A new national traffic safety study concludes that car accidents in urban areas can be dramatically reduced by relatively simple, inexpensive fixes to the roadway.

The study by the Insurance Institute for Highway Safety found that simply adding a green-arrow left-turn signal to an intersection, extending a merge lane or moving a bus stop by a few hundred feet could trim the number of accidents at dangerous intersections.

Working with engineers from the Virginia Department of Transportation, the safety organization chose six intersections in the Tysons Corner area for which police reports revealed a pattern in the type of crashes -- such as left-turning vehicles being hit by oncoming traffic or rear-end collisions.

VDOT made fixes to the intersections, which then were monitored for two to 41/2 years. The type of crashes addressed by the changes decreased at all six intersections, some dramatically, although traffic increased during the period.

At Leesburg Pike (Route 7) and the entrance to the Tysons Corner Center, for example, there had been several crashes in which vehicles turning left were hit by oncoming vehicles. VDOT converted the left lane to what it calls a "protected lane," in which drivers can turn left only on a green arrow, with oncoming traffic stopped.

After the change, the number of such collisions fell from nearly nine a year to none in two years.

"To see reductions in the magnitude that we see in this study is very gratifying," said Richard A. Retting, a senior transportation engineer at the Arlington-based institute that directed the study.

The Washington area ranked 11th nationally in the number of fatal car crashes, according to the study by the institute, which is a nonprofit research and communications organization funded by auto insurers. The study said that while most fatal auto crashes happen on the nation's rural roads, about 8,000 deaths and more than 1 million injuries occur each year on streets in and around the nation's cities. In 2003, the most recent year for which data were available, 490 people in the Washington area were killed in urban crashes. Countless hours were lost in traffic jams that resulted from accident scene cleanups and investigations.

Retting said the study was looking for "patterns of preventable crashes." He said this was the first time that researchers set out to determine whether specific types of crashes -- ones that could be prevented easily -- turned up frequently in the crash statistics for certain intersections.

For instance, at Leesburg Pike and Lewinsville Road, where left-turn oncoming crashes were common, engineers changed the left-turn lanes so that drivers were permitted to turn left only on a green arrow when oncoming traffic was stopped. The number of left-turn crashes went from about five a year to none in the 21/2 years after the change.

"In some cases, short-term, less-costly solutions can provide immediate relief with minimal or moderate effort," Retting said. "A lot of these projects can be implemented in-house with state or county employees."

Constance S. Sorrell, VDOT's chief of system operations, said the study is "going to be tremendously helpful." She said the state agency has plans to adopt the study's model to help reduce crashes at other problem spots. "We are definitely going to be using this," she said.