Hydroplaning (sometimes aquaplaning) in a road vehicle is an effect similar to planing in a boat. A layer of water between the rubber tires of a road vehicle and the road surface (or between airplane wheels and the runway) reduces the friction with the tires. They stop rotating, causing the vehicle to act like an unpowered and unsteered sled.

Causes of hydroplaning

The likelihood of aquaplaning increases if the momentum of the vehicle is high, the vehicle is imbalanced, the tread is worn or the water is deep.

Wheeled vehicles are designed to operate properly when there is friction between the rotating wheel surface and the road. Any frictionless substance can force a vehicle to hydroplane, should the substance separate the tires from the road.

The engine provides power through the wheels to provide a controlled speed. The wheels also provide steering and braking through the driver controlling the angle of the wheels relative to forward motion, and through the operation of the brakes. The tread in a rubber tire is designed to remove water from beneath the tire, providing high friction with the road surface even in wet conditions. This enables friction between the wheel and the road, allowing the wheel to rotate, and to provide rolling resistance, braking and steering power.

In a hydroplaning situation, increasing water pressure in front of the wheel means that the amount of water being dispersed by the tread is less than the amount being forced under the wheel. The vehicle then loses braking, steering and power functions because of loss of wheel contact with the road. The result is complete loss of normal control by the driver, and the vehicle will slide until it either collides with an obstacle or until wheel road friction is regained.

Hydroplaning is especially dangerous while the car is on Cruise Control, as the increased reaction time and lack of pedal feel can cause the car to accelerate without warning.