IIHS ratings show that visibility provided by headlights varies widely.

Updated: Oct 27, 2020

In 2016, the Institute released its first headlight ratings, and out of more than 80 headlight systems that were evaluated on the 31 the model year 2016 midsize cars, only one system received a good rating. As of March 2019, 14% of headlight systems tested on the model year 2019 vehicles received a good rating. More than half were rated marginal or poor due to inadequate visibility, excessive glare from low beams for oncoming drivers, or both.

So 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.

Another common problem is glare. Properly aimed headlights can illuminate the road ahead without getting in the other drivers’ eyes. Surprisingly, there are headlights that perform poor visibility and also cause excessive glare. That’s because there are headlights that meet the federal regulations, but don’t 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. Once the headlights are put on a vehicle, the regulations allow a wide range of mounting heights and widths but 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.

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