When driving in the rain, your car’s tyres need to displace and disperse the film of water on the road surface to maintain direct contact between the rubber treads and the tarmac.
That contact is essential to grip. And grip is essential to control of the vehicle. If the rainwater is not displaced and dispersed, a super slippery layer of compressed water develops between the tread and the tarmac. The result is called “aquaplaning”. Contact is lost. Grip is lost. Control is lost.
The car does not respond to steering or braking, and if either of those controls is needed to negotiate a corner or avoid an obstacle there is only one outcome. Bang!
Happily, car tyres are designed to displace and disperse water very efficiently. That’s the number one job of the fine slits and deep channels in the surface of the rubber that we call the “tread”. They can get rid of water, a lot of water, very quickly. And they need to.
At 100 kph in moderate rain, they need to displace and disperse 20 litres (wait for it) per second! That’s a hard ask, even for a well-designed tyre that’s nearly new.
If a tyre is half worn, it has no chance of sweeping away water at that rate, so speed (and, therefore, the amount of water to remove) must be proportionately reduced to avoid aquaplaning. How much slower? Well, if the tread depth is halved, the capacity to disperse water is halved.
On a bald tyre, almost any speed is too high for the (non-existent) tread slits and grooves to clear water, and maintain contact, grip and control. The law relies heavily on your understanding of these principles.
For a tread depth of just 2mm is “legal”, even though it is significantly less than half (!) the original.
So while you are permitted to continue using a well worn tyre, don’t assume that it will work just as well as a new one. In wet weather, you should adjust your cruising speed accordingly. And if your treads are anywhere near the legal minimum depth, that doesn’t meaning easing off a bit... it means slowing down a lot.
As a first sign that tyres are not clearing all the water and that the car is on the brink of aquaplaning, the steering will feel light and the car will tend to wander from side to side instead of maintaining a dead straight line ahead.
The need to slow down is immediate, but not (!) by using the brakes, which could induce a severe skid. Just release the accelerator completely, hold the steering steady and let speed decay until traction (and thus control) is regained.
Then stay well below that “brink” speed to allow a margin for variations in the depth of water on the road surface, which varies the amount of water the tyres must displace.
Aquaplaning can be deadly, but it is not malicious. If it happens to you even once, it’s not bad luck or bad karma — it indicates a serious misjudgment.
If it happens often, you need driving lessons and/or a new set of tyres.