At what temperature is the risk of both treacherous and extreme slipperiness the greatest?

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.

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.