When road projects are assessed in terms of their economic, social and environmental impacts, one factor to consider is the severance caused by the infrastructure and by the vehicles using the road. This paper outlines the way in which the concept of 'severance' has changed over time, and defines static and dynamic severance. Vehicular traffic acts as a 'dynamic' time-dependent barrier on urban roads and imposes problems of safety and delay on pedestrians wanting to cross that road. Although this is an old problem in traffic engineering practice, the gap acceptance based approach to pedestrian delay usually assumes that vehicle arrivals are random. This paper presents a model of the severance effects on busy urban roads where frequent traffic signals produce bunching of vehicles. The total delay to pedestrians crossing a street is conceptualised as the sum of the individual delays due to pedestrian interaction with vehicular headways. On arriving kerbside, downstream of a traffic signal, a pedestrian will face one or two possible traffic flow patterns: a bunched flow created by the traffic signal settings in the form of a block during which there are no gaps for pedestrians to cross; and/or a secondary flow - defined as random flow when there may be gaps to allow for crossing. Pedestrian delays are a combination of bunched and random flow. Field data on 4004 headways and 252 pedestrian crossing movements across main roads downstream of signalised intersections in Sydney support the greater accuracy of this model when compared with the estimates from Adams' delay. Simulation was used to compare the results of our model with those of Adams (1936), Tanner (1962), and Troutbeck (1986), confirming greater accuracy over the range of traffic flows from 100 to 900 vehicles per hour. (a).
Abstract