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.
The Highway Safety Manual 2010 provides a new set of methodologies to evaluate or predict safety performance on road sites. It is based on crash data from the United States of America. The paper gives a brief introduction to the Highway Safety Manual 2010 and its methodologies. The applicability of these methodologies has not yet been evaluated for South African conditions.
Two sections of route R44 (provincial road M 27) were analysed. Section 1 - between Klapmuts and Stellenbosch - is a single carriageway with shoulders and Section 2 - between Stellenbosch and Somerset West - is a dual carriageway road with at grade intersections. The respective safety performance functions (SPFs), modified by crash modification factors (CMF) were used to estimate the number of crashes. These were compared to the average number of crashes reported over the last 5 years, subject to the proviso that the reported crash data may not be as accurate as that of the USA.
On Section 1, the single carriageway road section, the observed number of crashes was 0,67 times higher than the predicted number, but the observed number of crashes for the intersections and road segments were 0,12 and 0,95 of the predicted number respectively.
The total number of crashes observed for Section 2 on the dual carriageway link sections. was about 4,7 times higher than number predicted. The number of crashes at stop controlled intersections was predicted, but as these intersections were not specified in the accident statistics, the values were added to the sections. The number of observed crashes at traffic signal controlled intersection was 1,1 times higher than the number of predicted crashes.
The evidence presented in this paper indicates that the safety performance functions that were investigated cannot be transferred to the South African situation directly from the USA where they were developed. The logic of the HSM 2010 methodologies seems to be robust. The ranges of values of crash modification factors seem acceptable.
This study did not attempt to explain the reasons why the predicted crash frequency differed from the actual number of crashes, as the road sections on which it was tested is not a representative sample. Local research into the shape and size of the safety performance factors and the calibration of crash modification factors should be promoted. The basis of such research is collision statistics, and every effort should be made to improve the quality of our data capturing system.
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