In an 8 hour day, that's one destination every 3 minutes 12 seconds. Is that realistic?

Yes, I wondered about that but then thought that, in the States, there will be a lot more people using couriers so the distance between stops will be shorter, parking spaces will generally be more readily available and roads less crowded, so I guess it's just about possible. It does seem amazing to me, though the Economist is pretty careful about their stats. Perhaps they work longer than 8-hour days?