After seeing him in ch5, I would not be surprised if Ralsei is one of those normally nice people who just get a serious case of road rage whenever they gets behind the wheel of a car (especially since I didn't get the impression that most Hometown drivers other then Toriel and Asgore care much about following regulations).
Wouldn't work, even if the knife pierced the ground (good luck), the car would either not be able to move, be absorbed into another darkworld, or lose its darkness when it moved off the fountain