network traffic flows, for the purposes of measuring the traffic impact of developments in congested road networks.