I used to ride a bunch of them in my commute into Boston many years ago. Boston traffic, so no, I never had to deal with a car stopping for me. Boston drivers don't do that. (Or at least didn't then.) The three o n my commute were at the bottoms of small hills in both directions so I could see everybody, pick my spot and enter going fast. I'd "park" myself on the bumper of a car knowing this was the one place n a Massachusetts roundabout that there wold be no other cars. I trusted that being on a fix gear, my reflexes would give me a few spare feet as soon as I saw that brake light come on. Yes, I was in my 20s, a bike racer and that was a little crazy but it worked very well. No close calls.
Ben