I leave mine on. It's better than not being seen by someone in a 3000 pound vehicle who's on their phone or texting or something. It'd be too distracting to turn it on every time I got to the back, then turn it off every time someone got behind me.

You need to point the thing kind of at drivers' eye height. That's usually a bit below someone that's right on your wheel but not much. Point it a bit to the left too, so it points towards the cars, not directly towards the other riders behind you.