JohnJ80

Car headlights are 700 lumens on low and 1200 lumens on high (halogen, LED are much brighter) - on that you're wrong. Cars have two of them. Here is a source for that information and here's another. In point of fact, a 9004 Halogen bulb (very common car headlight bulb and is around 1700 lumens). So, at 500 lumens, we're not even close on a bike.

Oncoming motorists are not now, nor have they ever, been blinded during the day by headlights during the day. Consequently, if we use this demonstrably safe number as the upper limit, 100 lumen tail lights and 500 lumen head lights are not going to be blinding anyone much less during the day. At 100 lumens, we have more than ample safety overhead. In point of fact, we're approaching almost an order of magnitude of difference. With a 500 lumen headlight on a bike, we aren't even near what a single car headlight puts out.

At 4m, if I blind someone with my wimpy and almost useless 40 lumen taillight, I have other far larger problems. Besides that, lux - the amount of lumens square area decreases as the square of the distance away from the light source. So if I go to 8m, the effect is 1/4th as strong. But other than that, go take a 40 lumen light out on a 45 mph road and see how far away it's useful. The vast majority of them are almost inconsequential at 500' (about 6-7 seconds of warning for a motorist at 45mph). Granted, better optics help a lot and really help a lower lumen light punch above it's weight for sure.

But here's the thing: As a cyclist, I'm far FAR more vulnerable than that motorist in their 3000 lb car. As such, if my little lights are perceived as annoyingly bright (it's an impossibility that they are blinding during the day), then my safety trumps their annoyance by at least an amount large enough to be in the "don't care" range.

I've been riding with headlights and taillights on bikes for the last 20 years. I've been riding with both during the day for the last 3 years. I can tell you based on experience of both following at other cyclists with whom I'm riding at a distance, and by observing motorist behavior with respect to me, that these are the level at which significant change happens. Some lights with better optics (only a very few lights) can do this with less, but in general, I think the rule holds.

And to put a point on it - no one is getting "blinded" with a wimpy 100 lumens during the day. If that were the case, we'd have had cars in ditches and head on collisions from blinded drivers for decades simply because that's about the output of car taillights and it's less than 1/5th the output of a car headlight. Motorists have been experiencing daytime driving with headlights for decades without consequence.

You can add to the visibility of the light by flashing or pulsing - on that agree and it also agrees with the research that Bontrager relied upon for the flashing interrupter patterns their Flare series of lights use.