theweathernetwork.com used to have a map called "highway forecast conditions". Either they removed it or i can't find it. They still have 'highway conditions', which shows a current snapshot, but its not the same. If i'm planning a drive from Montreal to Toronto which can take 6 hours (good weather) or 9 hours (black ice develops during the trip, roads closed), that is plenty of time for ice or snow to move in. Similarly, if i check at 6am and the roads have snow, they would usually be cleared later in the day if the snow has stopped.