A light rain begins after two weeks of dry weather. Why do experienced drivers treat the first stretch of rainfall as unusually slippery?

Why B is correct

During dry weeks, the roadway collects a thin film of engine oil drips, rubber dust and general grime. The first rain lifts that film and suspends it in the surface water, producing a greasy layer that is measurably more slippery than the same road under steady, established rain, which has had time to flush the residue to the gutters. The effect is worst in the opening minutes and in the wheel tracks of intersections where drips concentrate. The practical response is to treat the first rain as a warning, not a nuisance.

Why the other options are wrong

A. Rain always falls hardest in the first minutes of a storm

Storm intensity profiles vary wildly and have nothing to do with pavement friction. The hazard in the first rain is what the water mixes with, not how much of it falls.

C. Cold rain lowers tyre pressure enough to reduce grip

A cold-shock pressure drop in a tyre is small and gradual, and no match for the immediate, road-wide change in surface friction caused by suspended oil. The physics that matters here is at the contact patch, not inside the tyre.

D. Wet roads reflect headlights and blind oncoming drivers in the first minutes

Glare from wet roads is real but is a visibility problem, not the slipperiness being described. A film of mixed oil and water reduces grip whether or not any headlight is pointed at it, including at noon.

On the road

September in the city: two hot dry weeks, then the first spatter of a front at 5 p.m. The driver ahead carries corner speed through the first intersection and wobbles wide on the exit, front tyres chirping on the greasy film. You had already dropped a third off your approach speed for exactly this reason, and your line through the same corner is uneventful.

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