It's understandable why motorists would seek out underpasses when a tornado threatens. There's not much else that even remotely resembles shelter when you're driving across the rural plains. But the meteorologists have spoken: Don't do it!
This tornado survival myth likely started in 1991 when a television news crew, with cameras rolling, took cover under an overpass during a very weak tornado. They survived, and the resulting footage was widely circulated. It created a surprisingly persistent misconception that was still alive and well in 2013 when the movie "Man of Steel" depicted Clark Kent's dad taking shelter from a tornado under — you guessed it — an overpass.
So what's the big deal? For one, a tornado's winds intensify with height, meaning that even the short climb up under an overpass could put you at greater risk of injury from flying debris. What's worse, the overpass's design results in a wind tunnel effect, meaning gusts actually increase in speed as they're funneled through the narrow space under the end of the bridge. These dangers were tragically demonstrated in 1999 when a powerful tornado killed three people and severely injured others who sheltered under overpasses in Oklahoma.
The best thing to do if you see a tornado while driving is to get out of the way and find the nearest shelter — but only if traffic and road options allow. If this isn't possible, take cover in a ditch or culvert and cover your head with your hands.