Could I blind oncoming drivers?
I have a helmet mount very similar, and yes its totally possible to dazzle and annoy any other road user, from pedestrians to other cyclists to motorised vehicle drivers.
Benefits to a helmet light
- Its high up above the ground and higher than handlebar lights, so other road users can see it over cars.
- The beam follows your head, so you can light up the thing you want to see.
- Retro-reflectors everywhere! Since the light source is near your eyeballs, retro-reflectors work really well and from a very long way away. A sweep across a dark road will illuminate all those car bumpers and street signs and mailboxes and jogger's jackets and sports shoes.
Disadvantages of and risks due to a helmet light
- Drivers might get dazzled. I've had a bus driver yell at me from his rolling bus because of my head light. Admittedly he was more annoyed by the flashing, so after some research I decided to leave it on solid.
- Retro-reflectors can be dazzlingly bright when lit up at close range, destroying your own night-sight.
- Snaggage risk - your torch/flashlight is a fairly respectable-sized hook and can catch on low hanging branches. I've caught mine on a washing line at low speed and basically "clotheslined" myself with the chinstrap.
- Helmet Function may be compromised by the holder. Michael Schumacher's skiing accident was exacerbated by the gopro camera mount that focused impact forces on one point rather than spreading them over the head like a helmet is supposed to. If you were crashing, then the mount should snap off. What if it didn't snap, or if you impacted vertically onto it?
- Weight - torch batteries are dense and the added weight can be noticeable.
I have mine mounted fairly far back on the helmet, so the torch front barely clears the top. This reduces snag risk.
The mount is held on by a minimum of velcro, and there's a separate hand strap used as a second line to stop it getting lost.
As mentioned, I use it on solid mode now, however the focus is set fairly tight. At night my normal posture puts the lights on the road surface around 4-5 metres in front of my wheel. I tend to look up with my eyeballs rather than my neck.
However, if a car presents in a manner that crosses my path and may require them to give way (to yeild) then I will point the beam below the driver's door. If I'm passing a parked car that has a person moving in the drivers seat I might light up their wing mirror.
I'd rather annoy someone than get doored by their inattention.
While I won't deliberately dazzle anyone or anything, my priority is everyone using the road arrives home safe.
EDIT Here's a pic showing what I do, for clarity.
Have a friend wear your helmet with the light mounted on it like you planned. Then go to a small residential street at twilight and see for yourself whether it is too bright. Bring a car and try passing each other a few times, both in the twilight and when it gets completely dark. If it blinds you, see if dimming the light or aiming it down helps. If not, find a different light.
Problem with most flashlights is that they have a circular and symmetrical beam, ie they throw as much light up as down. This is good for MTB as you want to see branches flying into your face. For a bicycle, this is not what you want. You need special optics, which the Germans normalized (google StVZO bike light).
Result is something like this (as an example):
Notice the beam's sharp cutoff, very little light is sent upwards, to the eyes of incoming people. This is what you need.
Now, you should find a light of this type, small and light enough to mount on your helmet.
I began to rather dislike helmet mounted lights on oncoming cyclists. Fortunately I don't see them very often riding in a city. Yet from the dozen or so I met last Spring and the Autumn before two brought me in uncomfortable situations.
In both cases I was descending on a bi-directional multi use path (1% slope) at moderate speed (25 to 35 km/h).
In the first case a cyclist quite a bit down the way was looking at me and shining their light directly into my eyes. Opposite traffic was moderately dense cyclists were overtaking. Being dazzled I could not tell how much room I had to the left, and went as far to the right hand edge. However, I had difficulty identifying the edge of tarmac as well. I went a bit too far, onto wet fallen leaves and rough ground, yet could stay stable.
In the second case a slow rider with a head mounted light cycled that path up. A rider behind with weak or no light was preparing to overtake. I mistook the high mounted light of the first rider for the light of the second rider and did not notice that they were already overtaking and I had to move to the edge of the way. This led to a very close, fast path with the second cyclist.
Much more often I am dazzled by badly adjusted, very bright, fixed bike lights. This is especially a problem early in the Autumn when the weather is still fair but the sun sets early. Lots of people are still riding but are inexperienced with their lights. As soon as it gets cold, only experienced riders are left (or at least they were yelled at often enough to fix their lights.)