The brightest bicycle lights out there are blinding indeed. When faced with an oncoming 1000-lumen mini-star aimed straight at my forehead I can't see the trail, the pedestrians, or the hand in front of my face unless—well, unless I put my hand in front of my face for shielding. And the problem with a bike trail is that it's difficult to illuminate that narrow ribbon of pavement in front of you without also illuminating the oncoming traffic.
Biker behavior varies. North of the UW, most riders sporting bright headlights will shade them as they approach each other, though it's hardly universal. But on the southern stretch between Fremont and the UW the ambient lighting is bright enough that oncoming headlights aren't totally blinding, and no one bothers to shade.
As usual in the biking culture, aberrant behavior invites vocal criticism. I'm no exception, having informed a great many passing riders that their light was luminous to the point of discomfort, and would they mind ever so much shading it upon subsequent encounters. Actually, my wording might have been somewhat more terse. Although I try myself to err on the side of shading more than I need to, I've also been occasionally chastised for my 200-lumen Cygolite Hi-Flux LED headlight, which is aimed, I admit, somewhat higher than necessary (I'll explain why later).
A recent Cascade Bicycle Club blog by Miss Panniers gives some bicycle lighting guidelines that are eminently reasonable, though the online version doesn't discuss whether to shade or not to shade, as the print version did. The print version claimed that if your light is properly oriented to illuminate the surface in front of you, it will not be blinding to oncoming traffic. Since shading my light is no fun and even a little dangerous (try hitting a pothole while holding on to the handlebar with just one hand), I thought I'd put that assertion to the test.
I turned off all outside lights this evening and parked my bike such that its headlight was aimed straight down my driveway. Then I walked down the driveway and turned and looked, and it was, in fact, rather blinding. I then readjusted the light to point more downwards, walked back out, and the light seemed less blinding.
All well and good, but that was kind of unsatisfying. How much less blinding was it? I needed some way to measure. So I set up a camera on a tripod and took some pictures (f2.4, 1 second exposure). The camera was about 35 feet away from the headlight. Here are the results:
Glare when the light is aimed straight ahead
Glare when the light is aimed at the pavement about 20 feet in front
Glare when the light is aimed at the pavement about 12 feet in front
Clearly there's a massive difference between aiming the light straight ahead (top picture) and aiming it downwards. There's less of a difference between the two downwards-facing cases, although aiming the light at a spot 12 feet in front is noticeably better than aiming it 20 feet in front.
I don't know where I originally had the light aimed (I neglected to measure it before I started fiddling with it), but it was closer to 20 feet than 12, and probably even farther out than that. The reason I had it aimed so far out was that the first part of my commute is a rapid sprint down steep, dark Perkins Way in Lake Forest Park on which I effortlessly achieve speeds of 25-30 mph. A light aimed 12 feet in front of me would give me all of 0.27 seconds to react to upcoming potholes, downed branches, and crossing squirrels and raccoons (all of which I've seen). That's simply not enough time. I need at least 25-30 feet of warning.
This raises a dilemma. I want to be a good bicycling citizen; but I also want to be safe. Neither re-aiming my light to 12 feet nor shading my light and riding one-handed are safe for my particular commute. A better option would be to have a way to click from one setting to another, but my light doesn't offer that. Another possibility would be to build some kind of slideable shade; I may try that (though my mechanical abilities are nothing to shout about).
In the meantime, I'm keeping my light fastened loosely enough that I can manually re-aim as needed. I'll try aiming it outwards while I sail down Perkins, and then aiming it downwards once I get on the trail. If it turns out I can get away without shading my light, then I'll look for a more permanent solution. However it turns out, I have no doubt my friendly fellow-commuters will let me know how well it's working.