From: zowie@daedalus.stanford.edu (Craig "Powderkeg" DeForest)
Subject: Re: Personal strobes
Organization: Stanford Center for Space Science and Astrophysics
	<ZOWIE.93Jan6103814@daedalus.stanford.edu>
	<1993Jan06.193901.3271@i88.isc.com>
Date: 6 Jan 93 22:05:28

Me: (with two prominent typos and lots of falutin' jargon)
   For a given time-averaged luminance, a strobe
   light of any color is much more visible than a similar-luminance DC light.
   That's because (A) you can get much higher (momentary) luminance with a 
   strobe, and (B) your visual system percieves rapid changes (light flashing
   lights) differently, and more effectively, than static objects.

jeq: 
   If you have a gas discharge strobe, yes, you'll get more output because
   you're integrating the input power.  With LEDs, you're just switching on
   and off, reducing average power.

egreen:
   [equallly relevant comment about bulbs deleted]

Yep.  We're all right. of course.  A reason flashing lights are better
for cyclists than is DC, is the problem of how long the darn batteries
last.  So, by strobing, as jeq points out, you get a lot more
high-visibility duration for the same Energizer buck.

Of course, as jeq and egreen pointed out, you have to use a brighter
LED or a bigger bulb to get the higher luminance.  But you don't have
to carry a big honkin' battery around.

Also, there *are* studies that show that flashing lights are more visible,
both in terms of being more attention grabbing and in terms of being more
discernible in poor visibility.  I can't remember the ideal frequency, but
it's probably somewhere between three and ten Hz.  You can actually see
things better (ie percieve a dimmer average light) flashing at these
frequencies.  [sorry to not have a reference handy for this assertion;
I'll try to find one later...] 

And, as egreen mentioned, flashing lights *do* tend to distract attention,
and probably *are* illegal.  

I just checked through my handy CA Vehicle Code, and there are lots of 
provisions for flashing lights on motor vehicles (and a blanket prohibition
for cases not covered), but no mention of flashing lights on bicycles
at all.  So they're probably legal, since the blanket prohibition only 
applies to motor vehicles. 

jeq:
   The few times I've been behind a bicyclist with one of these strobes
   at night, I found that a) it was difficult to estimate range, because
   the strobe was the _only_ thing he had - no taillight, no reflectors,
   no light clothing - and b) because of (a), I tended to fixate on
   the cyclist and when I finally saw him, I was driving closer to the
   shoulder than I would have been had I not seen him at all.  Please, if
   you use one of these things, use it in combination with other
   conspicuity (non-tm) devices.

Excellent point!  I hate that.  BTW, The CA veh. code is pretty
specific (sec. 21201 (d)).  You need:
     (1) a lamp emitting a white light which ... illuminates the
highway in front of the bicyclist and is visible from ... 300 feet in
front of ... the bicycle;
     (2) a red reflector on the rear which shall be visible from a
distance of 500 feet to the rear when directly in front of [hi-beams];
     (3) a white or yellow reflector on each pedal...; and
     (4) a white or yellow reflector on each side in front of the
center of the bicycle, and a white or red reflector on each side to
the rear of the center of the bicycle.

So rear flashers clearly don't cut it alone.  

Of course, this is pretty far afield from motorcycles, which fall under 
the blanket prohibition.  

Oddly enough, you *are* allowed to put an amber flashing light on the back
of your motorcycle, that flashes at a frequency proportional to your rate
of deceleration!  

--
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