One of the many reasons overpasses are not safe is because of the funneling effect. During this time, winds are faster under a bridge, meaning debris can fly between the bridge and the road - possibly hitting your car. Strong winds can even blow people from under a bridge into the open.
If you are in your car during a storm, safely pull-off to the side of the road, out of the traffic lanes, turn on your hazard lights and keep your windows rolled up. Do not touch any metal or anything else that may conduct electricity.
If you are in your car during a tornado warning, however, the situation to find a safe spot may become more urgent. If you have time, find a sturdy shelter, if not, find a low spot, lay flat on the ground and far away from the road to avoid flying cars. Get as far away from windows and windshields as possible. Keep your seat belt on and crouch down in your car while covering your head.
To read more about storm safety in cars, check out the below links: