A guide by the Stormwater Manager's Resource Center in Maryland (funded by the EPA et. al.) states that roads can be as narrow as 22 to 26 feet and still allow access for emergency + large vehicles (Kyoto's Sanitation Services: "duh"). The guide is a bit paranoid contingent and not quite narrow enough for me, but it's a start. It still doesn't overcome the whole chicken-and-egg thing, either: streets, it says, must take into account existing traffic volume. But streets invite volume, especially if built too wide. Narrower streets, when dovetailed with excellent public or individual transport options, can give people solid reasons to give the car the day off. Read more.