Dangers of Hydroplaning
When driving on a dry road in the summertime, drivers hardly have to think about their tires. When a thunderstorm breaks or a persistent summer rain fills the grooves in the road, it is quite a different ballgame. The danger of hydroplaning can be felt in the steering: it is difficult to control the car, especially if the tires are in poor condition or worn out. The best way to prevent hydroplaning is to use new tires. Even new tires do not completely eliminate the risk of hydroplaning, but it is possible to control the car as long as you adjust the driving speed to the conditions.
From the viewpoint of traffic safety, it is important to have tires with the proper groove depth and properties to suit the weather conditions. When there is excess water on the road and the driving speed exceeds a certain limit, the tread pattern of the tire no longer pushes aside the water from underneath the tire. The feel between the tire and the road will be lost, and so will the grip.
When there is less than 4/32 of an inch of tread on the tires, the tires' wet grip and hydroplaning properties essentially deteriorate; the risk of hydroplaning, in particular, greatly increases. Furthermore, the breaking distance is longer and the car will skid easier.
In recent tests conducted by Tekniikan Maailma (5/2018), worn tires started hydroplaning at speeds as low as 47 mph. The best new tire in the test only started hydroplaning at 55 mph.
Lost contact area
When the driving speed increases and the tires wear out, the contact area between the tire and the road is dramatically reduced. The below figure shows the size of the contact area of a summer tire with various groove depths when driving at different speeds on a road with a water thickness of 4/32 of an inch. The contact area of a vehicle with worn-out tires (thread 2/32 of an inch ) and at the speed of 78 mph is only 6 per cent when compared to a stationary vehicle.