For men and women who have fought in the country’s wars in Iraq and Afghanistan, death behind the wheel is becoming another lethal aftereffect of combat.

After they leave military service, veterans of the two wars have a 75 percent higher rate of fatal motor vehicle accidents than do civilians. Troops still in uniform have a higher risk of crashing their cars in the months immediately after returning from deployment than in the months immediately before. People who have had multiple tours to combat zones are at highest risk for traffic accidents.

The phenomenon has been revealed by multiple pieces of evidence – research as well as observations of soldiers, veterans and counselors.

The most common explanation is that soldiers bring back driving habits that were lifesaving in war zones but are dangerous on America’s roads. They include racing through intersections, straddling lanes, swerving on bridges and, for some, not wearing seat belts because they hinder a rapid escape.

That’s probably not the whole story, however. Post-traumatic stress disorder, suffered by thousands of veterans, increases aggressive driving. Drunken driving and thrill-seeking are also more common after combat, according to a few studies and the testimony of many veterans.

If further research supports the observations, motor vehicle crashes will join suicide and interpersonal violence as a fatal, if indirect, consequence of the war on terrorism.

Motor vehicle crashes have long been a serious problem in the military armed services. From 1999 through 2012, a period spanning peacetime and the two wars, as many active-duty military personnel died in noncombat motor vehicle crashes both on and off duty (4,423) as were killed in the Iraq war (4,409).

LEADING NON-BATTLE INJURY

“Before suicides became the leading cause of non-battle injuries, motor vehicle injuries were,” said Bruce Jones, a physician and epidemiologist who heads the Army’s injury prevention program at Aberdeen Proving Ground, in Maryland.

War, however, worsens the problem.

Men who served in the wars in Iraq or Afghanistan have a 76 percent higher rate of dying in vehicle crashes, and women a 43 percent higher rate, than similar people in the general population, according to an unpublished study by Han Kang, an epidemiologist with the Department of Veterans Affairs. The same phenomenon was seen in Gulf War veterans and took five years to dissipate.

Fatal motorcycle crashes in particular spiked during the wars. They accounted for 14 percent of military traffic deaths in 2001, but 38 percent in 2008. The absolute rate of motorcycle deaths also tripled over that period.

“A lot of people come home and buy a motorcycle to have that adrenaline rush again,” said Steven Acheson, 27, a former forward observer in the Army and an engineering student in Wisconsin.

He spent time at Fort Stewart in Georgia, where the post put vehicles from fatal crashes on display as a form of warning. “There was once six or seven completely mangled motorcycles out in front of the gate,” Acheson recalled.

One of the best pieces of evidence that combat raises a person’s risk for car crashes comes from an in-house study by USAA, an insurance company in San Antonio, Texas, that covers thousands of military personnel.

The company offers a reduced-price insurance premium if vehicles are stored securely during deployment. Almost all of the company’s active-duty customers opt for it. As a consequence, USAA has before-and-after records for 171,000 deployments to Iraq and Afghanistan.

Troops had more at-fault accidents in the six months after their return from deployment than in the six months before they left. The increase was highest for people in the Army (23 percent) and in the enlisted ranks (22 percent).

Tellingly, there was a “dose-response relationship” between deployment and risk. Soldiers with three deployments had 36 percent more accidents, compared with 27 percent more in the twice-deployed and 12 percent in people deployed only once.

NOT JUST COMBAT BEHAVIOR

But the problem isn’t just a carryover of habits.

One-quarter of the Iraq and Afghanistan veterans enrolled in a 60-day residential treatment program for PTSD in California said they drove after drinking. One-fifth said they used seat belts “less than sometimes,” in part because they get in the way of a rapid escape from a vehicle.

“Failure to adapt the unique combat driving behaviors used in the current conflicts cannot be the only explanation for deployment-related risky driving behavior and excess . . . mortality,” wrote Mark Zamorski, a Canadian military physician, and Amanda Kelley, a civilian U.S. Army psychologist, in a report to NATO on the subject.

“All of the likely mechanisms . . . could be mediated by distress or mental disorders,” they wrote.

Todd Nelson was an Army logistician riding in the front seat of a Toyota Land Cruiser in Kabul in August 2007 when a car in an adjoining lane blew up. He lost his right eye, broke both jaws and had burns on 18 percent of his body surface. He’s had 43 operations under general anesthesia.

When he was finally well enough to get back on the road, he drove for several months before his wife “made a comment about how aggressive I was being. She said, ‘You’re scaring me,’” Nelson, 40, recalled recently.

He sped. He swerved whenever he saw a vehicle with a low-hanging rear end suggesting heavy load. “Hey, I drove much worse than this over there and nothing happened,” he says he told himself.

BREAKING THE HABIT

Nelson, who works in the recruiting department of USAA, broke the habit on his own.

He timed how long it took him to drive to college each day in San Antonio and proved to himself that going the speed limit added little time to the trip. He made a game of counting the number of cars he would let merge on the way in and the way back. After a month, he said, he was back to driving pretty normally. He now goes out of his way to talk about it to other veterans.

The military is beginning to pay attention to the risks facing veterans who resume driving when they get home.

The Army gives out a brochure with tips on how to increase self-control; one is to tape a drawing by one’s child to the dashboard.