Automakers seeking to improve safety with high-tech headlights might literally be leaving their customers in the dark.

A new study by the Insurance Institute for Highway Safety rates the headlights in just one of 31 new midsize cars as good: Toyota Motor Corp.’s Prius V. One-third of the models were deemed “poor,” the lowest rating on a four-point scale, including two from Mercedes-Benz cars made by Daimler AG. BMW AG’s 3 Series with halogen lamps had the lowest-scoring of all tested lighting systems.

The institute’s first headlamp study found that automakers rushing to roll out new technologies such as high-intensity discharge or light-emitting diode lamps, or systems that swivel according to the direction of the wheels aren’t following through to ensure the light will illuminate the road. In the case of the BMW, a driver might not have enough time to react to avoid a crash at speeds greater than 35 mph, IIHS said.

“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, executive vice president and chief research officer of the Arlington, Va.-based research group.

Government standards for headlights allow too much variation in the amount of illumination to adequately measure safety in on-road driving, the institute said. The group devised a new set of night-time road tests at its track in rural Virginia based on an analysis of real-world crashes. Cars were measured by how much light they projected while going straight, in gradual curves and in straight curves.

The BMW 3 series illuminated only 130 feet on the right side of a straight stretch of road in the IIHS tests. Its high beams didn’t reach 400 feet. The Toyota Prius V lit up nearly 400 feet on low beams and more than 500 feet on high beams.

New technology didn’t necessarily translate into better results. The Honda Accord earned an “acceptable” rating with its basic halogen lights. An optional LED system didn’t test as well. Curve-adaptive systems offered by General Motors Co.’s Cadillac, Kia Motors Corp. and Mercedes-Benz didn’t spare those models a “poor” rating.

Mercedes-Benz was “greatly surprised” by the IIHS rankings and is analyzing the results, company spokesman Rob Moran said in an e-mailed statement. “Consistent with our long-standing tradition of safety and innovation, Mercedes-Benz vehicles have been distinguished with top performance in lighting tests for many years,” Moran said. 

Kenn Sparks, a spokesman for BMW, didn’t immediately respond to an e-mail seeking comment.

Some of the problems amounted to poor vertical aim, IIHS said. That’s a problem for consumers who don’t know how to change the angle of the headlights, or who may not even be aware that the lighting isn’t doing what’s it’s supposed to do.

By Jeff Plungis