1 mile visibility is really easy to get. Any of the higher end taillights will do it. My $30 MagicShine will do it. There's a stretch of road about 3/4 mile long and straight, with bends at either end, and I have someone from work mention at least every couple of weeks when it's dark that they saw me about to disappear around the far bend when they just came to the straight stretch. I was bright enough for them to notice from that far away.

I'm actually thinking about getting an entire 2nd set of lights, both front and back, another set of MagicShines since I've been using them for close to 2 years now and the only problem has been the Y adaptor cable frays every few months and needs replacement, but those are $3 - and the new ones are heavier and don't appear to fray at all.