Limited guidance is currently available in Australasia on typical friction coefficients when calculating braking distances on unsealed roads. An experimental programme was undertaken to determine friction characteristics of representative New Zealand unsealed roads in various conditions. The tests utilised the GripTester fixed slip skid survey device and a car specially instrumented to record decelerations and stopping distances under emergency braking. The measurements established that dry and wet friction coefficients of unsealed roads are independent of speed. A recommended design friction coefficient was suggested for calculating stopping sight distances on unsealed roads, on average 15m greater than those recommended for sealed roads. It was also found that ABS braking is less effective than conventional braking on unsealed roads, particularly in loose material. The GripTester in its present form proved unsuitable for monitoring the skid resistance of unsealed roads, as it fails to accurately reflect actual vehicle braking performance. Shell rock appears to be an ideal material for providing unsealed roads with skid resistance levels comparable to those of good condition sealed roads. (a) For the covering entry of this conference, please see ITRD abstract no. E202275.
Abstract