Weather directly affects drivers’ ability to control their vehicles. Increased crash risk due to weather is quantified in multiple ways leading to different estimates. According to the Transport Canada’s Canadian Motor Vehicle Traffic Collision Statistics, in 2021, approximately 18% of fatal collisions occurred in adverse weather or on slippery roads, (i.e., wet, snowy/slushy, or icy pavement). However, a more detailed study concluded that the critical reason, which is the last event in the crash causal chain, was attributed to the road environment in only about 2 percent of crashes and assigned to the driver in 94 percent of the crashes (https://ops.fhwa.dot.gov/weather/q1_roadimpact.htm).
The most frequently studied weather condition is precipitation in the form of falling rain, snow, and hail. A wet road surface increases braking distances and makes steering, especially around corners, more difficult. Water on the road surface increases the risk of dynamic hydro- or aquaplaning, when a layer of water can lift a moving tire off the pavement. On roads that have been dry for a long time, a light rain can lead to viscous aquaplaning if drops of oil and dust mix with water to produce a thin liquid film on the road surface. During winter, ice on the road surface greatly increases braking distances and the risk of drivers skidding out of control. Black ice is a thin transparent layer of ice that takes on the colour of the road surface and is difficult to detect. Black ice forms when temperatures climb above freezing, melting some snow, and then drops again. Black ice can also form from dew or fog when temperatures drop below the freezing point.
Weather also negatively affects drivers’ ability to clearly see the road environment. Heavy rain and snow, especially when combined with strong winds, or fog can reduce visibility to the point where driving at the posted legal speed may be unsafe because braking distances will exceed the distance the driver can see ahead. Strong crosswinds combined with icy roads make steering difficult, especially for long trucks. Precipitation can reduce visibility in passenger vehicles from water splashing on the windshield from passing trucks, high humidity during rain can fog up windshields and windows, and at night, glare can increase from the headlights of oncoming vehicles reflecting on wet road surfaces (http://www.pik-potsdam.de/~luedeke/xSoSe09/Verkehr/FS_Influence_of_weather.pdf).
Despite the driving challenges created by adverse weather conditions, crash risks can be successfully reduced through safer driver behaviour. Specifically, drivers need to prepare themselves and their vehicles for adverse weather conditions and continuously exercise greater caution while driving by reducing speed and increasing space between their vehicle and other road users. The following websites list best practices for driving under adverse weather conditions: