Planet Bike, Serfas and a couple others make rack mount rear LEDs that are bright enough for nighttime and dim daylight use, and incorporate red reflectors as well.

I have the same Planet Bike Rack Blinky 5 shown above and it's quite bright within a narrow aperture of view -- directly behind the bike, level with a typical car driver's eye line. Subjectively speaking it's as bright as a typical car taillight, but with a much smaller area of glow. So two or more would probably be safer, as well as giving drivers a better visual indication of relative distance and speed.

I like the lights that run on AA or AAA batteries -- they last much longer between battery changes, compared with my USB rechargeables. But I'm planning to add a Cygolite Hotshot 80 or better -- those are bright enough to be seen in daylight too.