Rain and hydroplaning — Idaho Permit Test

Roads are more dangerous at the start of a light rain, when road oil and water mix to form a greasy film on the road.

The most slippery moment is the first few minutes, before the rain has washed the oil away.

roads are more dangerous at the start of a light rain when road oil and water mix to form a greasy film on the road

At speeds up to 35 miles per hour most tires grip the road surface even when it is wet.

Below that the tread can still move the water out of the way; above it, the water starts to win.

At speeds up to 35 miles per hour, most tires grip the road surface even when it's wet

At 55 miles per hour the tires may lose all contact with the road and start to hydroplane, riding on top of a film of water. When that happens you cannot brake, accelerate or turn.

Nothing the driver does with the controls reaches the road, because the tires are no longer touching it.

At 55 miles per hour, the tires may lose all contact with the road

To reduce the chances of hydroplaning, slow down in rainstorms or on slushy roads, slow down where there is standing water, replace tires as soon as they become worn, and keep tires properly inflated.

Worn tires have no grooves left to move water out of, which is the whole of how a tire works in rain.

Replace tires as soon as they become worn

If your car starts to hydroplane, maintain a steady speed, keep moving in a straight line, and slowly take your foot off the gas pedal.

Any sudden input while the tires are not touching the road takes effect the instant they touch it again.

Maintain a steady speed and keep moving in a straight line

Do not try to stop or turn quickly until your tires are gripping the road again, and if you must use the brakes, pump them gently unless the car has anti-lock brakes.

Locking a wheel that is already floating means it stays locked when it lands.

If you have to use your brakes, pump them gently (unless your car has an anti-lock braking system [ABS])

Wet roads can be as dangerous as icy roads, so always reduce your speed in wet weather, because you will need more distance for stopping and may skid on quick turns.

Drivers plan for ice and not for rain, and the stopping distance on a wet road is nearly as long.

Wet roads can be as dangerous as icy roads

When are Idaho roads most dangerous in the rain?

At the start of a light rain. The most slippery moment is the first few minutes, before the rain has washed the oil away. July 2026 Idaho Driver's Handbook, p.111: “roads are more dangerous at the start of a light rain when road oil and water mix to form a greasy film on the road”