Abstract: A method of warning a vehicle driver when the vehicle may be driving on a potentially hazardous road surface, the vehicle having a wheel or wheels at front left and right and rear left and right positions on the vehicle, comprising repeating the steps of measuring the instantaneous speed of the wheel or wheels at each of the four positions, calculating the ratio of the speeds of the wheels on the left and the ratio of the speeds of the wheels on the right and deriving and storing the difference between these ratios, comparing the current difference with the previously stored difference and in the event that the absolute value of the comparison between the differences exceeds a predetermined value issuing an alarm to the driver that the road surface may have varying frictional characteristics.