Drivers in the Indianapolis area last October were undoubtedly shocked to see a fireball explode on one of the area’s busy interstates. The explosion was caused by a propane-laden tanker truck that rolled over and struck a guardrail. The tractor separated from the tank trailer and took out a pillar of a bridge abutment. A trailing motorist that drove through the fireball had the fenders melted off her car, but was otherwise unhurt. Fortunately, there were no fatalities.

Over 100,000 loaded tanker trucks take to the nation’s highways daily; annually, an average of 1,265 are involved in truck rollover accidents. Heavy tractor-trailer truck traffic and the frequency of rollovers have not escaped the attention of the National Transportation Safety Board (NTSB). The NTSB is presently considering whether rollover accidents can be prevented by electronic stability systems. Automobiles are presently equipped with sensors that warn the driver if the vehicle is susceptible to a rollover. Computers then automatically apply the vehicles brakes until the vehicle regains its equilibrium.

The “Free Surface Effect”

Tanker trucks are particularly vulnerable to rollover because of their liquid cargo. When the tank is partially full, its liquid cargo is free to slosh about – which creates the “free surface effect.” The free surface effect, combined with the fact that a tanker’s center of gravity is located well above the trailer’s wheels, can cause overturns when tanker trucks negotiate roadway curves.

Most new tanker trailers are equipped with electronic stability systems. Unfortunately, many tractors that pull them, as well as older trailers, do not have this equipment. Currently, the trucking industry believes that it is impractical to retrofit tractors and older trailers due to the considerable expense.

The NTSB held hearings in early August 2010 to evaluate the tanker explosion last year. The Board hopes to release some of the conclusions drawn from this hearing and similar, prior hearings, later this year.