Why do slippery roads occur?

Most of us understand that some parts of a road are more slippery than others. But we often don’t realise just how much the road status is affected by the surrounding environment. And why.

 

So why are some parts of the road network more slippery than others?

There are a number of factors that influence how the temperature and the risk of slippery conditions vary on the road network. Here we explain a little more about the factors that matter most. If you want to know more, contact us and we will be happy to provide you with more information.

 

Altitude differences and topoclimates

Temperature conditions in the air layers closest to the ground are often affected by the altitude conditions around the road. The effect of this is most evident in clear, calm weather in the evening and at night. The temperature of the ground surface drops, which results in a low air temperature immediately above the ground. This cold air is relatively heavy and will therefore primarily fill the low-lying areas in the terrain. This creates so-called cold-air pools that can have a significant local impact on slippery conditions. During converse weather conditions (windy and cloudy), on the other hand, it is higher altitudes that are more affected by slippery conditions.

 

Close to water

When a road passes close to lakes, bays, rivers or other areas of open water, or runs adjacent to marshes, the risks of slippery conditions increase during the autumn due to the increased humidity locally. The high moisture content of the air means condensation easily forms on the road surface. This can then freeze, forming ice. If temperature conditions allow, water vapour in the air can, in certain situations, soon cause ice on the road in the form of frost.

 

The local climate in forest areas

When roads pass through forest areas, the width of the unwooded area has a significant impact on the air temperature above the road surface and the temperature of the road surface itself. When a road passes through a forest area, the road is shielded from solar radiation, so any ice on the road takes longer to melt within such areas. Clearings next to the road can cause pockets of cold air to collect in the evening and at night, similar to small cold-air pools.

 

See figure below

a. When the trees are close to the road, the heat radiation from surrounding trees is so great that the air temperature is kept relatively high.

b. A wider clearing leads to a greater risk of slippery conditions, as the heat radiation is less significant, while the air near the ground is disturbed relatively little.

c. In a wide clearing, the turbulent agitation of the air layers closest to the ground increases, as the wind can get to them more easily, reducing the number of instances of low temperature close to the ground.

 

Urban climates

The city has an impact on the air temperature, which is almost always slightly higher than in the rural surroundings. The differences are especially obvious during relatively calm, clear evenings. The temperature difference between the central parts of the city and the countryside can then amount to several degrees, and in extreme cases differences of 7–10°C can arise. All the houses and streets in the city store heat during the day, which then radiates back out in the evening and thus maintains the temperature. In rural areas, on the other hand, the surface of the ground and the near-ground air layers cool quickly. Urban areas also affect the formation of slippery conditions because relative humidity is often lower than in the surrounding areas. This leads to less frost and, at temperatures of around 0°C, any precipitation falls as snow in the countryside, but as rain or sleet near the city.