Daytime Running Lights: No Statistically Significant Effect On Safety

By James Baxter, NMA President

You’re not likely to see any bold headlines, and CNN will not be interviewing Bush administration DOT officials, but the seven year long saga of trying to prove that Daytime Running Lights improve highway safety is coming to a close.

The official results are couched in terms like “not statistically significant,” meaning they don’t prevent crashes or make our highways safer.

The motivation for the study was a General Motors petition to make Daytime Running Lights (DRLs) mandatory on all vehicles sold in the United States. For several years prior to this GM had been fitting its vehicles with DRLs and expected the masses to respond to this patronizing gesture with their checkbooks open.

Instead, random iconoclasts and other non-believers complained about glare, blanked out directional lights, wasted fuel, and just simple irritation with the DRLs concept. They apparently went elsewhere with their checkbooks and bought cars that allowed the operator to decide when to turn the lights off and on.

Two studies preceded this last effort and they indicated some safety benefit attributable to DRLS. However, the DRL critics were just as relentless in pointing out the flaws, and bad assumptions inherent in these studies. The third effort was an attempt to address these earlier failings.

To some degree the last study was an improvement over the prior efforts, but it too failed to address many of the more difficult questions, particularly those questions that suggested DRLS not only did not improve highway safety, rather they detracted from highway safety. Fittingly, the sample population of vehicles investigated in the study were GM products.

Skipping to the final results first, the researchers could find no statistically significant evidence that DRL equipped cars and trucks were less prone to be in accidents where daytime running lights could have been a factor.

Within the study they found random blips where DRL equipped vehicles were disproportionately involved in fewer — or more — accidents than not DRL equipped vehicles. This was largely due to small sub-sample sizes where random variations will cause distortions over short time spans. When the subgroups were combined and the number of sample vehicles increased, the differences between DRL and non-DRL vehicles evaporated.

There were primary and basic assumptions made that may not be valid.

For example, the study design was based on the premise that comparing accident involvement of the two types of vehicles; those equipped with DRLs and those not equipped with DRLs would answer the “safety question.” However, the concerns raised by DRL critics are only peripherally related to actual accident involvement of DRL equipped vehicles. It was their contention that DRL equipped vehicles were causing accidents, even though these same vehicles may not have been involved in the actual accidents.

One such subset of DRL opponents consists of motorcyclists who believe DRL equipped autos and trucks diminished or confused the visibility of motorcycles that typically operate with headlights on during daylight hours. (A Japanese study that explored the effect of daytime headlight use by motorcyclists found no benefit, but that’s another topic.) The correspondingly significant increase in motorcycle accidents has added emphasis to this claim. However, the concurrent increase in motorcycle ownership and use tracks this same increase in motorcycle accidents.

Complaints of “glare,” obscured turn signals, confused distance perception (“is that one headlight close by or two headlights far away?”) and failure to use headlights during periods of low visibility were oft mentioned but not seriously explored.

Issues that were raised in the US, but given short shrift, gathered considerable momentum in some European countries. Namely, does the visual dominance of DRL equipped vehicles mask or obscure pedestrians and bicyclists? Again, the recent NHTSA study mentioned these possibilities, but did not pursue them. In all fairness this would be a difficult task. Conversely, mandating that millions upon millions of vehicles burn headlights or auxiliary lights during daylight hours is not without considerable cost.

Given the finding of “no statistically significant benefit “in this most recent study it would seem GM could better spend its time petitioning Canada to repeal its DRL mandate.

Read the full study (PDF – 3.2MB)

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