Riding on a road that was dry and is now wet from rain — what changes most immediately?

FIRST RAIN DANGER is a well-documented traffic safety phenomenon:

Dry accumulation: During dry periods, vehicles deposit oil, brake dust, diesel residue, and tire rubber on road surfaces; these accumulate primarily at intersections, in traffic lanes, and at traffic lights.

Initial wetting: The first rain creates a slurry of oil and water on the surface — which has LESS friction than either pure water or dry pavement.

Later improvement: After 20-30 minutes of steady rain, the contamination is flushed away and the wet surface, while less grippy than dry, is more predictable than the initial oily mixture.

Riding response: Dramatically increase following distance; reduce speed; avoid sudden inputs; be especially cautious at intersections; lean the bike as little as possible in turns.