Fatal traffic accidents increase sharply in Israel on the third day after a terrorist attack, and researchers are searching for an explanation why.

An 18-month study found no notable change in fatal accidents on the day of an attack or the first or second day after. But fatalities jumped 35 percent on the third day, or a startling 69 percent after a terror attack that killed 10 people or more.

"Why traffic fatalities increase on the third day after a terror attack remains a puzzle," Guy Stecklov of the Hebrew University in Jerusalem (search) and Joshua R. Goldstein of Princeton University (search) write in Monday's issue of Proceedings of the National Academy of Sciences.

They studied terrorist incidents in Israel from Jan. 1, 2001, to June 22, 2002. During that period there were 63 attacks that killed at least one person.

Traffic volume, they found, was stable on the day after an attack, then declined for four or five days. Minor accidents declined by 6 percent decline on the day after an attack. Police suggested that might have been because an increased police presence immediately after an attack encouraged more caution in drivers, the researchers said.

For fatal accidents, there was no change on the first two days after an incident, then the increase on the third day.

Overall there were 689 traffic fatalities in Israel during the almost 18-month period, and based on a 35 percent increase on the third day after a terror incident, 28 of the deaths may be attributed to the terror incidents.

"Some fraction of the increase in traffic fatalities after terror attacks may be attributable to covert suicides and/or increased aggression on the road," Stecklov and Goldstein reported.

They note there are records of a similar third-day increase in traffic fatalities following well-publicized suicides and an increase in homicides after major boxing matches.

Another possible explanation, they say, may be that this time period coincides with people trying to return to their normal routines before they are psychologically ready.

Or, they add, the three-day lag might reflect a counterreaction to the collective bonding that occurs after a terrorist incident.

If increased stress is responsible for the increase in traffic fatalities, Goldstein and Stecklov conclude, "This stress may also have long-term consequences for stress-related illnesses such as heart attacks."