It takes more than speed, physicist says

Like any theoretical physicist worth the parchment of his Ph.D., Alex Kuznetsov pondered a question in 1996 and started scribbling math formulas on the back of an envelope to find an answer.

In this case, he wondered whether the old highway safety warning, "speed kills," was an absolute truth.

Not exactly, his formulas indicated. The maxim was true enough on an empty highway, where the risk of injury or death increased exponentially solely because of rising velocity.

But few people drive down an empty road. So Kuznetsov, then a postdoctoral researcher at Ohio State, added traffic to the equation.

Heavy or light, the presence of other cars also heightens a driver's risk, along with the distance and time spent on the highway, he reasoned. When Kuznetsov put pencil to this modified equation, he found traffic density to be the most important factor in accident risk -- not speed.

Drivers who travel at the same speed as the surrounding traffic decrease their risk even as the velocity of the pack increases. The reason: They pass fewer cars and aren't passed as often.

Those who travel above or below the speed of "the flow" substantially increase their risk -- slowpoke or speed demon. Kuznetsov says both types are "suicidal."

Kuznetsov's scribblings weren't intended as a serious scientific study. But his mathematical proofs illustrate one of the three major theories of the relationship of speed to auto wrecks. They also buttress the view of groups such as the National Motorists Association that say speed limits need to be easily adjusted to reflect road conditions and the prevailing speed of the majority of drivers.

"Your chances of getting hurt is really a matter of how fast you're traveling in relationship to other cars and how fast they're going -- it isn't just speed alone," said Kuznetsov, now a Wall Street trader with Credit Suisse in New York.