Measurements of skidding resistance are affected by the following two classes of factors: (1) factors affecting the slipperiness of the road, such as the state of polish of the surface. (2) Factors affecting the skidding measurements themselves, such as the accuracy of the calibration of the test instrument. Factors affecting the slipperiness of a road surface are considered in this report and estimates are given of their magnitude. These factors include seasonal variation, temperature, type and composition of the road surfacing material, type and condition of the road age, traffic density, climate and any contamination of the road surface. Methods of reducing variations due to these factors are discussed, particular emphasis being placed on the desirability of using mean summer values.