ITHACA, N.Y. -- Beyond the tragic deaths on Sept. 11, 2001, there were more lives lost as an indirect result of the terrorist attacks. Using airline passenger and highway statistics, nearly 1,200 more people died in the months subsequent to the attacks when they switched their travel plans from flying to driving, according to Cornell University economists.

Between Oct. 1 and Dec. 31, 2001, the economists found 725 driving fatalities linked to travelers changing their plans from air to less-safe car travel, which the economists have dubbed "The Sept. 11 Effect." An additional 400 to 500 people died in the first quarter of 2002 resulting from the Sept. 11 effect. (The researchers excluded September 2001 data.)

The economists -- Garrick Blalock and Daniel H. Simon, Cornell assistant professors of applied economics and management, and Vrinda Kadiyali, associate professor at Cornell's Johnson Graduate School of Management -- gathered National Highway Traffic Safety Administration data from 1994 to 2003 on road fatalities to isolate a pure Sept. 11 effect over other possible road fatality causes. The researchers removed biases for gasoline prices, weather, road conditions and commercial driving fatalities. The researchers found there was no increase in commercial road fatalities following the Sept. 11 tragedy. Over time, for automobile passengers, the Sept. 11 effect diminished as tighter airport security measures decreased the fear of flying.

"The existence of the Sept. 11 Effect is consistent with theoretical models in behavior economics … our results show that the public response to terrorist threats can create unintentional consequences that rival the attacks themselves in their severity," write the authors in their paper, "The Impact of Sept. 11 on Road Fatalities: The Other Lives Lost to Terrorism."

The researchers explained two reasons why the Sept. 11 effect occurs. The first is simply a fear of flying, where travelers place too much weight on the possibility of another terrorist attack.

The second reason is the inconvenience of flying. Air travelers experienced many changes in airport security procedures, such as arriving at airports two hours before takeoff for domestic flights and negotiating through numerous checkpoints. Given the deaths arising from increased automobile travel, the economists called the airport hassle "a regrettable irony given the inconvenience resulted from measures taken to prevent (air travel) deaths from terrorism."