> But when it comes to cars in very dense areas, there is a significant discontinuity in social cost between zero cars (pedestrians can freely use the streets,

> But when it comes to cars in very dense areas, there is a significant discontinuity in social cost between zero cars (pedestrians can freely use the streets, no traffic lights, etc) and a few cars (like in congestion pricing schemes). In my experience that's not quite the case. I lived in Barcelona's old town for about a year, and while it was technically legal to drive on many of the narrow, medieval-era streets, in practice it was enough of a pain that drivers rarely did. Because of the high volume of pedestrians, the few cars that did come through were forced to travel very slowly and the burden was on them to avoid hitting anyone, not the other way around. So I don't think you have to go all the way down to 0 cars to get most of the safety benefits – there's a tipping point somewhere earlier.