Some folks don't know/realize how invisible they are to others. I see this all the time on my walks and rides on our trail system.

How about a friendly - "I would be able to see you, and it would be safer, if you had some lights" I do this, and sometimes get results.

When I walk in the dark, I often strap on a red rear flasher, as well as a flashlight, and a couple of other walkers do, also.