Thursday, June 30, 2011

Should You Flash Motorists? The Moth Effect and The Bicycle

Nowadays many Chicago bicyclists equip their bikes with red rear flashing lights that alert motorists coming from behind of their presence.  This is a good thing.  Generally, when a motorist sees a bicyclists he or she will make every effort to steer clear.  Illinois law does not require rear facing lights on bicycles.  Only front headlamps and rear reflectors are legally necessary.  However, taking the extra step of equipping both the front and rear of a bicycle with lights is an excellent idea.  There is some debate, however, regarding the best and safest way to illuminate the bicycle rear:  Should the rear light blink or remain steady?

The debate arises from something dubbed the "moth effect."  Studies considering when emergency vehicles should and should not utilize flashing lights at a crash scene have sometimes demonstrated this effect.  Like a moth to a flickering flame, a human being behind the wheel will be attracted to a light blinking in the darkness.  The implication is that blinking lights on vehicles, and on the back of bicycles, may be more dangerous than steady lights.  Rather than alerting and repelling the motorist, a flashing light may actually draw the approaching vehicle to the light's source causing a collision.  Apparently, steady lights do not have such an effect.  There is little science, however, that supports the existence of this supposed effect.  According to James D. Wells Jr., who conducted a comprehensive 2004 study of the moth effect, "There are no known studies that have not been disproven that substantiate the actual existence of this effect in real world driving."  Furthermore, even if there is some evidence for the moth effect in the emergency vehicle context, there are no studies I'm aware of considering its application to bicycles, which generally do not emit nearly as much light as say an ambulance with all lights a-blazin'.

In a dense urban atmosphere bicycles at night at competing to be seen with a lot of illuminated objects, i.e. cars, street lights, flashing pawn broker signs, etc. Moreover, bikes are small in comparison to other vehicles on the road and bike lights can offer relatively little candle power compared to these other illuminated sources.  It seems that using a flashing light would help the bicyclist been seen best in the urban road discotheque.  The bicyclist should, of course, make up his or her own mind on the subject.  Either option is perfectly legal in Illinois.  I recommend reading an excellent treatment of the moth effect by human factors expert, Marc Green, complete with study references.


  1. This is why I use both. I run a PlanetBike Superflash in steady red mode (it's bright), and two PlanetBike amber Blinky3s in flashing mode (think hazard lights).

    Can the rear light pull double-duty as the reflector? I took my reflector off years ago as useless in the presence of my lights.

  2. Kristian,

    Yeah, I've done the same with my rear reflector. If you've got a functioning rear light, a rear reflector is probably unnecessary.

    Thanks for the comment and question.

  3. Interesting posting. I read recently of the phi phenomenon and now ride with two flashing Radbot 1000s but with them flashing in two different modes.

    I had had to drive a little bit of late at night (winter here) and one thing I noticed was that I easily lost sight of cyclists whose front lights, whilst bright where not flashing. This caught me by surprise as I had been of the view that steady front lights where better. These where cyclists BTW with very bright lights, not the basic "i hope you can see me" type so maybe that makes a difference.


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