Friction Coefficient, Rubber Tyre on Wet Asphalt

Water does not lubricate rubber the way oil lubricates steel; it interferes with the adhesion component of grip and, above a threshold speed, floats the tyre entirely. Wet friction is therefore not a single number — it depends on water depth, speed, tread depth and pavement macrotexture. Good tyres on fresh open-graded asphalt at moderate speed can still make 0.6; a worn tyre at 2 mm tread depth on polished pavement in standing water can drop below 0.3, and a fully hydroplaning tyre is essentially at zero.

The design consequence is that wet stopping distance is roughly 1.6 times dry, and that road geometry — superelevation, curve radius, sight distance — is set by wet friction, not dry. Highway design values are deliberately conservative, in the 0.3–0.35 range for the pavement-tyre combination at design speed, precisely because the honest uncertainty on this coefficient is a factor of two. The first minutes of rain are the worst, when water lifts accumulated oil film before washing it away.