Fact

Weather - In which ways does weather affect road safety?

Weather conditions affect both crash rate and the exposure to traffic hazards. This influence is strongest for the conditions of precipitation (including snow and hail), fog, low sun, wind, ice forming, and hot temperatures.

Precipitation

Research has shown that motorists adjust their road behaviour during showers. They overtake less, driver slower, and increase their following distance (Hogema, 1996; Agarwal et al., 2005). However, the risk of a crash during rain is still greater than in dry weather. The changes in driving behaviour are, apparently, insufficient to compensate for the greater risk during bad weather (Thoma, 1993).

Road users can have problems with reduced visibility during periods of precipitation. This can be reduced to approximately 50 meters during heavy rain or snow. Splashing water, particularly from lorries, can interfere considerably with the visibility of other motor vehicle drivers (Terpstra, 1995). Clouded windows and windscreens as a result of high humidity during rain can also reduce visibility (Fokkema, 1987). Furthermore, blinding can occur at night because the headlights of oncoming vehicles reflect in the water on the road surface (Ellinghaus, 1983).

The more rain, snow, or hail falls, the less the friction of the road surface. Rain can lead to dynamic aquaplaning. A layer of water on the road surface can cause the vehicle to lose contact with the road surface and to skid. The chance of aquaplaning depends on the skidding resistance of the road, but of course also on the vehicle's speed and tyre tread depths (Ellinghaus, 1983; Terpstra, 1995; Van Ganse, 1981). When it has been dry for a long time, a drizzle can lead to viscous aquaplaning if drops of oil and dust, together with water, produce a thin liquid film on the road surface. When the rain gets heavier, the chance of viscous aquaplaning lessens because the road surface is swept clean (Terpstra, 1995; Eisenberg, 2003).

Fog

In a fog the droplets of water are so small and light that they remain floating in the air. This leads to a reduction in visibility because the light is diffused by the fog droplets. In general, fog occurs when the humidity is 100%. When this happens people generally drive somewhat slower, but simultaneously keep a shorter following distance to the vehicle in front of them. In combination with the decreased field of vision, this increases the risk of crashes (Fokkema, 1987; Oppe, 1988). Fog can also cause viscous aquaplaning when water droplets provide a thin film on the road surface (Terpstra, 1995).

Low sun

Sunrise and sunset can greatly hinder the view that road users have of other traffic. The sun blinds the most when it is low on the horizon. This is the case until about an hour after sunrise and from about an hour before sunset. Motorists can still look through their windscreen, but cannot see clearly anymore. Also indirect sunlight which is reflected by, for example, a glass building, noise barriers, or other cars can be problematic. When lighted by the sun, dirt on the windscreen is more visible, thus hindering driving. Visibility is affected even more when the road surface is wet and reflects the sunlight (Fokkema, 1987).

Wind

Gusts of wind can push relatively high vehicles such as busses, delivery vans, camper vans, caravans, and lorries off course and, under extreme conditions, can even cause them to roll over. This happens mainly on bridges and viaducts. Objects carried by the wind, fallen trees, and broken-off branches can also cause traffic disturbance (Ellinghaus, 1983). Pedestrians and two-wheelers can be troubled by strong gusts of wind and therefore disturb other traffic.

Ice forming

If a road surface has an open structure, such as porous asphalt, wet parts of the road surface will freeze quicker than surfaces with a closed structure. When there is black ice, a thin layer of ice forms so quickly on porous asphalt that the asphalt loses its friction (CROW, 2000). Roads that have just been laid also have a greater risk of slipperiness: the layer of black bitumen has a lower temperature and is thus more sensitive to wet parts freezing. In due time the risk of slipperiness will lessen because of the wear and tear of the upper layer (CROW, 2006).

Temperature

High temperatures especially have a psychological and/or physiological effect on a driver. However, a lot less is found in the literature about the physical effects of weather conditions. According to a German study, emotions rise with the temperature, people are more irritable to others, they get tired, lose their concentration, and their reaction time increases (DVR, 2000). French researchers found an increase in the number of crashes during heat waves. Their explanation was that people possibly drive at other times of the day and that they sleep shorter or less deeply because of the high night-time temperatures. This results in them being more tired when they take to the road (Laaidi & Laaidi, 2002).

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