Method and apparatus for determining road surface condition

A method, featuring robustness against changes in tire size, is provided for determining a road surface condition by dividing a time-series waveform of tire vibrations into windows without resorting to detection of the peak positions or measurement of the wheel speed. A time-series waveform of tire vibrations detected by a tire vibration detecting unit is windowed by a windowing unit. Time-series waveforms are extracted from the respective time windows, feature vectors X are calculated therefor, and then likelihoods Z for road-surface HMMs (hidden Markov models) are calculated. The likelihoods Z1 to Z5 calculated for the respective road-surface HMMs are compared with one another, and a road surface condition corresponding to the road-surface HMM showing the highest likelihood is determined to be the condition of the road surface on which the tire is running.