Since road surface temperatures are usually much colder than the ambient temperature, you should expect roads to be slippery as soon as the thermometer shows +4 °C or colder.
The most treacherous road surface arises when the weather clears up after rain or fog and the temperature is around 0 °C. In these conditions so-called black ice may form. Black ice can occur very quickly when moisture on the road freezes to ice. Black ice is most common in winter but can also occur on cold days in spring and autumn as well.
Note that the road can be slippery at temperatures above +4 ° C as well, but not because of frost, snow and ice. Fog, wet leaves, newly laid asphalt, soil and mud from agricultural machines and pools of water are just a few examples of things that can lead to slipperiness, even though it is far from winter road conditions.
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