Abstract: Apparatus and a method for determining the tire pressures of vehicles as y pass an instrumented checkpoint on a roadway. Rigid corrugations on the roadway set the tires into vibration with a waveform which is a function of tire pressure. The complex waveforms from each tire of a given vehicle are subjected to a spectral analysis and the results of such analyses are compared to each other to determine which, if any, of said tires have produced a spectrum different from the normal spectra produced by the other tires. This invention requires no apparatus mounted on the vehicles being checked.