This is the reason I ride with such ridiculous taillights. I figure that there are people out there glancing at the road maybe for a half a second once every 4 or 5 seconds. I need to not just be visible but rather so bright it's effectively impossible to ignore, and it needs to be so from 10 seconds away, or about 700 feet.
I went through a lot of the lights on the market, but eventually I gave up and bought a Designshine.
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Work: the 8 hours that separates bike rides.