Those lights you see may not be as bright as you think. They probably just have a proper lens to focus the flash as it need to be. So don't worry so much about lumens.
Just get something that says it's intended for daytime flash. I have some very old rechargeable Bontrager Flares. The ones that are just little somewhat square little cubes. They seem to fit all my daytime needs.
IMO, when distant you just want enough flash to let others know you are there. When close up, you don't want to blind them so they run into you because you disoriented them with a bright flash. Besides, for close up, I feel bright clothing and bicycle that doesn't blend in with the surroundings does a better job.