Many people think an overpass, with its reinforced cement construction, would be a good option for shelter when a tornado is bearing down on you. Actually, it is one of the last places you would want to be.
On April 26th, 1991 a camera crew took shelter from a tornado under an overpass in southern Kansas and walked away unharmed. Their dramatic experience became national news and lent widespread credibility to the misconception that an overpass is a safe place to be during a tornado. But a hard lesson was learned on May 3rd, 1999, when an F-5 tornado, tracked across Oklahoma, intersecting with 3 interstate overpasses along its path. At each of these overpass sites, there were people taking cover, and at each site, someone died. Many more suffered horrific injuries.
There are three main reasons an overpass should not be considered a safe place to take shelter. First, the slope of the overpass can actually serve to funnel debris toward the corners where people usually take cover.
Second, wind speeds actually increase when forced into a narrow passage, so the tornado may be even stronger under a bridge or overpass. Finally, tornadic winds are often stronger just above ground level, so it is imperative that you be as low as possible.
This means the best course of action if you are on the roads and cannot escape a tornado is to abandon your vehicle, get into a ditch, and cover your head.