The operational analysis of two-lane rural highways

Abstract

Paper presented at the 23rd Annual Southern African Transport Conference 12 - 15 July 2004 "Getting recognition for the importance of transport", CSIR International Convention Centre, Pretoria, South Africa. A large proportion of the South African rural road network consists of two-lane highways. Some of these highways are carrying relatively high volumes of traffic with the result that some roads are operating at low levels of service. Currently, the Highway Capacity Manual of the Transportation Research Board is used for the analysis of the operations on these roads. Concern has, however, been expressed that the methodologies described in the HCM may not be appropriate or adequate for South African conditions. An alternative model has been developed for the operational analysis of two-lane rural highways. This model utilises queuing theory to model platoons of traffic along a highway. The average platoon length is determined which can be utilised to determine evaluation criteria such as percentage followers and to predict travel speed. This model can also be used to model highways with wide shoulders where the shoulders are utilised by vehicles to provide passing opportunities. The Highway Capacity Manual uses "Percent time spent following" as the measure of effectiveness in the evaluation of two-lane highways. This measure has the limitation that it can not readily be observed in the field. It is therefore proposed that this should be replaced by "percentage followers", although a new measure termed "follower density" should be used for the evaluation of two-lane roads.