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Do rear daytime running lights cause accidents?

Rear daytime running lights are now popping up on Volvos, Hondas & Toyotas. I witnessed an accident today in which a Honda Accord with these always-on red rear LEDs suddenly slammed on the brakes. I saw three cars consecutively abruptly stop. Then behind the third was a motorcyclist — it flipped he and his bike over. I am afraid these always-on rear lights serve to desensitize the actual braking when it occurs. — John

That Honda driver had his headlights on during the day. And, that just might be why he wasn't rear-ended when he slammed on the brakes, says Young Drivers of Canada

"If you hit something in front of you, you're obviously too close," says Angelo DiCicco, the driving school's GTA general manager. "And I can tell you conclusively from 28 years of teaching drivers that if you're driving during the day with your low beams on and your taillights on, you'll get a better following distance from the people behind."

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Front daytime running lights (DRL) are required by law in Canada. But rear DRL aren't.

We checked with car companies. Most, including Honda and Toyota, say they don't have rear DRL. Taillights only turn on when the headlights are on — whether you've turned them on manually or they come on automatically when it's darker outside.

Volvo is an exception — if lights are set to auto, then the tail lights come on with the front DRL, a company spokesman said.

For 30 years, Young Drivers has recommended that drivers turn on low-beams during the day, even if they have DRL. While front DRL make cars more visible from the front and reduce head-on collisions, there's a dark side.

Because those lights are only in front, you don't get any added visibility from the rear. And 33 per cent of crashes are rear-enders, DiCicco says.

"Having your taillights on makes you more visible to the other vehicles around you," DiCicco says. "And that way, as it starts to get dark, you're not one of those ghost cars who scares the crap out of everyone."

DiCicco says the idea that taillights desensitize drivers to brake lights is "an old wives' tale."

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"To blame a rear-ender on somebody having their lights on is a cop out," he says. "Physics is physics."

We couldn't find studies on rear DRL. But there is research on LED taillights.

A 2013 study by the U.S. National Highway Traffic Safety Administration (NHTSA) showed no difference in crash rates between cars with LED and standard taillights.

"But there's every reason to think LEDs should be safer," says Dr. Michael Flannagan, a researcher at the University of Michigan Transportation Research Institute. "More research is needed."

LEDs light up virtually instantly, while incandescent bulbs take a quarter of a second to heat up. That difference can be significant at highway speeds, Flannagan says.

And, LED tail lamps, with their concentrated dots, seem to be more visible, even though they're exactly as bright as incandescent bulbs. All lights are required by law to have a set minimum and maximum brightness.

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"When you measure LEDs with a photometer, they're the same brightness as incandescent," Flannagan says. "But anecdotally, people perceive them as being brighter."

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