These two events illustrate clearly the danger created by blocking traffic flow on highways beneath and in the vicinity of severe thunderstorms, trapping people in life-threatening situations. One of the co-authors was driving one of the vehicles for project V.O.R.T.E.X. on I-35 near Gainesville TX on 26 April 1994. Numerous vehicles were stopped under an overpass, with many people up on the overpass embankment under the bridge. As additional drivers encountered the bridge, many of them decided to stop, simply parking their vehicles directly in the traffic lanes because there was no more room to pull over onto the shoulder. In the process, that highway was nearly blocked at a location in the path of a tornadic storm.
Another, more glaring, example was documented in central Texas during the Jarrell TX tornado event on 27 May 1997. The photo in the slide was taken looking northwest. The beginning of the Jarrell, TX, F5 intensity tornado is seen as the slight cone-shaped lowering in the cloud base. As this tornado was developing, vehicles can be seen in the foreground parked near the overpass on I-35, apparently to seek shelter from the storm under the bridge. In addition to the traffic jam in the photo, the Texas State Patrol created an even bigger one when they stopped both northbound and southbound traffic on I-35 in anticipation of the tornado moving southeastward, and crossing the highway. Although the intentions of this action were certainly good, the huge chain of vehicles at a standstill created by this action could have resulted in a major disaster had the tornado deviated from its south-southwestward movement and moved southeastward. Nearly 5 miles of stopped traffic created a would-be sitting target for this tornado!
This was a very unique case, where the tornado moved south-southwestward and paralleled the highway. For its entire life-span, the tornado remained approximately the same distance west of the interstate. For the hundreds of people trapped in traffic jams on I-35 just east of the path, this was certainly a fortunate occurrence! However, it also shows how blocked traffic on highways around severe thunderstorms can lead to disaster.