About half of all fatal crashes in the U.S. occur in the dark, and more than a quarter occur on unlit roads. Headlights have an obvious role to play in preventing nighttime crashes, but not all headlights perform their job equally. Differences in bulb type, headlight technology and even something as basic as how the lights are aimed all affect the amount of useful light supplied.

Headlight technology has been developing rapidly in recent years. LED and high-intensity discharge (HID) lamps have begun to replace the traditional halogen ones. Many automakers offer curve-adaptive headlights, which respond to steering and swivel according to the direction of travel. Many also offer high-beam assist, a feature that can increase the use of high beams by automatically switching between high beams and low beams based on the presence of other vehicles.

At the same time, government regulations about headlights don't guarantee consistency when it comes to the amount of illumination they provide in actual on-road use. This has resulted in large variation in headlight performance. Many vehicles sold today have inadequate headlights, despite the recent strides in lighting technology.

How headlights are tested

IIHS engineers measure the reach of a vehicle's headlights as the vehicle travels straight and on curves. Sensors on the track measure how far from the vehicle the light extends with an intensity of at least 5 lux. A lux is a unit of illuminance, or the amount of light falling on a surface. For comparison, a full moon on a cloudless night illuminates the ground below to about 1 lux.

Both low beams and high beams are measured on five approaches, shown in the graphic below:

  • Straightaway

  • Gradual left curve (800-foot radius)

  • Gradual right curve (800-foot radius)

  • Sharp left curve (500-foot radius)

  • Sharp right curve (500-foot radius)

road curves graphic

On each approach, visibility measurements are taken on the right edge of the roadway. On the curves, measurements also are taken on the left edge of the travel lane. On the straightaway, the second measurement is taken at a point corresponding to the left edge of a two-lane road. This allows the engineers to gauge the illumination on both the right and left side of a straightaway, which are typically quite different. With most headlights, there is a steep drop-off in light on the left side of a straight road in order to prevent glare to oncoming vehicles.

Glare for oncoming vehicles is also measured from low beams in each scenario and must fall below certain thresholds to be considered acceptable.

Headlights are tested as received from the dealer. Although many headlight problems could be resolved by adjusting the aim of the lamps, IIHS doesn't change headlight aim. Few vehicle owners adjust the vertical aim of their headlights, so leaving the aim the way it was set at the factory makes the testing more realistic. Horizontal aim also is important, but in most vehicles it can't be changed after the initial factory setting.

Readings are taken 10 inches from the ground for visibility and 3 feet, 7 inches from the ground for glare.

How ratings are assigned

IIHS engineers compare the results of the testing with a hypothetical ideal headlight system. Using a system of demerits, they apply the visibility and glare measurements 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 the curves because crashes are more common on straight sections of road.

A vehicle with no demerits doesn't exceed the glare threshold on any approach and provides illumination to at least 5 lux over the distances shown in the graphic below. Longer visibility distances are required on the straightaway compared with the curves because vehicles tend to travel at higher speeds while going straight. Similarly, greater visibility is required on gradual curves compared with sharp curves.

headlight range graphic

Vehicles equipped with high-beam assist may get their low beam visibility demerits reduced. This credit is given only for approaches on which the glare threshold isn't exceeded and on which the high beams provide more visibility than the low beams.

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