When driving on a dry road in the summertime, drivers hardly have to think about their tyres. When a thunderstorm breaks or a persistent summer rain fills the grooves in the road or a slush-weather occurs in between harder winter conditions, it is quite a different ballgame. The danger of aquaplaning and slush planning can be felt in the steering: it is difficult to control the car, especially if the tyres are in poor condition or worn out. The best way to prevent aquaplaning and slush planning is to use new tyres. Even new tyres do not completely eliminate the risk of aquaplaning or slush planing, 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 both winter as summer tyres with the proper groove depth and properties to suit the weather conditions. When there is excess water or slush on the road and the driving speed exceeds a certain limit, the tread pattern of the tyre no longer pushes aside the water or slush from underneath the tyre. The feel between the tyre and the road will be lost, and so will the grip.
When there is less than four millimeters of tread on the tyres (in Norwegian: dekk), the tyres' wet grip and aquaplaning as well as slush planing properties essentially deteriorate; the risk of aquaplaning and slush planing, in particular, greatly increases. Furthermore, the breaking distance is longer and the car will skid easier. Tests conducted by Nokian Tyres show that with a worn-out along
tyre (tread less than 1.6 millimeters, approximately 5 millimeters of water on the road), aquaplaning or slush planning will occur when driving in a curve at the speed of 80 km/h, whereas the aquaplaning and slush planning speed for new tyres is 95 km/h.
When the driving speed increases and the tyres wear out, the contact area between the tyre and the road is dramatically reduced. The contact area of a vehicle with worn-out tyres, 1.6 millimeters, is only 16% when compared to a stationary vehicle.