I heard about a version that used springs as well. Basically a pull-back car: When you break, it puts tension on the springs, then it releases that tension when accelerating. Apparently it was very good for city-driving, since you get an absurd number of cycles with very small "charges", which makes it very good when you do a lot of start/stop driving in slow traffic. I'm not sure why I haven't heard any more about it in more recent years.