Originally Posted by
TimothyH
Laws aside, many people ride with lights on the back of their helmet or jersey and these are perfectly visible. The bike itself moves as one pedals and so there is little practical difference. Modern LED lights are visible to the point where motorist complain that they are too bright. I don't see visiblity of the lights as a problem.
???
Yes, bicycles move but that isn't actually much of a problem with lights that are set up properly. Nothing is perfect but what you are describing is often worse.
The issue is that mounting lights on places like seat bags often causes the lights to persistently be pointed downwards making them mostly worthless. Jersey mounted lights often have the same problem (though, those can be pointed uselessly upward).
And lots of people have no idea that their lights are set up badly.
Most people use lights that no motorist would complain about (that's mostly a made-up concern).
If the light is mounted on the bike, it's easy to check that it's set up reasonably and reliably. Lights in other places might be useful as a secondary but they are often less effective than a light mounted on the bike.