The Toyota Prius V is the only midsize car out of 31 tested to draw the highest rating of “good” in the first-ever headlights evaluations by the Insurance Institute for Highway Safety.

The best available headlights on 11 cars received an “acceptable” rating, while nine drew a “marginal” rating, IIHS said. Ten of the vehicles can’t be purchased with anything other than “poor”-rated headlights.

(Scroll down for headlight evaluation results. To watch a video report about the evaluations, click on the photo or link below the headline.)

The Prius V earned a “good” rating when equipped with LED lights and high-beam assist. To get those headlights, buyers must purchase the advanced technology package, which is only available on the highest trim level. When equipped with regular halogen lights and without high-beam assist, the Prius V earned a “poor” rating.

“The Prius V's LED low beams should give a driver traveling straight at 70 mph enough time to identify an obstacle on the right side of the road, where the light is best, and brake to a stop,” explained Matthew Brumbelow, an IIHS senior research engineer. “In contrast, someone with the halogen lights would need to drive 20 mph slower in order to avoid a crash.”

Surprisingly, many of the “poor”-rated headlights belong to luxury vehicles.

“If you're having trouble seeing behind the wheel at night, it could very well be your headlights and not your eyes that are to blame,” said David Zuby, IIHS executive vice president and chief research officer.

The ability to see the road ahead — as well as spot pedestrians, bicyclists or obstacles — is obviously essential for drivers. However, government standards for headlights, based on laboratory tests, allow huge variation in the amount of illumination that headlights provide in actual on-road driving, according to IIHS. With about half of traffic deaths occurring either in the dark or in dawn or dusk conditions, improved headlights have the potential to substantially reduce road fatalities.

In many vehicles, high-intensity discharge (HID) or LED lamps have replaced halogen ones. Curve-adaptive headlights, which swivel according to steering input, are also becoming more common.

Research has shown advantages for the new headlight types, IIHS said, but they don't guarantee high-quality performance.

“The Institute's headlight rating system doesn't favor one lighting technology over the other, but simply rewards systems that produce ample illumination without excessive glare for drivers of oncoming vehicles,” IIHS said in a news summary of the evaluations.

Researchers evaluated the headlights after dark at the IIHS Vehicle Research Center track in Virginia. A special device measured the light from both low beams and high beams as each vehicle was driven on five different approaches: traveling straight, a sharp left curve, a sharp right curve, a gradual left curve and a gradual right curve.

Evaluators also measured glare for oncoming vehicles from low beams in each scenario to make sure the glare wasn’t excessive.

Although the vertical aim of headlights can be adjusted on most vehicles, IIHS didn’t change headlight aim because few vehicle owners ever do and some manufacturers advise consumers not to.

After a vehicle was tested on the track, IIHS engineers compared its visibility and glare measurements to those of a hypothetical ideal headlight system and used a scheme of demerits to determine the rating. Results for low beams were weighted more heavily than high beams because drivers use them more often. The readings on the straightaway were weighted more heavily than those on the curves because more crashes occur on straight sections of road, IIHS noted.

Vehicles equipped with high-beam assist, which automatically switches between high beams and low beams depending on the presence of other vehicles, were able to earn back some points taken off for less-than-ideal low beam visibility. This credit was given only for approaches on which the glare threshold wasn’t exceeded and on which the high beams provided additional visibility compared with the low beams.

A vehicle with excessive glare on any of the approaches couldn’t earn a rating above “marginal.”

Since vehicles can be equipped with different headlights, there are a total of 82 headlight ratings for 2016 models even though there are only 31 vehicles. IIHS plans to rate every possible headlight combination as it becomes available from dealers.