...also because some of the roads I ride on are twisty narrow strips of asphalt out in the country, where there's just enough space for 1 car and 1 bike side by side, and ditches on both sides - I'm concerned by the risk of blinding an incoming driver and end up in the ditch because of too much light instead of too little.
All my neighborhood streets are like that except in the new subdivision, as are all county roads. I understand not wanting to blind an oncoming driver (often a neighbor) into becoming an incoming driver.
It isn't so much a question of too much light output as too much spill (light where you don't want it) and not enough light on the road. When LED outputs were marginal, or to boost runtime by running lower currents, getting all light produced into a long and narrow beam as the German standard lights do, actually makes for a more useable light than many circular beams at higher power that send too much light above the horizon and to each side. Have you looked at
www.peterwhitecycles.com beamshots of different lights he sells? That will give you an idea of brightness and beam shape of German standard lights and some others meant for mountain biking but not road use. swhs's blog is another good reference.
Another thing to consider is being seen. Drivers on cross streets here often don't come to a full stop (try to avoid it like the plague, actually), so don't look well enough to notice that a cyclist is approaching at speed. A narrow beam helmet light that points where you look, can give you a long distance beam to announce your approach and a steerable one to make sure drivers entering the intersection are aware you are there. Since they have not launched they can avert their eyes, and if you do dazzle them a bit to save them running over you, it does both of you a big favor. If you have surprised them, they should get that concept. You dip your helmet and look right to be nice to oncoming traffic. Works a treat. They yield as they should.
If you have two small hot spot beams, one tighter say, 5 degrees than the other, say 10 degrees, you can aim the larger one closer and the smaller brighter one, farther, making sure the hot spots are on the pavement. Both will be about the same size on the pavement, if aimed right You may need to adjust the far one on grades. You can use hoods to cut the wasted above horizon and excess side spill to be kinder to oncoming traffic:
A video of them in action (from about 3:00 minutes on the main lights are on low and the helmet is on high, and shows the beams best on the video. The sign lighting is my helmet light which I had aimed a bit high in this video.):
http://imageshack.us/clip/my-videos/545/2cg.mp4/
The output of the headlamps on low is 1100 lumens, and I am controling maybe 10% of the output, so about 1000 lumens on the road. 1400 on high (for wet pavement), and the helmet is about 600 lumens in a tight beam on high. Total output is in the range of a car's low beam. The camera has a sensitivity cutoff so the road is better lit out in front for farther than shown and the near hot spot not as bright. I can see sticks and potholes now, but I have 'older' less sensitive eyes.
The side spill control is so effective that they aren't as good to allert cross street drivers when used as DRL's in the day.
Take home story is that a low power light spraying photons all over, blinds oncoming drivers better than it lights the road. A high power light that does that is a traffic hazard, is the dark side of the force if you will. As Yoda said, "Control! You must learn control!" So I got the narrowest beams I coould squeeze into the housings, and tamed the rest. The German standards allow a bike light less light above and to the sides than is permitted for vehicle lights, maybe because of close proximity of cyclists meeting on 2-way bike paths, and limit total output below what I think is safe to ride the shoulders of high speed roads here in my part of NA. Being able to change output if you have situations where 'less is more' is a very good option to have. So if we had nice bike paths in town I can power down to glowing embers, if I want.
Buy the best light(s) you can afford. Figure on an upgrade in a year or two.
BrianMc
