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.

Some new types of lighting technology show promise. High-beam assist increases high-beam use by automatically switching between high beams and low beams, depending on whether other vehicles are present. Curve-adaptive headlights pivot in the direction of travel to improve visibility on curvy roads.

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.

Headlight performance

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.

About 1 in 3 headlight systems tested on model year 2022 vehicles earn a good rating. About 40 percent 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 in the IIHS headlight test have 19 percent fewer nighttime single-vehicle crashes than vehicles with poor-rated headlights (Brumbelow, 2021). Acceptable and marginal headlights are associated with reductions of about 15 and 10 percent. 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.

Why would headlight performance make such a difference? It takes 1.5 seconds for a driver to react to an unexpected event under ideal conditions (Green, 2000). At a speed of 55 mph, a car travels about 120 feet during this brief period. Once the driver applies the brakes, it takes more than 144 feet, on average, to stop at this speed (Jernigan & Kodaman, 2001).

The low beams of many headlight systems with poor ratings don't provide enough light for a driver going 55 mph on a straight road to stop in time after spotting an obstacle in his or her lane. They provide even less illumination on the left side of a straight road and when driving on a curve.

Glare is another common problem. Properly aimed headlights can illuminate the road ahead without getting in other drivers' eyes. It's also possible to have headlights that provide poor visibility and also cause excessive glare.

There are federal regulations on headlights, but headlights that meet the regulations don't necessarily have similar on-road performance. Under the standard, a headlamp is placed on a test rig, and light intensity is measured at different angles relative to the center of the lamp. Measurements are taken for visibility and glare, but the standard permits a large range of intensities and the angles can be adjusted within a relatively large tolerance.

In addition, once the headlights are put on a vehicle, the regulations allow a wide range of mounting heights and widths and don't say how they should be aimed. As a result, two vehicles could be equipped with the same headlights but have a large difference in the distances illuminated.

Headlight innovations

Between 1940 and the mid-1980s, almost every vehicle in the United States was sold with standardized sealed-beam glass headlamps (Moore, 1998). Beginning in the 1970s, these were filled with halogen gas to improve performance. Since the mid-1980s, most vehicle models have had customized headlights with replaceable bulbs.

Halogen bulbs now have competition from LED and high-intensity discharge (HID) lamps. These newer light sources are more efficient than halogen bulbs (and LEDs are much more efficient than HIDs), so they can produce more light with the same amount of energy. They also produce light with a more natural color than halogen bulbs.

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 are intended to make it easier to see on dark, curved roads.

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. Curve-adaptive headlights usually have LED or HID lamps, while fixed lights can be LED, HID or halogen.

HLDI has studied curve-adaptive headlights offered by several automakers and found that claim rates generally fell (HLDI, 2020). In most cases, the effect was consistent for claims under property damage liability coverage, which pays for damage to other vehicles and property. A limitation of these studies is that the time of the crash was unavailable.

A study with model year 2010 Mazda 3 vehicles found that nighttime collision and property damage liability coverage claim rates fell by 10 percent and 15 percent, respectively, for Mazda 3 models with curve-adaptive lighting compared with those without. Daytime claims were unaffected by curve-adaptive headlights (HLDI, 2016).

In an experimental study with volunteers driving a vehicle with three different lighting systems, curve-adaptive high-intensity discharge (HID) headlights allowed drivers to spot a hard-to-see object on a dark, curvy road about one-third second earlier than with conventional fixed headlights (Reagan et al., 2015).

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.

Using high beams when conditions allow makes it easier to spot obstacles in time to avoid them. For example, a driver of a 2019 Honda Insight should be able to spot an obstacle in the left lane 174 feet ahead while using low beams and 458 feet ahead while using high beams. When traveling 55 mph, this is the difference between having 2.2 seconds to detect and react to an obstacle using low beams and 5.7 seconds using high beams.

High beam assist is not yet widespread enough to determine if it reduces crashes, but 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 percent of drivers who were isolated enough to make use of their high beams did so.

A telephone survey of drivers in the same region found that a majority (54 percent) of drivers believe that other drivers use high beams as much as they should. This shows that drivers are unaware of the extent to which high beams are underused (Reagan & Cicchino, 2016).

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.

Headlights with an adaptive driving beam, not to be confused with curve-adaptive headlights, are similar to high beam assist. However, instead of switching the high beams on and off, 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 lead 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).

Until recently, adaptive driving beam systems, which have been available for about a decade in Europe, were prohibited in the United States because regulations required separate high- and low-beam systems. However, in 2022, the National Highway Traffic Safety Administration amended the regulations to allow such systems on vehicles sold in the U.S (NHTSA, 2022).