a road testing vehicle, scrim, has been developed by the transport and road research laboratory to provide a routine method of measuring the resistance to skidding (sfc) of wet roads. measurements of skidding resistance are affected by the two following classes of factors:- (1) factors affecting the slipperiness of the road, such as the state of polish of the surface. (2) factors affecting the measurements themselves, such as the calibration of the test instrument. factors affecting sfc measurements made with scrim are considered in this report and estimates are given of their magnitude. these factors are machine variability including repeatability, reproducibility, tyre resilience and wear, calibration, recorder drift, tyre pressure and water flow; machine operation including tracking, speed and location; test conditions including water-film thickness, evenness of road surface and turning of scrim; and variability associated with the processing of results. there are only relatively small "errors" associated with the use of a single scrim to compare a number of sections during a limited period of time, and steps can be taken to allow more precise comparisons to be made over longer periods. an appendix gives the results of co-operative trials that were made using seven scrims from several different organizations.(a) for part i: guide to the use of scrim, and part ii: factors affecting the slipperiness of a road surface, see irrd nos 224039 and 224038, respectively.

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