Highway Overpasses as Shelters

Much of the following material is taken from "Highway Overpasses as Tornado Shelters", by Daniel J. Miller, National Weather Service Forecast Office-Norman, and Charles A. Doswell III, Harold E. Brooks, Gregory J. Stumpf, and Erik Rasmussen of the National Severe Storms Laboratory.

Over the past 20 years, public perception that highway overpasses offer sound shelter from tornado winds has increased substantially, particularly as result of a widely distributed video from April 26, 1991 on the Kansas Turnpike. However, it appears that highway overpasses offer, at best, questionable shelter not only from tornadoes, but severe storms in general: three people in Oklahoma lost their lives while seeking shelter near or under overpass bridges, including one here in Moore at the Shields Blvd. bridge over I-35.

It is our contention that highway overpasses are inadequate tornado sheltering locations for the following meteorological reasons.

• First, ALL tornadoes have some amount of debris within their near-surface flow. In the case of a strong or violent tornado, much more debris would be present, traveling at much higher speeds, especially when debris from man-made structures is involved. In strong and violent tornadoes, typically harmless everyday items such as shingles, boards, pop cans, dishes (or pieces thereof) become dangerous missiles and are responsible for most tornado casualties.

• Second, by climbing up under an overpass, people will be exposed to higher wind speeds and more flying debris.

• Third, the narrow passage underneath an overpass might cause an increase in the wind speed under the bridge. The extent to which this is true, and the circumstances under which it could happen are not known, but this is at least a possibility.

• Fourth, most overpasses don't have girders or support beams for handholds or small ledges into which to crawl.

• Finally, if an overpass is directly in the path of a tornado, the wind will change direction nearly 180 degrees as the vortex passes. Thus, if one side of the overpass was protected from the highest wind speeds as the tornado approached, that same side of the bridge will be completely exposed to the wind and flying debris as the tornado moves away and vice-versa.

Taking shelter under a highway overpass is to become a stationary target for flying debris, with a substantial risk of being blown out and carried by the tornado winds. Safety in such a location is merely an illusion. Click on the graphic to see an animated explanation of this, courtesy of USA Today (graphic ©copyright 1999 USA Today, a division of Gannett Co. Inc.)

Other extremely important reasons why seeking shelter from tornadoes and severe storms underneath overpasses are the non-weather issues.

When a traffic jam results from blocked free flow of traffic because a multitude of vehicles parked near an overpass, this forces many others into the same desperate situation, unnecessarily. This happened at Shields Boulevard on May 3, and has also happened in several other severe storm situations. Once people start to seek shelter under overpasses, a potentially lethal series of consequences begins:

• First, the free flow of traffic is blocked.

• Then other potential escape routes are blocked by this stalled traffic, especially on limited-access roads such as interstate highways.

• There is also the potential for a major traffic accident in the form of a chain reaction during low visibility and/or nighttime events, where oncoming traffic might not be able to see the stopped vehicles in time to prevent collisions, adding to the blockage.

• A tornado hitting such an involuntary assemblage of vehicles will leave a deposit of rubble and vehicles, rendering the road impassible for several hours.

• The blocked road would prevent emergency vehicles from gaining access to the affected area.

• We experienced every one of these consequences in Moore on May 3rd!

Despite all of the incredibly valid meteorological reasons why overpasses are very bad places to be, the traffic blockage aspect may be just as important of a reason why we need to minimize this practice! We must avoid forcing others into the same desperate situation by blocking the road!

Finally, there were also a few people on May 3 who ACTUALLY LEFT THEIR HOMES TO DRIVE TO THE NEAREST HIGHWAY OVERPASS TO SEEK SHELTER! As has been stated before, the current guidelines for people in buildings are extremely effective when practiced properly, and in almost no instance should one leave a soundly constructed building that has an interior room in a tornado situation!