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Abstract
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The Insurance Institute for Highway Safety (IIHS) recently announced the results of vehicle headlight evaluations, widely reported in the media, especially because so few of the systems were rated as good or acceptable. The IIHS evaluation protocols [IIHS 2016] and rationale [IIHS 2015] are available for review, as are test results on individual vehicle's headlighting systems [Karush 2016].
The IIHS [2016] evaluation procedure involves measuring the amounts of light reaching photosensors positioned to represent potential roadway hazards as well as the locations of oncoming drivers' eyes, in order to characterize both visibility and glare from headlights. Systems receive demerits if they fail to illuminate hazards from certain distances or if they direct excessive light that could contribute to glare for other drivers. Bonuses are provided for headlights that swivel into curves or that automatically switch high beam headlights on when no other vehicles are present because these features likely contribute to increased safety [Bullough 2014].
Importantly, IIHS does not adjust the vertical aim of the headlights on the vehicles it measures but rather evaluates the headlights as received. This is reasonable, because most drivers do not adjust the headlights on their vehicles and misaimed headlights are not uncommon on both in-service [Flannagan 2011] and brand-new vehicles [Skinner and others 2010]. It matters, though, because headlights aimed too low will impair a driver's visibility, whereas headlights aimed too high will increase glare to other drivers.
Indeed, the results of IIHS's rating evaluations support this latter point. Setting the bonuses from swiveling headlights and automatic high beams aside, Fig. 1 shows demerit scores for each of 64 vehicle headlighting systems evaluated by IIHS, plotted as a function of the average misaim of each vehicle's headlight. Notwithstanding a good deal of spread among the systems, a modest but statistically robust (coefficient of determination r2 = 0.173, P < 0.01) trend is observed; demerit scores tend to be lower (better) when the measured headlights were aimed properly and higher (worse) when headlights were aimed too low or too high. Post hoc adjustment of the demerit scores to account for this effect would double the number of systems rated as good (demerit score ≤ 10) or acceptable (10 < demerit score ≤ 20). So, what are we to take away from this?
Even accounting for the possibility of a relationship between vertical aim and evaluated headlight performance, differences among the systems would still be large, indicating that some headlights do outperform others at providing visibility while minimizing glare to others. Nonetheless, it may not be obvious to consumers that having their headlights aimed properly on a regular basis can also have a meaningful safety impact. Kudos to IIHS for making their test protocols and results available. They provide an important resource for understanding something we all aim for--improved nighttime driving safety.
Language: en |