Aquaplaning (or hydroplaning as it is sometimes called) is a rare phenomenon in which a complete layer of water builds up between the road surface and one or more of a vehicle’s tyres, causing them to lose direct contact with the road. When this happens tyres no longer have any grip, and cannot be used to steer, brake or drive the vehicle, and even simply staying on course may become impossible.
Despite its rarity, the term ‘aquaplaning’ is commonly heard. However, it is not mentioned in the UK Highway Code, and is widely misunderstood. It is not the same thing as simply skidding on a wet road.
When travelling in the wet a vehicle’s tyres have to maintain direct contact with the road in order for it to be able to steer or brake. For this to happen water has to be cleared away from under the contact patch between the tyres and the road. This is achieved by the water being forced out of that contact patch, through gaps between the road stones and through the grooves in the tyre tread pattern. The task of clearing the water becomes greater as the speed increases, since there is a larger volume of water per second to be shifted from the path of the wheels, and is all the more so when there is a measurable depth of water on the road rather than the surface being simply ‘wet’.
The problem of clearing the water is an every-day occurrence when driving in wet conditions and is managed by ensuring that roads have a good surface texture and tyres have at least a minimum depth of tread. But the effect of vehicle speed is still there, and it remains the case that the higher the speed of the vehicle, the less grip there is, which causes both the maximum braking rate and the speed at which bends can be taken, to become less. However, this reduction in (but not loss of) a vehicle’s capacity for manoeuvre is not the same as aquaplaning.
In a full investigation of the problem, which is necessary when dealing with road vehicles, the determining factors are:
- tyre pressure
- road surface texture
- tyre tread depth
- water film thickness
- the aspect ratio of each tyre’s contact patch with the road.
Clearly, if the thickness of the water film is less than the tread depth, aquaplaning will not occur. Similarly, if the road has a good texture which allows water to drain away between the stones, there will rarely be a significant film thickness on top of them. Only when there is poor texture, heavy rain or flooding may there be enough of a film for aquaplaning to be a possibility: it is difficult to generalise, but typically, with good tyres a film of more than 5 mm thickness is needed for it to be possible, although with poor tyres that could drop to 3 mm, and with bald tyres it could be as little as 0.3 mm. Even if the circumstances suggest aquaplaning to be a possibility, careful investigation of all the circumstances must be undertaken before it can be regarded as probable.
The aspect ratio of the contact patch, which is its width divided by its length, is a further complicating factor. For example, a narrow tyre will ‘cut’ through water more readily than a wide tyre, but if the contact patch of a wide tyre is also long, that will mean the water has to penetrate further back along the patch to lift the tyre clear of the road.
With all these contributing factors, there is clearly no particular speed at which aquaplaning becomes a real possibility, but when investigating a loss of control on a wet road one should not in any case expect it to happen below about 50 mph (80 km/h), and indeed the common experience of driving without difficulty in rain on a motorway at around 70 mph (110 km/h) should tell us that even at that speed there is unlikely to be a problem on a well made road and with good tyres.
TRL has a very long history of researching what determines the amount of friction which will be present between tyres and wet roads, and our experts are particularly well placed to investigate and advise in incidents where the degree of wetness, the condition of the road and tyres, and the manner of driving are thought to have led to a loss of control and a collision.