A road is slippery but you cannot slide on it; you slide on the ice that is not neccessarily slippery. Slide refers to a normal motion, but slip to anaccident.

if you have the experiences of driving on the icy road, you know well the car's tendency of not going straight forward. This is called skidding. Even though it is actually caused by the tire surface slipping on the road, but we don't call it “car slipping”.

That's why people compile dictionaries