The Toyota Prius v is the only midsize car out of 31 evaluated to earn a "good" rating in the Insurance Institute for Highway Safety's (IIHS) first-ever headlight ratings.

The best available headlights on 11 cars earn an acceptable rating, while nine achieve only a marginal rating. Ten of the vehicles can't be purchased with anything other than poor-rated headlights.

"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," says David Zuby, IIHS executive vice president and chief research officer.

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, the Institute says improved headlights have the potential to bring about substantial reductions in fatalities.

About half of traffic deaths occur in dark, dawn or dusk conditions. Image credit: Pixabay.About half of traffic deaths occur in dark, dawn or dusk conditions. Image credit: Pixabay.In many vehicles, high-intensity discharge (HID) or LED lamps have replaced halogen headlights. Curve-adaptive headlights, which swivel according to steering input, are also becoming more widespread. Research has shown advantages for the new headlight types, but they don't guarantee good performance, IIHS says.

The Institute says that its rating system rewards systems that produce ample illumination without excessive glare for drivers of oncoming vehicles. Headlights are evaluated on the track after dark at the IIHS Vehicle Research Center. A special device measures the light from both low beams and high beams as the vehicle is driven on five different approaches: traveling straight, a sharp left curve, a sharp right curve, a gradual left curve and a gradual right curve.

After a vehicle is tested on the track, IIHS engineers compare its visibility and glare measurements to those of a hypothetical ideal headlight system and use a scheme of demerits to determine the rating. Vehicles equipped with high-beam assist, which automatically switches between high beams and low beams depending on the presence of other vehicles, may earn back some points taken off for less-than-ideal low-beam visibility.

"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," says 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."

Among the headlight systems earning a poor rating, the halogen lights on the BMW 3 series are the worst, according to IIHS. A driver with those headlights would have to be traveling 35 mph or slower to stop in time for an obstacle in the travel lane. A better choice for the same car is an LED curve-adaptive system with high-beam assist, a combination that rates "marginal," IIHS says.

However, curve-adaptive systems don't always lead to better ratings. The Cadillac ATS, Kia Optima and Mercedes-Benz C-Class all earn poor ratings even when equipped with adaptive low and high beams.

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