Paper presented at the 32nd Annual Southern African Transport Conference 8-11 July 2013 "Transport and Sustainable Infrastructure", CSIR International Convention Centre, Pretoria, South Africa.
Skid resistance values found in older design manuals were typically based on studies
where the worst case scenarios of bald tires, smooth pavement and wet conditions were
used. The graphs depicting these values were first published in the AASHTO Policy for the
Geometric Design of Rural Highways, 1954. In the AASHTO Policy for the Geometric
Design of Highways and Streets, 2001, skid resistance ceased to define braking distance
(the second component of Stopping Sight Distance) and a standard deceleration of 3.4 m/s2, based on comfort, became the design norm at all speeds.
These design values are of little use in the reconstruction of crashes. The skid resistance
for specific scenarios must be determined. Values obtained from published sources can
serve as guidance to orders of magnitude and as reference values. The stopping -performance of modern cars under ideal conditions can be in the order of 8 to 9 m/s2. With
GPS loggers and accelerometers now generally available, field tests can be done with
similar vehicles under comparable conditions at a fraction of the cost of the formal testing
equipment. The acceptance of these values has not been tested in court and the credibility
of the results will have to be agreed by the experts acting for the Court or be tried by the
court based on the facts presented.
Values for road surfaces contaminated by gravel (sand to stone chips) could not be found
in literature. Tests, using a Race Technology DL1 data logger with accelerometer, were
conducted on a good gravel road and on an abandoned section of sealed road with sand
and crushed stone gravel on. All the tests were done in dry conditions. The gravel roads
were included to establish a measure of comparison. The skid resistance of the gravel
roads were found to be 0.53 to 0.63. The skid resistance of the sealed road with sand
contamination were found to be between 0.36 and 0.44. The skid resistance of 13.2 mm
and 6.7 mm stone used for seal resurfacing on the sealed road varied between 0.37 and
0.47.
The skid resistance of dry gravel roads and surface roads contaminated with sand and
gravel are still greater than the comfort-based design value of 0.35. This does not imply
that such areas are safe for all vehicles (motorcycle and bicycles being especially
vulnerable). The skid resistances of surfaced contaminated roads do not meet the brake
performance of 4.4 m/s2 (skid resistance of 0.44) required in the National Road Traffic Act,
Act 93 of 1996.
