In general, crash risk is higher in bad weather conditions than in good weather conditions. Bad weather conditions are mainly rain, snow/hail, fog, strong winds, slipperiness, low sun and high temperatures.
Bad weather conditions can lead to longer braking distances (slippery conditions due to rain, snow or hail) and cause the vehicle to be less stable (wind). Visibility can also be reduced, not only by fog, but also by splashing water, fogged-up windows or glare from low sun. Heat/high temperatures can cause people to think and act less effectively. However, people do appear to compensate for poor weather conditions, for example by driving more slowly or adapting their headway distance. This partly reduces the increased crash risk.
The weather also affects mobility, such as choosing whether or not to go out, the length of a trip, the choice of transport mode, the time of departure, and the route . For example, when the weather is fine, people are more likely to walk, cycle or ride motorbikes instead of driving cars or using public transport (mode choice). Fine weather also leads to more recreational trips (additional mobility) by pedestrians, cyclists and motorcyclists, for example. This leads to more casualties among these groups, simply because of the greater exposure to road hazards. Conversely, bad weather will result in fewer casualties among these groups.
Weather versus climate
Due to climate change, extreme weather conditions such as heavy precipitation, severe storms and more extreme drought and heat are expected to become more frequent. This means that weather effects should increasingly be taken into account when studying road safety developments. On its website, CROW offers practical information for road authorities and others on how to deal with the consequences of climate change, or 'climate adaptation’ . The online CROW knowledge base also provides information on climate adaptation, for example by means of a series of fact sheets .