For a long time, I've been pretty vocal about cyclists and safety - not their own safety, on which they're the experts, but their effect on others' safety. Most recently, it being winter and dark early where I live, I've been in a few discussions about the specific sub-issue of lights. Since I've put in some effort to learn what I can, I might as well share it. One thing I've learned is that it's very important to keep several distinctions in mind.
- Illumination vs. visibility. Illumination is about your ability to see hazards and obstacles, including but not limited to other people. Visibility is about others being able to see (and react to) you. Sometimes these are at odds.
- Headlights vs. tail lights. Headlights are primarily about illumination, though they affect visibility as well. Tail lights are entirely about visibility.
- Any lights vs. reflective clothing. Reflective clothing is mostly like tail lights, but subject to slightly different concerns. I'll get to those later.
- Individual vs. collective safety. Some things can be (or seem) better for individual safety, but be bad for safety overall. I put blinding or distracting other road/trail users in this category.
I'm going to address these issues primarily from the perspective of cyclists on mixed-use trails (like the Minuteman Commuter Bikeway near where I live), interacting with pedestrians. I believe there are some differences for riding on the road with cars when it comes to this particular issue, but I have a lot less knowledge or experience there.
Let's start with headlights. There are three basic factors that determine headlights' effect on safety: angle, brightness, and flashing vs. steady. Angle and brightness are mostly about illumination. How much of the path ahead can you see? Well, how much do you need to see? A lot less than many cyclists seem to think. On a trail (no cars), there are two cases to consider: are there others ahead of you, or not? Your reaction time plus stopping distance create an "unsafe zone" around you, which varies in size according to your speed. If there's significant cross-traffic or congestion you should slow down to keep that unsafe zone smaller. At such speeds, you do not need your light to project very far. The only thing high-angle lights will do is blind other people. Don't. Angle them down so that there's minimal light shining from handlebar height to walking eye height, even at the edge of your circle.
The other case is when you have a clear path ahead for the moment, and need to make sure it stays clear. How far should your "horizon" be? The three-second rule works for bikes as well as for cars. At 20mph, which is faster than most cyclists, that works out to about 58 feet. The best information I can find says that actual stopping distance (not counting reaction time) is about 25 feet without risk of skidding or overloading the front wheel so you do a header. Add them up and that's pretty close to the "25 yards" one interlocutor claimed he needed, though he was talking about congested conditions so he was Just Plain Wrong anyway (and I'll bet he never gets up to 20mph either). But hey, let's be generous and pad it out to an even 100 feet. That's the distance at which your headlight should be casting barely any light on the ground. You should use a light that fits within that limit, or angle a brighter light further down if you want to see closer things more clearly. Those 600-lumen headlights pointed right at onlookers' eyes, which I unfortunately encounter all the time, are not safety aids. They're weapons.
OK, we should be riding downhill from that contentious topic. How about flashing headlights. Again, no. Here's the thing about flashing lights: they create less illumination, but higher subjective brightness for onlookers. Bad for your vision, bad for their vision. Yes, they're more visible, but there's practically no chance of that being an issue from the front anyway. Flashing headlights are just bad, which is why many places ban them. They should.
For tail (or side) lights, brightness is still a concern but less so than for headlights. Remember, these are for visibility, not illumination. They don't need to illuminate anything, though they do need to be visible from a distance through rain/snow/fog. Flashing is also an issue, but for a more complex set of reasons than with headlights. In addition to increasing subjective brightness, flashing lights are also more effective at drawing attention. That's why so many cyclists swear by them, and those are good things, right? Not so fast. Flashing lights also make it harder to gauge distance and direction, and that's bad. While making sure others see you at all is the most important thing, making sure they avoid you can be nice too. This equation also changes significantly when you switch from riding on a road with cars to riding on a trail with pedestrians.
That brings us to color, and a tradeoff of visibility vs. vision. Green or yellow provide the greatest visibility, for reasons related to the prevalence and sensitivity of the different cone cells in the eye. They can be seen from the greatest distance (they also penetrate fog etc. the best), but that means they're also most likely to exacerbate any brightness problems. Again, how far do you need to be seen from? No more than the same 100 feet mentioned above. Not on a trail, anyway. (This is one area where road riding is probably different, because cars are moving much faster and have much longer stopping distances. On the other hand, the combination of reflective clothing plus their lights probably creates more visibility than any light you can project, and anyone who's driving at night without lights is probably so much of an idiot that they won't notice your light anyway.)
For a mixed-use-trail situation, red or orange lights provide thoroughly adequate visibility, and preserve others' vision well for the same physiological reasons mentioned earlier. Blue is just bad. It's very hard to have a blue light that's bright enough to create good visibility but not so bright that they cause overstimulation and blindness. I really hate the way blue LEDs are overused in electronics, and they're not much better on bikes. White is the trickiest. Mostly it's similar to green, but has the additional advantage of allowing color discrimination. Scraped your knee and want to see if that's dirt or blood? Good luck with a green light. Want to see all the colors on a paper map? Good luck with a red light. I think it's a good idea to have a white light on you somewhere, but not for visibility. Keychain lights are great, and fit easily into a repair kit.
So, what do I think people should do? Well, definitely not a bright flashing green light. A moderate red light will provide all the visibility you need. Shifting toward orange is even OK. If you have to pick just one, flashing is probably the way to go despite the distance/direction issue, but here's the thing so many cyclists seem to overlook: you can have two. A steady red and a flashing red seem to provide all of the benefits with none of the drawbacks, as long as the overall brightness is reasonable.
Reflective clothing is cool. I love it. It's also generally good for safety. For trails and (definitely this time) even more so on roads, it's really hard to go overboard. The drawback is that reflective clothing relies on light coming from somewhere else. While it's criminal for someone to drive at night without lights, and probably unwise for someone on a bike, it's not all that unreasonable for pedestrians to do so. More importantly, even if you disagree, it's too common to dismiss. If neither of you are emitting light, even if you're both wearing all sorts of reflective stuff, you'll still be nearly invisible to each other. So you can't rely on reflective clothing alone. Wear as much as you like. Even a full vest/jacket of super-reflective white or green (I have a soft spot for Illuminite because I used to work right above them) is no worse than a street sign when illuminated by a passing car's headlights, and unlike the dazzlers some people mount on their handlebars the moment tends to pass quickly. You'll look dorky, but not dangerous.