Rain, spray and standing water

What you need to know

A wet road does not feel very different at a constant speed in a straight line. That is the trap. The difference appears the moment you ask the tyres for something — braking hard, steering quickly, or both at once.

The single number to remember is this: in the wet, allow at least double the stopping distance. Everything else follows from that.

• Your two-second following gap becomes at least four seconds.
• Your approach speed to a bend needs to be lower than it would be in the dry.
• Your braking should start earlier and be gentler.

See it on the road

The first ten minutes of rain after a dry spell are the worst. Oil, rubber and dust that have built up on the surface lift and mix with the water, and grip drops further than the rain alone would explain. Painted lines, metal drain covers and tram rails become slippery out of proportion to everything around them.

That matters most for motorcyclists and cyclists, who have two small contact patches instead of four. If you are following one, give them room to move around those hazards.

Aquaplaning

If there is enough standing water, the tyres can stop touching the road at all and ride on a film of water instead. This is aquaplaning, and it has a very distinctive feel:

• the steering goes light and vague, as though disconnected;
• the engine note may rise as the driven wheels spin up;
• the car stops responding normally to small steering inputs.

What you do next decides whether it is a non-event or a spin.

Ease off the accelerator. Keep the steering still. Do not brake.

Let the speed fall until the tyres bite again — you will feel the weight come back into the steering. Then, and only then, steer or brake as normal.

Braking or steering while aquaplaning is what turns it into a crash, because the moment grip returns, the car does whatever you were asking of it, suddenly.

Deep water, floods and fords

Standing water deep enough to reach the engine bay can destroy an engine, and water moving across a road can move a car. If you cannot see how deep it is, the right answer is to turn round.

If you do go through:

1. Check the depth first, and watch what happens to a vehicle ahead if you can.
2. Drive slowly, in first gear, keeping the engine speed up and slipping the clutch. That keeps water out of the exhaust.
3. Do not stop in the water.
4. Test the brakes gently once you are out.
5. Dry the brakes by applying light pressure while moving slowly — check your mirror first so the driver behind is not surprised.

Wet brakes are noticeably less effective on the first application. Finding that out at a junction is a bad way to learn it.

Why it matters

Rain does not just reduce grip. It also reduces what you can see: spray from other vehicles, a windscreen that is never quite clear, and headlights that scatter. So the two things you rely on — seeing a problem early and being able to stop for it — get worse at the same time.

Doubling your gap fixes both at once. It buys you distance to stop and it moves you out of the spray.