ROAD SURFACE CONDITION DETERMINING METHOD AND ROAD SURFACE CONDITION DETERMINING DEVICE

A road surface condition determining method and a road surface condition determining device. A road surface condition determining method includes: a first step of acquiring over time a signal including information on acceleration, a strain rate, or strain output from a sensor disposed on an inner surface of a tire when the tire rolls on a road surface and obtaining time-series data X(t) when the tire has rotated at least twice; a second step of obtaining an autocorrelation function R(τ), where τ is time, from the time-series data X(t) obtained in the first step; a third step of extracting positive local maximum values having a predetermined magnitude or more at τ> for the autocorrelation function R(τ) obtained in the second step; and a fourth step of determining unevenness of the road surface by using the positive local maximum values extracted in the third step.