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A rash of accidents on a major freeway immediately after it was treated with a pavement rejuvenator prompted the British Columbia Ministry of Transportation and Highways (BCMoTH) to commission a set of traction tests by the author. A series of 31 maximum braking tests were run at speeds of 50, 70, 90, and 120 km/h on wet normal and treated pavements, and two additional tests were run on dry pavements at 50 km/h.
A 1991 ABS-equipped Lincoln Town Car was outfitted with a Lamar™-5th wheel, g-ANALYST™, and two bumper guns and was driven by a police-trained driver. The tests were divided more or less equally between maximum ABS and slide-to-stop braking.
The untreated surface had an average coefficient of friction at 50 km/h of 0.71 for peak (ABS) braking and at 75 km/h of 0.61 for maximum locked wheel braking. The standard deviation in both cases was 5% of the mean value. The similar conditions for the treated surface were 0.68 for the peak ABS at 50 km/h and 0.53 for the locked wheel at 75 km/h.
An interesting outcome of this testing is in the experience with the equipment. It was difficult to determine when and where the onset of braking actually started and stopped. The devices failed to function at a rate of 6 to 12 in 100. It was found that all measuring devices were needed to provide sufficient data to confidently estimate the average coefficient of friction.
traction tests, coefficient of friction, skid testing, wet pavements
Professor and co-ordinator of accident investigation team, University of British Columbia, Vancouver, BC