See and be seen: What every cyclist needs to know about daytime running lights

Daytime running lights header

Visibility on the road was once considered the antithesis of coolness on the road. However, growing safety concerns revolving around driver inattentiveness have quickly changed those attitudes, with fluorescent-colored clothing and bright flashing lights suddenly the norm for hardcore roadies who are more concerned with making it back home alive than Strava KOMs.

The thinking behind this seems obvious enough — more visibility should equate to more safety, right? However, not everyone is on board with day-glo clothing, and changing up an existing cycling wardrobe can be prohibitively expensive. By comparison, compact LED flashers are both fashion-neutral and far cheaper to purchase.

But when it comes to lights, there are few guidelines on what’s actually effective, and what isn’t. How bright is bright enough? Can a light be too bright? What’s best, steady or flashing lights? Answers aren’t uniform, but the information is out there — and it might just save your life.


Canadian experimental psychologist Dr. Marc Green is an expert in visual perception — how the human brain processes incoming information gathered by our eyes. According to him, our retinas don’t simply collect visual information and then pass it on to be analyzed in totality.

Such a cache of data would be so monumental that it would overwhelm our brain’s ability to evaluate it. Instead, visual perception is essentially a matter of resource management.

“A fundamental property of human nature is its ability to adapt and to improve efficiency by making behavior more and more automatic (requiring less attention) through experience,” he wrote on his web site, Human Factors. “At every opportunity, people switch behavioral control from the conscious and resource-draining processing of attending the light entering the eyes, to an automatic and unconscious mental model. In a sense, we develop packaged subroutines that we can trigger and forget because they run off with minimal supervision.

“In other words, we act primarily on expectations. When viewers are in a familiar situation and/or engaged in a familiar behavior, they rely more on stored knowledge in the mental model.”

Daytime running lights 2

Put into the context of operating a motor vehicle, familiar and repetitive events — like driving on well-known roads and during regular hours of the day with consistent traffic patterns — can practically be interpreted by your brain as insignificant background noise. Consider the last time you moved to a different address. How many times did you drive to your old house after work by mistake, seemingly on autopilot? And when you got there, did you even remember doing it? Could you recall what you saw along the way?

People are also conditioned to look only for the things that they judge to be most applicable or important at the time — something commonly referred to as “selective attention.” On the road, this means that drivers of four-wheeled motorized vehicles are primarily looking out for other cars and trucks, not motorcyclists or cyclists. Because of this conditioning — as well as the unconscious mental model to which Dr. Green refers — drivers may see, but not perceive those other road users.

This phenomenon can be easily illustrated by those popular “invisible gorilla” videos that regularly circulate on the internet. Despite someone in a gorilla suit walking in plain view across the screen, roughly half the people watching the video for the first time don’t see it.

Drivers on the road only see those people passing the ball around. If you are a road cyclist, guess what? You’re the invisible gorilla.


Now, what if you attached a light to that gorilla? Would you notice it then? That’s the thinking behind daytime running lights (DRLs), which were first used on four-wheeled motorized vehicles in various forms in the late 1970s, with more widespread adoption occurring two decades later.

More than enough time has passed since then to evaluate the effectiveness of DRLs at reducing the frequency of crashes. Although exact figures vary depending on region and study specifics, most investigations have concluded that there is at least some benefit to vehicles using them on the road, generally hovering around 5-15% reductions in crash frequency.

That same line of thinking is what has driven current trends in cycling visibility, and the theory makes sense when viewed in the context of selection attention and Green’s visual comprehension theory. By more acutely standing out against a background, an observer (a driver, in this case) is more likely to see you.

Research is backing up that theory, too.

A Danish study published in 2013 compared the accident rates of 3,845 cyclists — roughly half equipped with DRLs, the other without — over a 12-month period, and concluded that the former group was significantly less likely to be involved in an accident than the latter group.

“The controlled experiment gives evidence that the use of permanent bicycle running lights will significantly improve traffic safety for cyclists due to the improvement of visibility,” the authors stated. “The results of the project indicate that the occurrence of bicycle accidents with personal injury to the cyclist is 19% lower for persons using a permanent bicycle running light than for persons not using a permanent running light.

“The results indicate that the bicycle running lights may reduce the occurrence of multi-party bicycle accidents involving motor vehicles as the counterpart, as well as the occurrence of multi-party accidents involving other cyclists and pedestrians as the counterpart. The safety effects are especially related to daytime multiparty bicycle accidents.”

Daytime running lights 3

A study recently conducted by Clemson University offered similar conclusions.

“The fundamental finding from this study is that bicycle taillights can provide significant conspicuity enhancements during daylight conditions,” stated a presentation given by Clemson researchers ahead of the paper’s publication. “In all three manipulations (Placement, Intensity, and Mode), at least one configuration that included an active taillight was rated as being significantly more conspicuous than when the taillight was either absent or turned off. This finding is encouraging in that it indicates that, even in daylight, active lighting can improve the conspicuity of bicyclists viewed from the rear.”

But as far as the lights themselves are concerned, there isn’t sufficient consensus (at least among the bicycle industry) on the technical specifications on what constitutes an effective DRL. How bright is bright enough, for example? And what sort of flashing patterns should be incorporated? Moreover, there are currently no restrictions on what can and cannot be described as a daytime running light.


When it comes to brightness in relation to daytime visibility, it seems intuitive that more light should be better than less. But how bright does a light actually need to be in order for it to be visible to other road users, and what metric should be used to measure that?

In the cycling industry, light brightness is most commonly reported in terms of lumens. However, that metric only describes the total amount of light produced by the source, not how it’s directed. Take a 500-lumen light bulb and a 500-lumen flashlight, for example: both emit the same amount of light, but the flashlight will be visible from a greater distance depending on how it’s aimed.

Alternatively, lux measures how those lumens are distributed on a surface, with one lux defined as one lumen per square meter. Using that same light bulb and flashlight analogy, it’s easy to see how they could have identical lumen ratings but vastly different lux measurements.

When it comes to bike lights, lux arguably provides more useful information in terms of real-world brightness and visibility, but it’s rarely used in the industry as it’s both more difficult to measure than lumens and also somewhat trickier to understand given the increased variability in how lux is measured.

“Lux is very hard to define properly, but it would describe the specific beam pattern more effectively,” said Aaron Murano, worldwide R&D manager for Lezyne. “For lux to make sense, you would need to specify the angular position in the field of view and the distance from the light. This gets very hard to compare and can be extremely misleading.”

Either way, few will argue that — all else being equal — more light is better, but even so, there’s little agreement on how bright is bright enough to provide increased safety over not using a daytime running light at all.

Lezyne’s Zecto Drive rear LED light, for example, offers a modest 20-lumen claimed output in “daytime” mode, while Cateye aims its new Rapid X2 Kinetic LED rear light for riding during daylight hours with a rating of 50 lumens. Other rear lights, such as Lezyne’s flagship Strip Drive Pro, can triple that output.

Is the Lezyne not bright enough? Can a light be too bright?

“It’s very difficult to say,” said Philip McAleese, CEO of See.Sense. “You need something that provides good contrast against you as the cyclist, and the background. As a rule of thumb, we think 100 lumens is a good starting point.”

Some companies have resorted to a more straightforward measurement — visibility distance. See.Sense claims their highest-power DRLs can be seen from up to 2km away, for example, and Knog says its top-end 44-lumen Blinder Mob V Mr. Chips rear light is viewable from up to 1.2km away. Similar visibility claims are hard to come by from most other manufacturers.


The amount of light produced by a daytime running light is important, but it’s not the only factor influencing how visible that light source will be to other road users.

As mentioned earlier, that selective attention phenomenon conditions drivers to look out for other motor vehicles; and since they’re already looking for them, it makes sense that those daytime running lights will help them stand out against the background.

Steady daytime running lights, similar to what are used on motor vehicles, may not be enough to make road cyclists any more visible than if they went without. Remember that that selective attention phenomenon conditions drivers to look for other four-wheeled motor vehicles, so the odds are already stacked against us. Motorcyclists have used daytime running lights for years, for example, and that user group is all too familiar with how steady DRLs offer only modest protection against the driver that “just didn’t see you.”

Motorcyclists have since moved more toward flashing front and rear DRLs, and cyclists have followed suit.

“Brightness at the source does not necessarily mean bright where it is intended to be seen from; lumens are not everything,” said Applegate. “The lensing, flash pattern, and range of the light are equally important. When designing effective daytime running lights, you need an interruptive flash pattern that draws the attention of drivers. You want not just to be visible, but to be recognized. There is a difference.”

Some companies take a even more active approach to attracting driver attention. Whereas most lights use fixed, pre-programmed flashing patterns, some other ones — such as See.Sense’s Icon and Garmin’s radar-equipped Varia — incorporate sensors that detect approaching traffic and adjust brightness and/or flashing patterns in response.

That said, local regulations may dictate a specific flashing pattern, regardless of what research has demonstrated to be most effective — or, sometimes, no flashing at all.

German light manufacturer Lupine offers just a single rear light — the Rotlicht — that boasts an eye-popping 160-lumen claimed output as well as a built-in accelerometer and light sensor that adjusts the output based on rider movement and ambient conditions. Nevertheless, the company is legally restricted from specifically designating the Rotlicht’s highest-power mode for “daytime” use.

“Flashing lights seem to annoy people a lot more than steady ones, so it’s safe to say they make riders visible,” said Dan Haus of Lupine North America, a subsidiary of German-based light manufacturer Lupine. “Flashing lights are illegal in Germany, but in the UK, their road-vehicle lighting regulation mandates a slow flash.”

McAleese also stresses the importance of side visibility — especially in urban settings — which often gets overlooked in favor of more obvious light features such as brightness and run time.

“We believe the key factor is side visibility,” he said. “Many light focus into a narrow rearward or forward facing beam, yet being involved in a collision at these angles is fairly rare. Side-on accidents are much more common, so that’s where you need the light to be going.”


Manufacturer claims on cycling-specific daytime running lights vary wildly, and consumers would benefit from some sort of industry standard to provide a level of confidence that what they’re buying will be effective.

“There is a standard for daytime running lights for cars that specifies the brightness and distribution of light required,” said McAleese. “It might be a little over the top for bike lights, but a simpler version would be a great step forward for the cycling world.”

Such a standard doesn’t yet exist. But even if it did, standards don’t always guarantee optimum effectiveness. Case in point: A recent study by the U.S.-based Insurance Institute for Highway Safety found that many car headlights didn’t provide sufficient brightness in real-world situations, despite satisfying regulations.

So in the absence of a mandated standard for cycling-specific daytime running lights, or any sort of widespread agreement amongst the industry for what is and isn’t effective, what is a safety-minded rider to do?

I suggest a more pragmatic approach. Keep an eye out riders using lights that seem especially attention-grabbing, and find out what they’re using. (As an added bonus, such research could entail fitness-boosting intervals as you attempt to catch a beacon-equipped rider far ahead of you on the road). After all, chances are that if they’re more visible to you, they’ll also be more visible to a driver.

Alternatively, ask your local shop to mount a few prospective models outside, fire them up in their brightest settings, make sure they’re properly aimed, and then move progressively further away. Take note of which lights seem most conspicuous to you, and which ones are hardly visible in bright, midday sun, and then choose accordingly.

Barring that, I’d argue that it’s better to err on the side of caution. Although there isn’t much consensus on the specifics, there’s widespread agreement in general — purchase the brightest light you can comfortably afford, and opt for one with more complex flashing patterns rather than a simple strobe. Other factors, such as ease of mounting and charging, run time, and weatherproofing should also be considered; a DRL won’t do you much good if you’re not using it, or if it isn’t working when you need it most.

Either way, make sure whatever you choose is properly mounted. Dim lights may provide a false sense of security, but so, too, will a bright light that’s pointed in the wrong direction.

Thanks to Cycling Tips and James Huang for the content.