A road testing vehicle, SCRIM, has been developed by TRRL to provide a routine method of measuring the resistance to skidding (SFC) of wet roads. This report outlines the basic principles underlying the design of SCRIM, describes the vehicle, test wheel, recording system and water supply and discusses ways in the results can be analysed. Causes of variation in SFC measured by SCRIM are discussed in relation to both research work and the monitoring of roads for maintenance purposes and methods are recommended that will keep variation to a minimum; these include the use of 'mean summer values', averaging results over at least 5 readings wherever possible and frequent calibration checks