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. Traditionally, rural roads in South Africa have been designed for the traffic volume that
occurs in the 30th highest hour of the year. Many urban roads, however, have been designed
(or evaluated) for the highest peak-hour traffic volume that occurs during a "normal" week
of the year.
Urban roads have a relatively flat annual traffic flow distribution and the differences between
the traffic volumes in the 30th and 100th highest hours as well as the highest normal
peak-hours are usually not significant. Rural roads, however, can show significant peaks
during the year with the result that the 30th highest hour traffic volume can be significantly
higher than the volume that occurs in the normal peak hour.
The 30 highest hours of a year in rural areas typically occur during school holidays, and then
only during a few specific weeks of the year. It is therefore probably not economical to design
roads to provide a high level of service during such hours, and a relatively low level of service
for these hours may have to be accepted as an economic necessity (particularly when the 30th
highest traffic flow is high relative to average flows). An alternative approach, discussed in the
paper, is to design a road to provide a higher level of service during the highest "normal"
peak hour, and to check whether the level of service is acceptable during the 30th highest hour.
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