This is an overly simplified, overhead schematic of what happened in the Kansas Turnpike video from 26 April 1991. The video was shot from under the overpass looking southwest, which is to the upper left in the schematic. The tornado moved northeast, roughly parallel to, and southeast of the highway. As this video has been shown over the years, it has been promoted in the light that these people ‘miraculously’ survived this tornado by ‘wisely’ seeking shelter under the bridge. Frankly, nothing could be farther from the truth. A series of circumstances, including the overpass construction scheme, the strength and relative position of the tornado, a bit of luck, and the fact that this happened in a very rural area all combined to make this a very deceiving video.
First, not only was this a relatively weak tornado, especially in comparison to the Oklahoma City tornado of 3 May 1999, but the people under the bridge were only on the fringes of its circulation. It should be noted, however, that despite its "official" F1 rating at the time it passed the overpass, there is evidence in the video that the tornado was actually stronger than that. Careful observers will note that as the tornado is approaching the bridge where the people are seeking shelter, a minivan several hundred yards down the highway is rolled several times by the tornadic winds. Also, many other vehicles, including large semi-trailer trucks, were rolled over and severely damaged by the tornado as it moved northeast, roughly parallel to the highway.
Second, the unique construction of this overpass allowed the people to crawl well up into a small area on a ledge underneath the bridge, and also provided something to hang onto during the period of strongest wind, thereby greatly decreasing their chances of being blown out of their "crawl space".
Finally, the fact that this occurred in a rather remote area of rural Kansas meant there wasn't very much flying debris within the tornado's circulation at the time it interacted with the overpass. Had this happened in an urban area, or if it had been a strong or violent tornado, the people would have been exposed to more flying debris that probably would have caused serious injuries. Why? Note in the schematic (this is also obvious in the actual video for those who have seen it) how the wind rapidly changes direction as the vortex approaches and then passes the bridge: at first, the wind is blowing from the east and southeast.
Thus, the people huddled underneath the southeast side of the bridge are relatively protected from the wind by the bridge embankment. However, as the tornado passes, the wind rapidly turns to the north, then to the northwest, then to the west, and finally to the southwest, all in the matter of a few seconds! In an urban setting, especially if the tornado had been more intense, at some time during the event, the people would have been completely exposed to the strong winds and flying debris.