Sheltering under an underpass is actually a myth. It is not recommended by the National Weather Service.  A strong tornado will  not be stopped. The major tornado in Oklahoma Cit, the F5, killed many under underpasses with 300 mile an hour debri, or simple sucked them out and shot them across the landscape.

If the tornado is visible to you, and its not moving towards you, relax. If it is heading toward you, and you have time yet, drive away, using right angles. Example if it is heading east, travel north or south. If its close, look for underground shelter, a dip in the ground. Your car will be thrown around, so you dont want to stay in it, or very near it.