Headlights

Nighttime visibility is critical to highway safety. About half of traffic deaths occur either in the dark or at dawn or dusk. Better headlights lead to fewer nighttime crashes.

Not all headlights do their job well. IIHS evaluations show that the on-road illumination provided by vehicle lights varies widely.

Good headlights shouldn't cause excessive glare. Properly aimed low beams light up the road ahead without temporarily blinding drivers of oncoming vehicles. IIHS headlight ratings take glare into account.

Some new types of lighting technology are helping. High-beam assist, curve-adaptive headlights and, more recently, adaptive driving beams all improve on-road visibility.

IIHS ratings show that visibility provided by headlights varies widely.

The Institute released its first headlight ratings in 2016. Out of more than 80 headlight systems available on the 31 model year 2016 midsize cars that were evaluated, only one system received a good rating.

Forty-four percent of headlight systems tested on model year 2024 vehicles earn a good rating. About 23% of the systems tested are rated marginal or poor because of inadequate visibility, excessive glare from low beams for oncoming drivers, or both.

Vehicles with good ratings for visibility in the IIHS headlight test have 19% fewer nighttime single-vehicle crashes and 23% fewer nighttime pedestrian crashes than vehicles with poor-rated headlights (Brumbelow, 2022). Acceptable and marginal headlights are associated with reductions of about 15% and 10% in nighttime single-vehicle crashes. These numbers are based on an analysis of police-reported crashes after controlling for differences in miles traveled, driver-related risk factors, road conditions and other variables.

While inadequate visibility is the headlight issue known to lead to crashes, excessive glare from other vehicles’ headlights is a common concern. Although there is no evidence that glare by itself leads to crashes, properly aimed headlights can illuminate the road ahead without getting in other drivers’ eyes. It’s also possible to have headlights that have both poor visibility and excessive glare.

Manufacturers have taken steps to reduce glare as part of their response to IIHS headlight ratings. For the 2023 model year, only 6% of the headlight systems tested had excessive glare, compared with 20% in the 2017 model year.

Beginning in the mid-1980s, most vehicle models had customized headlights with replaceable halogen bulbs. More efficient high-intensity discharge (HID) lamps appeared in the 1990s, and LED headlamps followed in the early to mid-2000s. LED technology has proliferated recently as it uses less energy than HID and is more reliable over the life of the vehicle.

IIHS headlight ratings are technology-neutral. There is a performance metric that headlights are tested against, whether they use halogen, LED or HID technology. Use of a particular technology does not guarantee a good rating. Similarly, there are examples of poor ratings for every type of headlight.

Some of today’s vehicles are available with curve-adaptive headlights. These headlights pivot in the direction of travel based on steering wheel movement and sometimes the vehicle's speed to illuminate the road ahead. They make it easier to see on dark, curvy roads. For example, an experimental study with volunteers driving vehicles with different lighting systems found that curve-adaptive HID headlights allowed drivers to spot a hard-to-see object on a dark, curvy road about one-third of a second earlier than with conventional fixed headlights (Reagan et al., 2015).

Studies show that curve-adaptive headlights reduce insurance claims, though it can be hard to tease out how much of the benefit is from HID or LED lamps and how much is from curve adaptivity.

HLDI has studied curve-adaptive headlights offered by several automakers and found that claim rates are generally lower for equipped vehicles (HLDI, 2023). While the studies in that compilation did not differentiate between crashes that occurred at night and those that occurred during the day, an earlier study of model year 2010 Mazda 3 vehicles did (HLDI, 2016). It found that nighttime collision and property damage liability coverage claim rates were 10% and 15% lower, respectively, for Mazda 3 models with curve-adaptive lighting compared with those without. Daytime claims were unaffected.

Another currently available feature is high-beam assist. This technology uses a camera to automatically switch between high beams and low beams, depending on whether other vehicles are present. The driver still has the option to manually switch between low and high beams. 

Researchers from IIHS and the University of Michigan Transportation Research Institute found that drivers in and around Ann Arbor, Mich., didn’t use their high beams enough (Reagan et al., 2017). Only 18% of drivers who were isolated enough to make use of their high beams did so.

Using high beams when conditions allow makes it easier to spot obstacles in time to avoid them. For example, a driver of a 2024 Subaru Solterra should be able to spot an obstacle in the left lane 202 feet ahead while using low beams and 533 feet ahead while using high beams.

High-beam assist could improve the high-beam use rate if drivers are simply forgetting to turn on their high beams, are unsure whether oncoming vehicles are far enough away to do so safely, or underestimate the effect of high beams on visibility.

Researchers from the University of Michigan and General Motors (GM) found that GM’s high-beam assist was associated with reductions in nighttime crashes (Leslie et al., 2021).

Headlights with adaptive driving beams, not to be confused with curve-adaptive headlights, are similar to high-beam assist. However, rather than switching between high and low beams, they continuously adjust the high-beam pattern to create a shadow around other vehicles. In this way, adaptive driving beams offer high-beam visibility except for the segment of the beam that is blocked out to limit glare for oncoming or leading drivers.

IIHS research showed that glare from a vehicle with adaptive driving beams was significantly lower than glare from an SUV with HID low beams and a sedan with curve-adaptive HID low beams (Reagan & Brumbelow, 2015).

Adaptive driving beam systems, which have been available for over a decade in Europe, were prohibited in the United States until 2022, when the National Highway Traffic Safety Administration amended the regulations to allow such systems on vehicles sold in the U.S (NHTSA, 2022).