The influence of gradient on saturation flow rate at signalised intersections

Abstract

Saturation flow rate is the maximum hourly traffic volume per lane that can be accommodated through an intersection, and is used in the design and analysis of signalised intersections and traffic signal plans. The most widely applied method to estimate saturation flow rate is provided by the Highway Capacity Manual (HCM), which proposes default values obtained from USA empirical data for application in saturation flow rate calculations. This study considers the applicability of the HCM proposed base saturation flow rates, and particularly the adjustment factors for gradient on saturation flow in the South African context. The data for this research was collected by author ES Viljoen for use in her dissertation conducted in the final year of the BEng degree at Stellenbosch University. The influence of approach lane gradient on saturation flow rate was investigated for application in the South African environment. The results of this research indicate that the HCM underestimates the influence of approach gradient on saturation flow rate, a similar finding to other international research. The standard use of a base saturation flow rate of 1 900 passenger cars per hour per lane for South African conditions was also commented on. HCM methods do not take into account the influence of speed limit on saturation flow rate, which has been shown to significantly affect saturation flow rate in numerous research projects, and is also commented on in this paper for zones with a 60 km/h and 80 km/h speed limit.