Or, Why This Is A Bad Approximation At Best
Ten feet?? A commuter cyclist riding at 10 mph covers ten feet in about two thirds of a second. Assuming Bicycling has its numbers right on light range, 2/3 of a second is barely time to think "oh, shit" as you note the pothole or piece of lumber you are about to hit.
There is a lot more to lighting then lumens and more to this story than lumens. For one, the shape of the beam and thus the focusing lens determines how the light is distributed, which affects lux, which is actually more important. Lux is a measure of how brightly something is illuminated, i.e., lumens per square meter, so a light that puts all of its lumens in a narrow beam will illuminate brightly for a longer distance but leave anything west or east of dead ahead in blackness. A well shaped beam concentrates illumination where you need it rather than in the eyes of oncoming traffic or out into space.The link from the Appalachian Mountain Club used the example of a full moon illuminating the earth to about 0.25 lux. Likewise, not all of us have equally good night vision and some of us need more lux than others. Luck, too. So let the buyer do some serious R and D before purchase.
But just taking that little graphic in the corner of pg 26 at face value, what can one say about how much light you need to get out to a distance of say, 1 to 3 seconds. 1 to 3 seconds is the broad measure of human reaction time in traffic. So what really matters to avoid that pothole is to be able to see it in time to do something besides hit it. I put Bicycling's range vs. lumens into a graph and used that relationship to solve for a 1 to 3 second range at various speeds. This is what I got.
|Bicycling's 3 points put on a graph|
|At a given speed (abscissa), the number of lumens (ordinate) will, in Bicycling's model, illuminate the road 1 to 3 seconds ahead|
Mind you, this is more of a thought exercise and not hard and fast advice, but if you are thinking of buying a light (and Fall is coming up fast), you need to think about how fast you are going, the roads you are on, and therefore, how far out you need to see. You also need to think about whether you need a light that distributes light side to side (to avoid those raccoons darting out from the side of the road, etc). That is a highly individualized decision and goes far beyond "...should work for most situations".