Originally Posted by q`Tzal
To dovetail upon Starkmojo's comments on inadequate lighting:
inadequate lighting is any lighting equipment that does not help to distinguish the cyclist from the objective visual background of a homo sapien. Our eyes are not perfect and biological shortcuts save on construction costs by assembling an image that is representative not accurate. Night vision is dependent upon contrast and that is what is sorely lacking. ...
The above excerpt describes part of the problem for visibility, people on the road, biking without lights and/or other visibility gear present. Without something to make distinct contrast between themselves and the background upon which they're seen by other road users, people on bikes can visually disappear even when in plain view by road users that are traveling with consideration for the conditions and concentrating on the road ahead. Depending upon what they are or aren't wearing, and what their bikes are or aren't equipped with, the effective image they present to other road users can be like camouflage.
Riding on a street with lots of street lighting, making it possible to see the road ahead well enough to be able to see the road ahead without relying on a headlight does not necessarily mean ambient light is sufficient to allow other road users to be able to adequately distinguish bike road users on the road from the background they're seeing them against.
It's probably easier to see this phenomena as a passenger in a car than as a driver. Lately, I've had a few occasions to do exactly this. That experience strengthens my appreciation for cyclists that run more than the minimum state legal requirement of front light and rear reflector: such things as, adding rear tail lights, multiple lights front and back, reflectors and reflective material of a wide range of shapes and sizes...and of course, hi-vis colors. Any of those things help a lot to have someone on a bike be far more readily distinguishable from a roadway background than someone on a bike with the minimum required front light and rear reflector.