1. A method for determining a condition of a road surface while a motor vehicle is travelling on the road surface, comprising: (a) continuously measuring a vertical movement of a body of said motor vehicle; (b) obtaininq a frequency distribution of said vertical movement; (c) dividing said frequency distribution into a plurality of different frequency regions; (d) calculating average values of said frequency distribution for a predetermined length of time within each of said frequency regions; and (e) specifying said condition of said road surface based on said average values of said frequency distribution for each of said frequency regions.