Nighttime visibility is critical to highway safety. About half of traffic deaths occur either in the dark or at dawn or dusk.
Few vehicles have headlights that 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.
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.
As of March 2019, 14 percent of headlight systems tested on model year 2019 vehicles received a good rating. More than half were rated marginal or poor because of inadequate visibility, excessive glare from low beams for oncoming drivers, or both.
About our headlight evaluations
Why does headlight performance matter? 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.