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My bike light, the Cygolite Metro 360, has a "Daylighting" mode which flashes very brightly and is intended to be used during the day. Similarly, many tail lights have flashing modes. Obviously one should bike with lights at night. But is it advisable to bike with lights on (in flashing mode) during the day as well, to alert cars to your presence? Or does it not make much of a difference? If there are any scientific studies or data on this, please reference.

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There are arguments both ways on this, and any evidence one way or the other is largely anecdotal, and at the very least dependent on the specific situation. –  Daniel R Hicks Jun 29 at 4:08
    
As far as I know, in the Netherlands it is prohibited to use flashing lights (at least at night). I don't really now why they even sell them here with that option. –  Bernhard Jun 29 at 7:35
    
Source for my statement (in Dutch): rijksoverheid.nl/onderwerpen/fiets/vraag-en-antwoord/… –  Bernhard Jun 29 at 7:38
    
The same is true in Alberta, Canada. The vehicles that are allowed to use flashing lights are clearly defined in the Highway Traffic Act. In practice, the law is rarely (maybe never?) enforced for bicycles. –  Travelling Man Jun 29 at 14:41
    
The very bright flashing mode can be dazzling even during the day if they're close enough - look out for one coming towards you on a cycle path or test it with yours. –  Chris H Jun 30 at 9:31
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4 Answers 4

up vote 17 down vote accepted

According to Safety effects of permanent running lights for bicycles: A controlled experiment. (Madsen JC1, Andersen T, Lahrmann HS.) they give about a 19% reduction in crash rates. There's a copy of the paper in Scribd as pdf. Every reference I've been able to find appears to refer to this one study.

The incidence rate, including all recorded bicycle accidents with personal injury to the participating cyclist, is 19% lower for cyclists with permanent running lights mounted; indicating that the permanent bicycle running light significantly improves traffic safety for cyclists

Wikipedia has a few articles, starting with daytime running lights (reports largely positive results) and bicycle lighting which gives a link in Danish that appears to refer to the Odense study above but claims 32% reduction.

Daytime running lights improve cyclist safety significantly A large-scale experiments with daytime running lights on bicycles among 4,000 cyclists in Odense shows 32% reduction in cyclist accidents. The result is so striking that Councilman

Wikipedia Daytime Running Lights links to Google books copy of OECD, International Transport Forum (2013), Cycling, Health and Safety, but I can't copy'n'paste out of it so here's an image:

enter image description here

A BicycleForums anecdote resulted in a largely positive discussion, and there's this rant that concludes they're essential. The Australasian College of Road Safety amongst others is strongly support of them for motor vehicles so it's fair to assume bicycle riders would gain something from the practice.

FWIW I'm convinced. I have dynamo lights on my commuter bike that I never turn off (they have standlights so they go off a few minutes after I stop moving).

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A head light addresses cross traffic and head on traffic. Too often vehicles are looking for vehicles and don't see a bike that is in plain site.

My experience is that yes a strong flashing light in daylight helps being seen. I live in a large city that is not particularly bike friendly and I definitely feel it helps.

I took these picture through my truck window in full sunlight at 1:00 PM. This is 400 lumens. enter image description here

A light is a safety tool. On cross streets and driveways don't assume the vehicle is going to stop and don't assume they see you. I wait for the vehicle to stop and I wait for them to look at me. With a light they don't just see you better it is easier to tell they saw you. They will hesitate on the light and then typically make eye contact. I feel like the super bright makes a difference. They don't just see you - you get their attention.

I know you ask about daylight but at night a powerful light puts out enough light for vehicles to see your light from a cross street.

You might think I don't need a $100 600 lumen light to see the road and at low speeds you don't. The higher power lithium ion led lights we have today I feel are much more effective for be seen.

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So, I've done a lot of (non-academic) research on bike lights for this site's community blog and more recently for the bike lights resource site I created, The Bike Light Database. There is a disappointing lack of hard scientific data on bike lighting at all, and essentially none regarding this specific question.

I can tell you from extensive anecdotal experience that a flashing light definitely helps during the day, both forward-facing and rear-facing. A small AAA-powered blinky light won't do much for you, but if you have a high-output light on during the day, drivers give you much more respect. From the rear, they tend to slow down more and pass with a wider distance. On the front end, I'm much less likely to get cut off by drivers pulling out of driveways or side streets who think they have enough time to make it but don't quite.

A flashing light is more useful during the day since there's so much more ambient light to compete with. The human eye responds to a flashing light more quickly than a solid light because it appears like movement. At night the flashing can be very distracting, but during the day there's not a high risk of disorienting drivers since again, there's so much ambient light.

When Cygolite first released the lights with their Day Flash mode they told me that they had done some studies that led them to selecting a certain minimum brightness for their day flash. I'll email me contact over there and see if I can find out what sort of research they did and if they can share any of that with us. I also met a university researcher who's doing some studies regarding bike lights, although last I talked with him it sounds like it was still a work in progress. I'll see if he's doing any work with daytime lighting and if he can share any of that as well.

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"they tend to slow down more and pass with a wider distance" mainly because they get more time decide what to do. –  andy256 Jun 29 at 5:12
    
@andy256 right, due to seeing me sooner because of the light, they have more time to determine and execute a safe and appropriate response. –  nhinkle Jun 29 at 5:20
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Most automobiles in the US now include daytime running lights, since there were studies showing that this did increase other drivers' awareness of them. If you want the scientific studies, knowing that the keywords are "automobile daytime running lights" is likely to help you find them.

It isn't entirely clear whether that effect fades when daytime running lights become the norm rather than an exception. It also isn't entirely clear whether it would apply with the smaller lights on bicycles.

However, I think this is a case of "It's not likely to hurt, and anything that makes a driver notice you might help."

(With the possible exception of drunk drivers, who have a bad habit of swerving toward whatever has caught their attention. But there are fewer drunks on the road during the day.)

Will it help enough to justify its cost in batteries? I haven't a clue. Sounds like a great opportunity to run another study, if someone's looking for a thesis project.

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Yeah, it's probably in the "can't hurt" category, so long as the light is not so bright as to annoy drivers. Most flashing lights can run a long time on a single set of batteries (5-10x longer than when steadily on). And my personal opinion (with admittedly no data to back it up) is that drunks and sleep-deprived drivers are less likely to swerve into a flashing light than a steady one. –  Daniel R Hicks Jun 29 at 4:11
    
As to running "another study", there's barely been a first one -- very little in the way of hard statistics is available for any aspect of bicycle safety, since government agencies don't choose to collect the information. –  Daniel R Hicks Jun 29 at 4:13
    
As I say, there's the automotive studies. But if you want a bike study, I can't believe that it would be much harder to conduct than any other serious multiyear study, and you might be able to get the manufacturers of bikes and bike accessories to fund it. If you care enough to want a formal answer, it's up to you to make it happen. –  keshlam Jun 29 at 4:17
    
Regarding batteries, most lights bright enough to be visible during the day are using rechargeable batteries anyways. –  nhinkle Jun 29 at 4:52
    
Also, some bike lights these days are brighter than motorcycle or even some car lights. –  nhinkle Jun 29 at 4:53
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