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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?

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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
    
@meagar, does my edit to the title reflect your question properly? (Please revert my edit if I'm off-base.) –  Neil Fein Dec 1 '10 at 0:21
    
@neilfein Looks good, except I don't generally like to duplicate tags (safety) in the title. –  meagar Dec 1 '10 at 3:36
    
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. –  Neil Fein Dec 1 '10 at 5:52
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I hate flashing lights... but that's not data... –  Murph Dec 1 '10 at 8:00

5 Answers 5

up vote 41 down vote accepted

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.

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I agree with most of your answer, but meagar's question specifically asks for data, not opinions. –  Neil Fein Dec 1 '10 at 0:18
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You are correct...updated –  Byron Ross Dec 1 '10 at 1:26
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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? –  Neil Fein Dec 1 '10 at 5:49
    
Thanks for the links to hard data, that's really what I was looking for. –  meagar Dec 1 '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 4 '10 at 0:22

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:

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.

                                      Drivers       Cyclists
Flashing lights on wrists/ankles   4.03 (0.96)    4.23 (0.84)
Bicycle lights                     3.3  (1.15)    4.5  (0.67)

Visibilty on scale 1 to 5; parentheses: standard deviation.

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:

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:

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 detecti on 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.

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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.

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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. birket.com/technical-library/144 –  Kurt E. Clothier May 13 '13 at 20:00

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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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, and right so. Stop using them!

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

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The question specifically asks for studies or data. Do you have data to support your assertions? –  KennyPeanuts 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
    
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
    
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 0:10
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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 3:46

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