INTERSECTION LEVELS OF SERVICE - A NEW APPROACH

The basic parameters representing the operational characteristics of signalised intersections are the volume-to-capacity ratio 'Y', the degree of saturation 'X' and average delay 'D'. The Guidelines published by the Traffic Authority of New South Wales includes levels of service for signalised intersections as well as unsignalised intersections assuming signal control. These levels of service are dependent on the Degree of Saturation 'X' of the intersection. The historical method of relating level of service to 'Y' and 'X" values and cycle length is difficult if not impossible to convey to other than traffic engineers and even some of them get it wrong. Average delay, however, the most important end result of the analysis, is easier to understand. This paper introduces levels of service based on average delay per vehicle. This attempts to rate the performance of an intersection in terms that the end user and adjudicators in development applications can understand. The paper puts forward a case for using 'average delays' per vehicle as the main criteria for each of the intersection control modes of traffic signals, roundabouts, give way or stop signs. The maximum practical cycle length for traffic signals that should be used for saturated conditions is between 120 and 140 seconds. Drivers and pedestrians are conditioned to expect these cycle lengths and the delays inherent with them when conditions require them. A cycle length of 140 seconds for an intersection just undersaturated will have an average vehicle delay of about 70 seconds. For oversaturated intersections the average delay will be greater than 70 seconds and for undersaturated will be related to the optimum cycle length calculated. Assuming this average delay of about 70 seconds to represent a poor level of service (level 'E'), levels of service for the three control modes are proposed. (A) For the covering entry of this conference, see IRRD abstract no. 843369.