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Headlights on a third of U.S. midsize car models do a poor job, and only one, the Toyota Prius v, performed well enough with an optional upgrade to earn a good rating from the Insurance Institute for Highway Safety in results released on Wednesday.

While they currently meet U.S. government standards, better headlights could reduce U.S. accident deaths and improve safety, the Arlington, Virginia-based group said.

In its first ratings of vehicle headlights, the insurance group tested midsize cars to see how well they illuminate the road and whether they create excessive glare for oncoming vehicles. The group's ratings carry weight with consumers so automakers work to win high marks in its tests.

Government rules allow for significant variation in the amount of illumination that headlights provide in actual on-road driving, IIHS said.

Of 31 midsize models it tested, 11 earned an acceptable rating, nine were rated marginal and 10 were poor. The Prius v was rated good when equipped with optional LED headlights and a feature that turns off high beams when there is an oncoming car.

Vehicles getting acceptable ratings include the Audi A3, Honda Accord and Nissan Maxima. Vehicles with poor ratings include the Kia Optima, Chevrolet Malibu and Volkswagen Passat.

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Many luxury vehicles have poor-rated headlights, and many cars only get higher ratings with option packages. For example, the base Prius v, which has halogen lights, gets a poor rating.

Some headlights are inadequate because they are not aimed properly at the factory. "Many headlight problems could be fixed with better aim," said IIHS engineer Matthew Brumbelow.

The U.S. National Highway Traffic Safety Administration said in a statement Tuesday that its revised new car assessment program will give incentives for automakers to improve headlight performance.

"NHTSA is committed to promoting a higher standard of safety, including in headlighting systems," spokesman Bryan Thomas said.

A spokesman for the Alliance for Automobile Manufacturers, a trade group representing Toyota Motor Corp, General Motors Co, Ford Motor Co, Volkswagen AG and others, declined to comment.

A 2007 NHTSA report found that 49 percent of passenger traffic deaths occur at night, while 25 percent of driving occurs at night. It's not clear how much the fatality rate is related to lighting, versus driver behavior at night like speeding or intoxication.