The First Rain Is the Worst Rain

Most drivers assume heavy, sustained rain is the dangerous kind. It isn't — the most dangerous moment is usually the first few minutes of the first rain after a dry spell, and the reason is what has been building on the road surface while it wasn't raining.

Through weeks of dry weather a road accumulates oil and diesel drips, rubber dust from tyres, and general grime. It sits there, bound to the surface, doing nothing. Then the first water arrives — and instead of washing it away, it lifts it into a thin, slick film floating on top of the tarmac. For a while the road surface is not wet tarmac; it is a layer of lubricant with water on it.

Grip under those conditions is dramatically worse than on settled wet tarmac — the curriculum describes it as approaching the slipperiness of ice, and braking distances stretch far beyond the wet figures you already know.

And then it improves. Sustained rain washes the film away, so the danger is highest at the start and falls as the rain continues — which is exactly backwards from how it feels. The practical rule follows:

When rain begins after dry weather, slow down immediately and give yourself far more distance — and don't relax just because the rain gets heavier.

This is also why the first rains of each season are so dangerous: the dry spell before them has been long, so the film is thickest.