Abstract: A method of measuring surface friction in which drag due to contaminant lying on a said surface can be isolated, and a continuous friction measuring device for effecting the measurement. The device has a pair of friction measuring wheels (36,37) in contact with the ground and a third wheel (41) which is freely rotatable during the friction measuring process, the axle (42) of the third wheel (41) being provided with a load sensor (55) to measure the horizontal drag load on the third wheel during the friction measuring process. A trailer or vehicle incorporating the wheels (36, 37, 41) is passed through contaminant lying on a surface and the horizontal drag on the wheel (41) is measured simultaneously with a continuous measurement of the friction coefficient of said surface.