"Two is one, one is none".
This thread has been an interesting read.
I used to keep basically one or maybe two crap lights in the car, I mean, it hardly ever gets used so I didn't want to "waste" my good lights there....until I had to repair a radiator hose in the Catskill mountains in pitch black during a freezing rain.
Now each of my vehicles has an
Energizer ELMCL21L (for general kick around and long duration use), an older Surefire G2 Nitrolon, and a Princeton Tec EOS headlamp. All of which are loaded up with Lithium batteries because my area does get cold in the winter. I also have a battery carrier holding a spare set of AA Lithium, CR123, and AAA lithium batteries.
Additionally I keep 5 road flares, a DOT approved safety vest and gloves, and a traffic wand that fits on the Nitrolon. If all of that seems like a lot you should keep in mind that my area is mountainous, curvy, and often foggy and you need to lay warning signals a significant distance away or someone WILL fly around a curve and nail ya.
Recently I put a couple of LED "Flares" in my Jeep as well, they aren't nearly as nice as the ones
mhpreston posted about and I really haven't put them to any kind of test yet.