Translating test results into ratings

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. In this system, the low beams are weighted more heavily than the high beams because they are used more often. The readings on the straightaway are weighted more heavily than those on curves because more crashes occur on straight sections of road.

A vehicle with no demerits doesn’t exceed the low-beam glare threshold on any approach and provides illumination to at least 5 lux over specified distances, ranging from nearly 200 feet for low beams on a sharp curve to nearly 500 feet for high beams on the straightaway.

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

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

One good rating out of 82

Most of the vehicles included in this release have multiple headlight ratings, so there are a total of 82 headlight ratings for 2016 models even though there are only 31 vehicles. IIHS is rating every possible headlight combination as it becomes available from dealers.

The Prius v earns a good rating when equipped with LED lights and high-beam assist. The low beams cover a distance of nearly 400 feet in the right lane while traveling straight and about 160-210 feet on the curves. The high beams extend more than 500 feet on the straightaway and about 180-220 feet on the curves. Neither the low beams nor the high beams are curve adaptive. The car’s performance on curves might be improved if that feature was added.

Consumers who want the good headlights on the Prius v need to buy 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 earns 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,” Brumbelow says. “In contrast, someone with the halogen lights would need to drive 20 mph slower in order to avoid a crash.”

Among the 44 headlight systems earning a poor rating, the halogen lights on the BMW 3 series are the worst. The low beams illuminate only about 130 feet on the right side of the straightaway. A driver with those headlights would have to be going 35 mph or slower to stop in time for an obstacle in the travel lane. The system’s high beams don’t reach 400 feet. A better choice for the same car is an LED curve-adaptive system with high-beam assist, a combination that rates marginal.

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

In the case of the Optima, a big problem is glare. Its curve-adaptive system provides better visibility than its nonadaptive lights, but produces excessive glare for oncoming vehicles on all five low-beam approaches.

One of the best headlight systems evaluated has none of the new technology. The basic halogen lights on the Honda Accord sedan earn an acceptable rating, while an LED system with high-beam assist available on the Accord earns only a marginal.