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My head and tail lights can be set to blink or emit a steady beam of light.

I usually set my rear (red) light to blink, because I believe it makes me more visible at night. My friend argues that it makes it more difficult for motorists to judge how far away I am.

Are there any studies showing which is the safer way to use a tail light?

Nik
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user229044
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    From what I've heard, it's pretty much a toss-up. Blink makes you visible quicker, steady lets motorists figure out what you are (and how far away you are) quicker. When it's dark, I have one blinky and one steady in the back. If it's rainy or dim I'll set both to blinking. – freiheit Nov 30 '10 at 23:49
  • I agree that the duplication looks odd. However, when someone does a search, they'll see a list of question titles with very little of the question text underneath the titles, so the title is pretty important. – Goodbye Stack Exchange Dec 01 '10 at 05:52
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    I hate flashing lights... but that's not data... – Murph Dec 01 '10 at 08:00
  • There's some missing context here. In Manhattan, there's a sea of lights for a driver to make sense of. On a N. Dakota country road, you will be the only light most of the time. – Jay Bazuzi Dec 04 '10 at 23:39
  • @jay, correct, but in 'large urban center' drivers are more likely to be aware and alert than on 'generic backroad'. Also , the speed of approach is generally higher outside urban centers, leading to less decision making time. – Byron Ross Dec 20 '10 at 01:15
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    Just as an addition: Note that in jurisdictions where bicycle lights are required by law, usually a non-blinking light is required (for example in Germany). In that case, a blinking light is only legal in addition to a non-blinking light. – sleske Feb 16 '11 at 10:31
  • I'm not sure I'd say "usually" - I think in most places in the US where a light is required, a blinking light counts as well. – zigdon Feb 16 '11 at 20:16
  • +1: check local laws. Several European countries have dropped the "nonblinking" requirement (Denmark, Holland, Austria at least). I'd have preferred an official stance so it would be consistent at least. – Torben Gundtofte-Bruun May 01 '11 at 21:13
  • And note that most US states only require a front headlight and rear reflector. And even that requirement is poorly enforced. – Daniel R Hicks Sep 06 '11 at 18:27
  • I set them to blink just because formula one cars have blinking tail light and the safety car has blinking head lights. I guess they're blinking for safety reasons. I think they look cool J – imel96 Apr 12 '13 at 23:14
  • As a driver, I would recommend flashing LEDs front and back since it warns me that a cyclist is there. Unfortunately, even motor bikes with their much better lights can disappear into the mess of other lights. – Doug Sep 06 '11 at 18:09
  • What about "twinkling" lights? I have one that alternates between two sets of lights in the same device. – PositiveK Oct 10 '13 at 01:55
  • All comments concern the bike with the light only not the ones who are blinded by excessive intense lights, similar with the head beam of a care shining at you. Lights on helmets are equally dangerous to the approaching biker being looked at by the one with the light. The rule should be the same as for mopeds or motorbike. –  Nov 19 '13 at 23:14
  • I believe adding one to my helmet (as well as bike) improves my visibility. – Mark W Nov 21 '13 at 13:52
  • @Uwe Last time I checked, mopeds and motorcycles aren't running a single tiny LED with a 9 volt battery for a headlight. – user229044 Nov 21 '13 at 14:59
  • Sidenote: In the Netherlands, blinking lights are not allowed (source, in Dutch). I never heard of anyone getting a fine over this, though (the fine seems to be €55). Not sure what the regulations in other countries are. – Martin Tournoij Apr 08 '14 at 11:55
  • I use a slight different technique to be visible to drivers from behind as well as to on coming traffic: I use two blinkers on both of my fork stems for drivers ahead to detect me as a very thin object rolling towards them (aka bike) with a head light on my handle bar. I use a tail light set in as a blinker for drivers coming up on me to detect me early while having a reflector right above it to make him judge the distance. – Ahbab Razzaque Aug 28 '14 at 01:59
  • I select blink on the visibility lights simply because there isn't strong evidence that it's significantly WORSE, and it saves battery life. – keshlam Aug 29 '14 at 19:23
  • Google "target fixation" + "blinking lights". Muy two cents. I turned blink off just in case... – gaurwraith Mar 06 '15 at 01:47
  • @gaurwraith That's really silly, and bad advice founded on nothing. You'd rather be less visible to every single driver on the road all the time, on the off chance that somebody will experience target fixation? Do you honestly think that makes you safer? What if the numbers came out that for every driver who experiences target fixation, a hundred other drivers never saw you to begin with? Just "google it, it's a thing" is not enough to actually form an opinion on. – user229044 Mar 06 '15 at 14:59
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    I was riding Vätternrundan two years back starting at midnight - the guy in front of me had a flashing rear light, made it near impossible to judge distance to his back wheel (making it very dangerous). So just to chip in (on this very old post) I'd say never use blinkers in a group ride! – rg255 May 06 '15 at 10:22
  • I'd recommend at least one of each, If your rear light goes out, you're generally unaware. A Redundant pair (or more) on the back is a very good idea. Some separation helps too, a high light and a lower one helps being visibilie. – Criggie Sep 30 '15 at 21:46
  • As a multirole road user, I find the ever increasing trend among cyclists towards smaller and smaller lenses with brighter and brighter bulbs, often aimed straight into other road users' eyes, is totally counterproductive. To be seen you need a large gently illuminated panel, an object bright and large enough to be seen but not dazzle - not an infinitely bright pin point of light. And no-one seems to be producing a cycle light that meets these criteria. – Grimm The Opiner Dec 15 '21 at 11:00

6 Answers6

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The short answer is that 'safer' is subjective and depends on your requirements.

You are both correct. Movement attracts the eye, so your blinking light is noticed. It is easier to judge the position of a steady state light.

For a motorist to pick out your tail light, particularly, from a sea of noise is very difficult. The surface area of the light is tiny, and it's all on its own. Your light is just not important enough to notice amongst the jumble of signs, traffic directions and other cars unless it does something to stand out, like flash.

My understanding is that the reason it's easy to see cars (apart from bulk) is that they have 2 lights moving together. Something to do with your brain automagically resolving the connected pattern. That's why it's hard to see a car with one working taillight, or a motorcycle or bicycle.

In terms of safety, my policy is always 'be seen and misjudged' rather than 'not seen'. So I always set both front and rear lights to blink at night. If you need illumination for the road, I'd strongly suggest a second forward light for that.

From Rear Lighting Configurations for Winter Maintenance Vehicles

Flashing lights will be perceived as having higher brightness than steady-burning lights, up to a flash frequency of about 15 flashes per second. Such brightness enhancement can aid in conspicuity, and several rear lighting systems have been designed to have a flash rate between 5 and 9 flashes per second in order to maximize their perceived brightness. While conspicuity may be greater with such configurations, an observer’s ability to make accurate judgements of relative speed or distance may be compromised when flashing or strobing lights are used. Croft observed that the judgments required in tracking an object were difficult to make under strobing conditions, yet very easy in steady-lighting conditions. Observations made during a study of service vehicle lighting for maintenance operations similarly pointed out that strobing and flashing systems designed for maximum conspicuity can at the same time reduce one's ability to judge relative speed and distance. Periodic sampling of the field of view in another study resulted in deterioration of one's motion-tracking ability that increased as the distance to the object of interest decreased.

Also from Selection and Application of Warning Lights on Roadway Operations Equipment

Flashes are bursts of light which, by definition, are unexpected because they do not occur in nature (save for lightning). This characteristic is their most important feature and why they are so good at capturing attention.

Byron Ross
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    I agree with most of your answer, but meagar's question specifically asks for data, not opinions. – Goodbye Stack Exchange Dec 01 '10 at 00:18
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    @Byron - Impressive! That gets my upvote. I'm wondering if, based on the rpi.edu paper, if we should be using one steady light and a blinking light. Would the blinking light "anchor" the steady light, making it easier to determine where it is in space, or would the blinking light confuse the situation? – Goodbye Stack Exchange Dec 01 '10 at 05:49
  • Thanks for the links to hard data, that's really what I was looking for. – user229044 Dec 01 '10 at 18:28
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    @neilfein: Seems like the ideal would be two steady lights fairly far apart to make distance and speed easiest to judge, and one blinking to get attention to you in the first place... – freiheit Dec 04 '10 at 00:22
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    It's not exactly comparable though. Snowplows look like other vehicles but are moving much more slowly so rear ends are likely. Bike accidents are mostly "didn't see you" not people running into the back of a bike because they are doing 15mph instead of 50 – mgb Dec 04 '10 at 04:40
  • @mgb, I'm pretty sure that the reason for the data on snow plows is that research funding is easier to come by - see the number of articles on police lights. I suspect people don't see snow plows the way they don't see bikes, but someone would need to fund the research! :) – Byron Ross Dec 20 '10 at 01:17
  • @neilfein, I suspect 'the more, the better'. As I said, my policy is that it's better to be seen and misjudged... It's the drivers that don;t see me that scare me. – Byron Ross Dec 20 '10 at 01:18
  • At one time I had a life vest strobe beacon on the back of my bike. It flashed a xenon strobe about once a second. I'm relatively sure all motorists saw THAT light. – Daniel R Hicks Sep 06 '11 at 18:29
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    Nice to have a citation, but it is not relevant for traffic situations. It is very disturbing to be on the same road as such a blinking light, because it attracts your concentration against your will, and will decrease the safety of everybody else. Don't blink! – user unknown Oct 19 '11 at 22:54
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    I agree to some extent with @userunknown. Though it is well established that flashing lights attract attention, and are often thus naively assumed to be "safer", there are other factors that need to be considered. Flashing lights are distracting and disorienting, and I expect that they make things less safe overall, certainly for everyone other than the rider, but probably also for the rider. Don't flash, please! – orome Sep 30 '15 at 01:04
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    Another advantage of flashing lights is that a light which is on for 0.05 seconds and off for 0.15 seconds will use only a quarter as much average power as one which is lit continuously with the same maximum brightness. – supercat Aug 10 '16 at 15:47
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Wood et al. (2009): Drivers’ and cyclists’ experiences of sharing the road: incidents, attitudes and perceptions of visibility. Accident Analysis and Prevention, 41 (4), pp. 772-776

About differences in the visibility as perceived by bikers and drivers (scale of 1 to 5):

3.4. Perceptions of effectiveness of visibility aids

The largest difference relates to the visibility of cyclists using lights on their bicycles, where cyclists rate themselves as significantly more visible when using bicycle lights than did the drivers. This difference, in turn, is much greater at night than during the day.

Table 4

Visibility Aid Drivers (sd) Cyclists (sd)
Flashing lights on wrists/ankles 4.03 (0.96) 4.23 (0.84)
Bicycle lights 3.3 (1.15) 4.5 (6.67)

So cyclists think they are both equivalent, with a possible small advantage for the steady light, while drivers thought the flashing lights to be more visible (but remember differences between detection and recognition, see below) to be better, but still less visible than the bikers thought the flashing light.

With regard to the distance:

3.6. Perceptions of visibility distance

An analysis was also performed with regard to the average distance at which drivers and cyclists believed that a cyclist would be visible to a driver using low-beam headlamps at night. On average, cyclists believed themselves to be visible from 110.3 metres (sd = 157.662), while drivers believed a cyclist would only be visible at 48.3 metres (sd = 58.69) on average (that is, at less than half the distance estimated by the cyclists), t (1424) = - 9.247, p < .001.

Probably even more important than deciding whether flash or steady light is better, is actually using the light:

Abstract

While the use of visibility aids was advocated by cyclists, this was not reflected in self-reported wearing patterns

*(emphases mine)s

Maybe this Cochrane review: Interventions for increasing pedestrian and cyclist visibility for the prevention of death and injuries is useful for background info, and they have a few comparisons of steady light vs. reflector and blinking light vs. reflector:

  • Blomberg 1986: A flashing light held by a pedestrian yielded a greater detection and recognition distance when compared with reflectorised accessories (420m versus 207m and 96m versus 92m respectively).
  • Watts 1984b: A rear bicycle lamp yielded a greater detection distance when compared with reflectors (306m versus 184m).
  • Watts 1984c: A flashing beacon on a bicycle yielded a greater detection but not recognition distance when compared with reflectors (588m versus 444m and 59m versus 71m respectively)

It also has a lot of comparisons of reflectors on moving parts vs. "static" parts: "biomotion" configurations are better detected.

Personally, I have steady lights both in front and rear (Germany), but I have an additional rear light that I switch to blinking when I judge the conditions particularly dangerous.

I've heard that rather than the absolute brightness of the light, the lighting area is important for visibility. Thus, lights with larger reflectors (the inner mirror) are probably better for visibility. Which is contrary to the current trend to smaller reflectors and LEDs which are brilliant, but basically point sources.

However, I could not find the study.

Michael come lately
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cbeleites unhappy with SX
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  • Fantastically thorough and well-referenced answer! – James Bradbury Sep 22 '16 at 08:09
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    There's also this 2008 study by the Dutch research institute TNO. It concludes a marginal increase in visibility for flashing front lights, but recommends against flashing lights because it inhibits accurately determining the cyclist's position and decreases emergency vehicle visibility. – Sanchises Jan 23 '18 at 15:12
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    @Sanchises: Your link is broken. But the study is still available at this link, thanks to the Internet Archive Wayback Machine. – unforgettableidSupportsMonica Oct 14 '18 at 16:25
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    This is interesting. I speculate that part of the cyclist/driver discrepancy is that cyclists think of "visible" as "my light caught their eye" whereas drivers think of it as "I've recognized what it is, which direction it's pointed, and how fast it's going." – CCTO Oct 20 '21 at 20:36
  • @unforgettableidSupportsMonica I gave up and made a Community Wiki answer on behalf of Sanchises. – Michael come lately Oct 03 '23 at 19:53
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A blinking light is disturbing. Even after you have been noticed, even from the opposite direction, it is hard to concentrate on something else. In Germany, these blinking lights are prohibited (StvZO §67 (4) 2.), and rightly so. Stop using them!

Maybe you are safer, but the rest of the traffic is less safe. If everybody starts blinking and flashing, driving will become impossible.

In dark circumstances, blinking makes it hard to estimate the distance when following such a light, and hard to estimate the change of the distance. Since the eye can't adapt to the changing light circumstances fast enough, the viewing possibilities are reduced. For emergency vehicles, you are expected to slow down and stop, so that's a different situation.

Michael come lately
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user unknown
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    The question specifically asks for studies or data. Do you have data to support your assertions? – DQdlM Feb 28 '12 at 15:36
  • @DQdlM: I only have a citation for the law in Germany and a second one – user unknown Feb 28 '12 at 17:07
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    The law may say that blinking lights are not allowed, but that doesn't necessarily mean that they are distracting or less safe than a solid light. Blinking bike lights are very common here and I don't find them to be particularly distracting, but they do make it easy to identify that the light is on a bike. Since cars and motorbikes have unlimited electrical power available, there's no reason for them to use flashing lights, they can (and do) just use brighter lights in the first place (and indeed, I have to shield my eyes from some LED brake lights on cars because they are extremely bright) – Johnny Jun 28 '13 at 23:04
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    I don't see how a bad light at a car justifies your bad light. It's an invalid argument. And while a blinking light might not be less safe for the one who uses it, they make the traffic less safe for everybody else. It's just childish. – user unknown Jun 29 '13 at 00:10
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    You keep saying that blinking lights are bad, but I haven't found any references that backs that up. You quoted a German law that bans the lights, but they are legal in many other jurisdictions, so that's not really proof that they endanger others on the road. – Johnny Jun 29 '13 at 03:46
  • Interesting. I probably crossed the German border with blinking lights hundreds of times and was never stopped by the border police because of it. – Nobody Jul 26 '16 at 17:23
  • They often act as "better a bad light, than no light at all", which is pretty common for cyclists, but declining, thanks LED. – user unknown Jul 26 '16 at 18:46
  • @Johnny I checked those "citations" and I cannot find any law prohibiting blinking lights. Maybe something is lost in the translation? – mchid Feb 08 '19 at 03:52
  • @mchid - I think you meant to reply to someone else (probably the user called "user unknown"), I didn't post any citations, and in my previous comment I said that the lights are legal in many areas. – Johnny Feb 08 '19 at 04:04
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    @Nobody That would be because cyclists are generally treated as outlaws: Neither protected by the law, nor expected to adhere to laws. And I'd argue, that we should change that: Make sure we adhere to the law, and require motorists to treat us as the law prescribes. We'll be much safer if we are treated with the same amount of respect that a motorcycle rider receives. – cmaster - reinstate monica Feb 08 '19 at 17:57
  • @cmaster That's an interesting and amusing notion and I can see its merits, but don't fully agree with it. Although I have to admit I rarely do long distance cycling in Germany, I only commuted through it for a couple of kilometers for a couple of years. So I don't really know the German situation that well. – Nobody Feb 09 '19 at 12:02
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Just a complement to Byron's answer.

Blinking too fast would be counter-productive. For instance, a 20 Hz blinking could get fuzzied, and you would end up with the equivalent of a half-powered steady light.

According to this NASA study (<blink>warning: not about traffic</blink>), the optimal frequency range for catching attention is 4-8 Hz (cycles/second).

NASA also cites "duty cycle", with an example where the light period is longer than the dark period. This increases overall luminosity, and could be a nice trade-off to both catch the attention and allow distance/speed perception.

Nicolas Raoul
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    Why 4 - 8 Hz is great for catching attention, it is also on the low end of the frequency spectrum commonly associated with causing seizures. http://www.birket.com/technical-library/144 – Kurt E. Clothier May 13 '13 at 20:00
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    But please, please, don't use those now-you-see-me-now-you-don't lights that only flash at around 2Hz and are dark most of the time! With these you can can move by several bike lengths while in the dark. As a fellow cyclist I find them incredibly annoying. Although I do notice them well, I have no chance of telling where the bike is going. – Emil Mar 08 '15 at 20:34
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    @Emil: pre-LED there were halogen strobes that had sharp, piercing flashes about once every five seconds. Useless to identify where a moving vehicle was, and if you happened to be looking at it when it flashed, your night vision was gone for a minute after. Thankfully those are long gone. – CCTO Oct 20 '21 at 20:40
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    @CCTO Now we (they) just stick a pair of them (sans blinking) on the front of cars as a fashion statement. – Alex Hajnal Dec 15 '21 at 00:15
  • Concur - I've got a couple of front white lights that have a "throb" mode rather than a hard-cut. So the light level grows and falls over ~2 seconds. It has all the benefits of "movement" without the eye-gouging of flashing, and as a rider I could still see with the light. A good compromise. – Criggie Oct 03 '23 at 23:48
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Sanchises noted in a 2018 comment that…

There's also this 2008 study by the Dutch research institute TNO. It concludes a marginal increase in visibility for flashing front lights, but recommends against flashing lights because it inhibits accurately determining the cyclist's position and decreases emergency vehicle visibility.

The paper helpfully has a summary of the findings in English:

For front lights we find:

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  • In most cases flicker raises the conspicuity of front lights.
  • There is no clear relation between flicker frequency and conspicuity.

For rear lights we find:

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  • Flicker does not contribute significantly to the conspicuity of rear lights.

Other conclusions:

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  • There is no need to increase the conspicuity of front light by using flicker.
  • Visual perception suggests the following disadvantages of flickering bicycle lights:
    • Flickering bicycle lights make it harder for other road users to estimate the speed and trajectory of a bicycle.
    • Flickering bicycle lights may reduce the conspicuity of emergency services.
Michael come lately
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    Additional thought - flashing of lights helps with battery life overall, especially with modern LEDs. A unit that lasts 10 hours on solid might get 20 hours at 50% flashing rate because the losses to switching are tiny. – Criggie Oct 03 '23 at 23:49
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No data points, but I would agree that in most cases the flashing light is better. A single, constant-burning light can easily get buried in "noise", and there's even a tendency for motorists to unconsciously follow a constantly-burning moving light ahead (especially if drunk), a fact that I suspect figured into a couple of nighttime rear-end collisions I've learned of over the years.

To improve the ability of the motorist to judge the location and speed of the light it's better if it flashes relatively rapidly -- maybe 2-3 times per second.

Re legality, 169.222 subdivision 6 of the Minnesota statutes states:

A bicycle may be equipped with a rear lamp that emits a red flashing signal. https://www.revisor.leg.state.mn.us/statutes/?id=169.222

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Daniel R Hicks
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  • Blinking lights suffer the same issues as solid lights from personal experience. The question is, which is better and that will require some pretty decent statistics. – Brian Knoblauch Jan 28 '15 at 14:52