In practical testing, the NIteflux was not as bright as the Dinotte 300R, and the Quad Red is significantly brighter than the 300R, despite being rated at just 200lm. The Niteflux is maybe a 150lm lamp.

The City Slicker calls itself "500lm" based on a raw estimated luminosity of the pair of CREE LEDs is uses. In the real world, the light is putting out ~100lm.

As I mentioned, a legitimate manufacturer will state an accurate rating, not the "maximum theoretical output of the elements under the best possible conditions."

This is why a 70lm Lezyne tail light will in practice be brighter and more visible than a "300 lumen super bright tail light!!" from eBay.

There is no such thing as a 500 lumen bicycle tail light. An incandescent brake light on a car is generally ~400lm-- and will be orders of magnitude brighter than an LED bike taillight, because it's putting out a broader spectrum. LED car tail lights suffer the same viewing angle problems as bike LEDs, offset by the fact that they run on 12V, use significantly bigger elements, and a lot more of them.