Differentiated speed limits that will work

Abstract

Paper presented at the 31st Annual Southern African Transport Conference 9-12 July 2012 "Getting Southern Africa to Work", CSIR International Convention Centre, Pretoria, South Africa.

Following a research report (Van As and Joubert, 1998) and a fatal bus accident involving British tourists in 1999 the then Minister of Transport reduced the maximum speed limit for public transport vehicles (buses and minibus taxis) to 100km/h. This meant that light vehicles and public transport vehicles could have different speed limits on the same road – something that already applied to heavy vehicles. The purpose of this paper is to determine the effectiveness of the different speed limits for different vehicle classes on the same road. The effectiveness is defined as the extent to which the drivers of the different types of vehicle keep to these limits. The paper describes a project in which the free-flow speeds of different types of vehicle were measured on roads where the geometry was such that it did not affect the speeds. The speeds of nearly 9 000 vehicles were determined on 12 sections of two, four and six-lane roads in the Western and Eastern Cape Provinces where the general speed limit (ie for light vehicles) is 120 km/h. It was found that 85% of the drivers of light vehicles and buses do keep to their respective limits, but that the drivers of heavy vehicles and minibus taxis to a large extent exceed their limits.