Photo: FCA

The Insurance Institute For Highway Safety says nearly half of all fatal crashes in the U.S. occur in the dark. To combat this, they’ve just introduced a new headlight safety test. And after rating 31 midsize sedans with their new methodology, the safety lab in Virginia has concluded that many modern headlights need to step up their game. Here’s how they tested them.

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The IIHS just put 81 headlights for 31 different mid-size sedans to the test, and only one of them scored a good rating: The Toyota Prius V with LED lights and High-Beam Assist.

IIHS says the Prius V with LED headlights allows a driver traveling 70 mph on a straightaway enough time to identify an obstacle on the right side and come to a complete stop without making contact.

The same cannot be said about the BMW 3 Series, whose headlights IIHS says are the worst of the bunch, allowing the driver to travel only 35 mph in order to see an obstacle and stop in time.

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That’s a huge difference in performance, one that IIHS says has a lot to do with automakers’ focus on styling over performance, and a lack of federal regulations specifying headlight aim.

But the 3 Series isn’t the only culprit, as 44 different headlight systems received poor ratings.

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How Are The Tests Run?

Photo: IIHS

Using a system of demerits, IIHS determines a headlight’s overall score by combining test data for two different categories: glare and visibility.

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Glare and visibility scores are determined by using photometers to measure illumination for five different vehicle approaches: a straightaway, 150 meter radius left and right turns, and 250 meter radius left and right turns.

Using crash data, satellite imagery, and typical rural roadway speed limits, IIHS determined this appropriate radius of curvature and also the length for their tests as well as the appropriate vehicle speed.

As the vehicles drive the five different approaches specified in the table below, headlight visibility readings are taken by the photometers, which sit 25 cm from the ground, and glare readings are taken at 110 cm off the ground (presumably to emulate the eye-height of an oncoming driver.)

How Does IIHS Assign Scores?

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Visibility

The image above shows distances at which the ideal headlight will illuminate an object with five lux. The distances represent approximately how far ahead a driver must see an object in order to stop before hitting it (when traveling ~60mph on straights and ~40mph on curves).

Visibility demerits are determined by driving vehicles along the straightaway or curve, and determining the distance from the vehicle that the luxometers begin to read five lux (a unit of illuminance.) IIHS says that the five lux rating was determined based on their research on how much illuminance in needed to identify low contrast objects, though they admit that there are many factors that play into how much light is needed to notice an object at night.

The distance at which the instruments read five lux, as measured along the center line of the travel line, is averaged between three tests and demerits are assigned based on how headlights stack up against the competition and against an ideal headlight (like the one represented in the plot above).

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Glare

Low-beam glare lux readings, which are measured at a higher vertical position on the roadway and in the oncoming lane, are rated based on thresholds detailed in Federal Motor Vehicle Safety Standard (FMVSS) 108.

IIHS says an acceptable glare reading may not exceed 10 lux from 5 to 10 meters away, and at longer distances, overall exposure distance (the distance traveled at a given lux reading) must be lower than those shown in the plot above.

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The idea here, is that when an oncoming car is closer, the larger angle between the driver and oncoming headlights means the driver can withstand more glare (in this case, 10 lux) without discomfort. As for farther readings, IIHS says setting the limit for total distance traveled at a given lux reading, rather than simply allowable lux at various distances from the light source, accounts for both “dosage” and illumination level.

Overal Rating

Visibility and glare findings are combined to create an overall rating in what IIHS calls a “system of demerits.” Based on crash data and knowledge of real-world usage, IIHS has weighed straight-line tests and low-beam values more heavily than curved and high-beam results.

In addition, cars with High-Beam Assist and adaptive headlights can have their demerits reduced for a higher overall score (though any vehicle with excessive glare can only score as high as marginal).

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You can learn lots more about the methodology on IIHS’s website. I think it’s an interesting rating system, and it will be interesting to see how it affects the ways automakers design (and most notably, aim) their increasingly-fancy headlights.