Though there has been some research into road rage among veterans, therapists and psychologists have only recently begun to view traumatic brain injuries and P.T.S.D. as factors in prolonging driving problems, probably by causing people to perceive threats where none exist — such as in tunnels, overpasses, construction crews or roadside debris.

“In an ambiguous situation, they are more likely to see hostile intent,” said Eric Kuhn , a psychologist with the Palo Alto Veterans Affairs Health Care System, who has studied driving problems. He said his research found that veterans who report more severe P.T.S.D. symptoms also tend to report being more aggressive drivers.

Experts note that driving problems are not always the result of the disorder. In some cases, returning troops may be reflexively applying driving techniques taught in Iraq during the height of the insurgency — for example, speeding up at intersections to avoid gunfire or scanning the roadside for danger instead of watching the road ahead.

In a study of Minnesota National Guard soldiers who returned from Iraq in 2007, Dr. Stern and fellow researchers found that a quarter reported driving through a stop sign and nearly a third said they had been told they drove dangerously in the months immediately after their tours. Both results were higher than the answers reported by National Guard cadets who had not been deployed.

Though driving problems seemed to decrease the longer the troops were home, they did not always vanish. Dr. Stern found that many Guard members remained anxious about certain roadway situations, including night driving or passing unexpected things.

“Those are things they associated with threats they saw in combat,” she said.

Ms. Max, a grandmother of four, was deployed at the age of 60 to Iraq, where one of her jobs was to carry large sums of cash to Iraqi reconstruction projects outside fortified American bases. She said she learned to be hypervigilant on those trips.

Upon returning to California, she struggled with P.T.S.D. and took time off from her nursing job. She also noticed feeling nervous for the first time in her life about driving — a major problem because she had to drive to visit patients.