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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?
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.
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.
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.
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.
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:
- […]
- 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:
- […]
- Flicker does not contribute significantly to the conspicuity of rear lights.
Other conclusions:
- […]
- 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.
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
