No matter the season; rain is always a likely prospect here in the UK. With our nation being treated to an average of 135 wet weather days a year and a tendency towards regular downpours, it’s easy to see rain as “normal” driving conditions. But whether riding a bicycle or driving a 4×4, every road user should be aware of the impact of wet weather on driving conditions.
In 2016, a Highways England safety campaign, “When It Rains, It Kills,” highlighted just how dangerous wet roads are compared to dry roads, and what road users need to do to keep themselves safe. Their shocking statistics revealed that people are 30 times more likely to be killed or seriously injured on the roads in rain than in snow. The then-Highways England’s Head of Road Safety, Richard Leonard, explained why:
“Most of us already slowdown in snow, ice or fog but when it rains, we consider it normal so don’t adapt our driving.
The sad fact is that 2,918 people were killed or seriously injured on the roads when it was raining [in 2015], and not slowing down to suit the current conditions was identified as a factor in 1 in 9 of all road deaths.”
Understanding the reasons why wet weather can make accidents and injuries far more likely is a key step in adapting driving/riding behaviour and minimising the risk.
Why is it Difficult to Drive on a Wet Road?
Wet weather can impede safe driving in a number of surprising ways:
- Reduced Friction: Water makes road surfaces more slippery by reducing the coefficient of friction, which decreases the grip of tyres on the road surface. This makes it far easier for vehicles to skid, especially during sharp steering manoeuvres or sudden braking.
- Longer Stopping Distances: The stopping distance for braking cars on wet roads is nearly double that found on dry roads. This means drivers who do not adjust their speed and the gap they leave between themselves and other vehicles, even if they are sticking to the speed limit and normal guidelines, are far more at risk of collisions.
- Aquaplaning: Aquaplaning occurs when water gathers in a wedge ahead of a vehicle’s tyres. As the car increases in speed, it is unable to escape quickly enough. This results in a force known as hydrodynamic pressure: a pressure buildup that grows to exceed the pressure from the tyre, causing it to lose all traction as it lifts off the road surface. When this happens, the driver can’t steer or brake.
- Poor Visibility: Rain and water spray can make it difficult for drivers to see clearly (motorcyclists in particular can struggle with wet and foggy visors). Standing water on the road can also obscure hazards, such as potholes.
- Other drivers: Even when a driver personally adjusts their driving behaviour to account for wet weather, if others on the road do not do so, they may be forced to drive defensively.
The Importance of Tire Tread in Wet Weather
New tyres have a deep tread (8mm), which is designed to provide traction and grip. The patterned surface found on all tyres helps to channel water away from where the tyre makes contact with the road and creates “biting edges”, helping drivers safely navigate their journeys without skidding. Over time, however, this tread becomes worn down and loses friction.
This is an issue in any conditions, but becomes especially problematic when it’s raining, which is why tyres are such a point of focus during a car’s MOT.
At What Temperature are Wet Roads Most Dangerous?
Wet roads are most dangerous after a period of dry weather – especially if it has been hot. Fresh downpours on dry road surfaces mix with dust, oil, rubber and debris to create a hazardous, slippery liquid. When the weather has been hot, this is made worse by softened asphalt.
In time, the rain will wash the other materials away, and the road surface will become less greasy. So while drivers should always be vigilant in wet weather, they should be particularly careful in the first fifteen minutes of rainfall.
Using Anti-Skid High Friction Road Surfacing
Areas where cars have to stop or slow down suddenly, such as:
- Junctions
- Pedestrian crossings
- Roundabout approaches
- Cycle lanes
- Bus lanes
- Fast bends
…can become perilous in wet weather. The application of high-friction road surfacing is one means by which authorities reduce the risk of accidents in these areas.
High friction surfacing works by combining a binder with an aggregate known as calcined bauxite, which can be applied either hot or cold. The aggregate creates smaller and more numerous points of contact with the tyres of a vehicle, known as high contact pressure points. These improve grip and reduce the chance of skidding.
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No matter how common rain is in the UK, the fact is that wet weather fundamentally transforms the driving environment – from spray against car windscreens making it much harder to see the vehicles ahead to slick road surfaces which make handling a vehicle significantly more challenging. Alongside sensible infrastructure modifications, such as the application of high-friction road surfacing, the most important factor in the reduction of wet-weather accidents is driver adaptation.
Taking more care by turning on your headlights, driving at an appropriate speed (which may be below the speed limit), and maintaining double the usual distance between yourself and the vehicle in front are all easy ways to stay safe on wet roads. Also, keeping a close eye on your tyre tread is extremely important, as well as exerting hyper-vigilance during that all-important first 15 minutes of rainfall.
By respecting the rain and making small, proactive adjustments, you ensure every journey remains safe and secure, regardless of the weather.
