Friction Coefficient, Rubber Tyre on Wet Asphalt
| Value | 0.5 — |
| Status | Measured: ± 0.2 — (0.4 relative) |
| Source | AASHTO Green Book / NCHRP wet-pavement friction studies |
| Categories | Material PropertiesEngineering & Trade |
| part per trillion | 500,000,000,000 ppt |
| part per billion | 500,000,000 ppb |
| part per million | 500,000 ppm |
| per cent mille | 50,000 pcm |
| basis point | 5,000 bp |
| per mille | 500 per mille |
| percent | 50 % |
| ratio | 0.5 — |
Learning zone
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.