General Motors has released a study praising the life-saving effects of daytime running lights (DRLs), headlights that automatically shine between dawn and dusk. The carmaker claims that its customers have avoided 37,000 collisions since 1995, when DRLs became standard on GM vehicles. How did they figure that out?
The study uses something called a "pairwise case-control design" to guesstimate the number of near misses. Simply put, the authors examined the collision rates of several post-1995 GM models, in 17 states. Each model was then paired with a vehicle with comparable "design and operator demographics" but without DRLs—say, a Ford with a similar body shape and weight. The "DRL effect" could then be calculated using a straightforward ratio:
DRL effect = collision rate with DRL/collision rate without DRL
A DRL effect under 1 corresponds to a decrease in the collision rate for DRL-equipped vehicles relative to their less luminous peers. The study, produced at GM's behest by Exponent Inc., notes that, "for example, a ratio of 0.95 is equivalent to a reduction in the collision rate of 5 percent." So, if a particular Ford model in the study was involved in 10,000 collisions, and a comparison with the GM it's paired with reveals a DRL effect of 5 percent, then GM can claim that daytime headlights prevented 500 collisions in this instance.
The next step for Exponent was to estimate the gross number of accidents avoided. The authors employed regression analysis, a complicated statistical technique used to factor in the impact of multiple variables at the same time. This made it possible to account for such variables as the presence of other safety features and state-by-state differences in traffic conditions. Given the complexity of this methodology, Exponent's researchers weren't exactly using slide rules at this point; all of the accumulated data was analyzed with the help of SAS statistical software. They also broke down their data sets by type of collision, ranging from multivehicle to car-meets-pedestrian.
The final estimate of 37,000 collisions prevented should help GM bolster its case for mandatory DRLs. Two years ago, the automaker asked the National Highway Traffic Safety Administration to require DRLs in all new cars, but the agency has yet to issue a ruling. Opposing mandatory DRLs is the Association of Drivers Against Daytime Running Lights, which criticized a previous GM study as too simplistic, failing to account for the increased use of antilock brakes, year-to-year weather changes, and other factors. The organization claims that GM is merely trying to make a buck by making lights burn out faster and upping fuel consumption (since lights do consume an iota of gas). The carmaker has countered that low-intensity DRLs do not significantly decrease bulb life and that the fuel-consumption hit will amount to less than $10 per year, per vehicle. Multiply that by 200 million vehicles in the United States, though, and it amounts to a healthy chunk of change.
Explainer thanks Jim Schell of General Motors.