In Germany only 1 out of 1,000 passenger cars involved in accidents has complete aquaplaning *

Since the phenomenon has been identified at the end of the 1950’s, tire hydroplaning is considered as a dangerous event for car drivers.

The question about the frequency of hydroplaning leads to long debates – the ones considering that most of the wet accidents are due to hydroplaning; the others considering it is a very seldom case, and that wet grip is much more relevant. The representatives of both positions have so far lacked reliable statistics. Although the phenomenon of hydroplaning is about 60 years old, it is almost impossible to find any scientific estimation of how frequently vehicle accidents can be caused by hydroplaning, especially in Europe.

To change this situation, the VUFO carried out a study* for Germany. Now, researchers are able to quantify the number of accidents caused by hydroplaning, using an objective and scientific way as well as representative data.

The results show that complete aquaplaning occured only in 1 (0.15%) of 1,000 passenger cars involved in accidents causing personal injury. This also means that in wet accidents complete aquaplaning occurs 100 times less frequently than "normal" wet grip situations (0.6% and 62% respectively), whereby it must be taken into account that partial aquaplaning can also exist in the wet grip situation.

Figure 1: Relevance of wet accidents in Germany and proportion of several grip situations on wet roads

The final results obtained from the analyses are summarized in the following table:

Table 1: Result summary for hydroplaning as a root cause of accidents in Germany

The study uses two data sources with a maximum of cases in order to have a chance to catch hydroplaning relevant situations.

The first data source used is an In-Depth Accident Database (GIDAS) which contains 24 577 passenger cars and, for each of them, relevant information to determine full hydroplaning occurrence (e.g. tire tread depth, initial speed, rain intensity, presence of ruts, etc.). With these parameters as well as information from the driver interview, the accident description and the accident reconstruction, it is possible to evaluate an objective probability that the accident was caused by hydroplaning.

The second data source used are police reports of the complete federal state of Saxony. Here, a time period of two years (July 2015 until June 2017) was considered, containing more than 204.000 accidents involving passenger cars. The description of accidents is less detailed but with appropriate keyword research it is possible to estimate the relevance of hydroplaning in the accident initiation phase.

The analysis of passenger cars in the GIDAS database was the key result source, as police reports do not necessarily provide information about hydroplaning and a bias is likely.

*VUFO study “Analysis of hydroplaning accidents”; 2018; Basis: GIDAS data from 2005-2017, n=22.783 passenger cars, thereof 5.810 vehicles in accidents with known rain intensity.

What is hydroplaning?

A tire travelling at a significant speed over a wet flooded surface pushes a little “bank” of water in front of it. The interaction of tire and water at the front of the contact patch causes a rise in the water pressure (called hydrodynamic pressure). If this pressure becomes greater than the tire-road contact pressure, the tire can no longer push and evacuate the water away and it lifts off the road surface. This is known as hydroplaning. Depending on the water depth, above a certain speed the tire will lose any contact with the road surface, drastically reducing its breaking and steering capacity.

The two major factors influencing hydroplaning are the vehicle speed and the depth of standing water on the road. Also, the tread pattern and the shape of the contact patch play a role in the evacuation of water from the contact patch and thus maintaining the optimal contact of the tire with the road surface. The road surface will also affect any hydroplaning process.

Hydroplaning is often mistakenly thought to be a binary process – either gripping or floating. However, that is not true. As shown on Figure 2, hydroplaning is a progressive phenomenon that, depending on water depth on the road and vehicle speed, gradually reduces the surface area in contact with the road and therefore the surface on which the grip can work. Often on wet roads hydroplaning is present in very small proportion (partial hydroplaning), which reduces the contact surface and thus diminishes the tire grip.

Figure 2: As speed increases, the contact patch decreases (water is shown in green to improve contrast)

How prevent or reduce the risk to be in hydroplaning?

At first, the condition and pressure of the tires should be checked regularly. Insufficient pressure greatly increases the risk of aquaplaning (and, additionally, increases fuel consumption and wear). If the tire is under-inflated by 30% compared to the recommended pressure, there is a real increase in the risk of aquaplaning. Second, the speed should be adapted to the weather and road conditions. Finally, if hydroplaning occurs, sudden braking or steering maneuvers should be avoided.