Why do people act like *** klowns on the road.

People in cars act completely differently to when they are out of their cars, as you found out. They think they are invincible, king of the road.
They stop and let you by if walking in the store or the street, but the same person will cut you up or refuse to let you in when they are driving in the carpark or out on the road. It's bizarre.
Then when they get in a road rage situation they quickly remember that they are normal human beings and often get the **** kicked out of them.