Sleet is AWFUL to drive in, but wow, does it take a LONG time to melt! In the Valentine's day storm, I was living in Lancaster, PA and we had SIX INCHES of sleet, and then several inches of wet snow...it all froze solid that night, and it took WEEKS of temps in the 40s and 50s to melt the literal glacier that formed. Cars that hadn't been cleaned out were pretty much frozen in place.
It was quite an incredible experience.