There's probably no such thing as a 500lm rear light. You don't need that much light in the rear: rear lights are red, not white, so that power isn't spread out across the visual spectrum the way white front lights are. 50-150lm is more than enough to be seen in the daytime, let alone nighttime. Also, don't forget that human vision is non-linear in its perception of brightness; you don't need as much power at night to be seen with a red light as you do in the daytime.

Finally, compare to car lights. I'm pretty sure a standard halogen automotive headlight isn't even 1000lm, and those are for vehicles traveling over 65mph.