Managing Director, Marks and Associates Limited

Skid resistance measurement and analysis is now a routine procedure in motor vehicle crash analysis. In fact it was one of the earliest investigative and analytical tools used for this work. Many different measurement methods are in common use, including: visual estimation assisted by friction tables; various dragged devices with friction force measurement and instrumented vehicle skid-to-rest testing, the last having become the preferred alternative for many investigators over the past decade or so. The skid resistance measurement devices at present commonly used by traffic engineers for pavement condition monitoring and maintenance intervention, such as SCRIM, ROAR, Griptester and the British Pendulum, are only rarely used for forensic purposes in New Zealand although in some other countries these may be applied as a routine procedure during crash-scene investigations. The interpretation and analysis of the results obtained by skid resistance measurement in the forensic context may seem to be an obvious process but it is not always straightforward. Uncertainties exist and there is considerable scope for fundamental error. The latter is of significant concern, given the potential adverse consequences of an analyst presenting flawed expert testimony in Court. This paper examines the most common skid resistance measuring methods used for forensic purposes and discusses the interpretation and analysis of the results obtained, the uncertainties involved and the expression of expert opinion in Court. It contrasts the differing needs of the forensic analyst and the traffic engineer in this area. It emphasises the need for crash analysts to have a thorough understanding of the physics involved, the empirical data obtained and the relationship between the analysis subsequently performed and the skid resistance measurement method used