We must do a better job protecting our waterways from excess runoff, but we have been using the wrong measuring-stick: now, we use cubic feet per acre; it should be cubic feet per person. Stormwater laws today typically require that a building site discharge no more stormwater once the building and sitework is complete than it did when it was an open field or forest. The most common way of doing this is to build a stormwater retention pond. Left to standard engineering practice, they’re normally surrounded with chain link fences topped with barbed wire and are soon littered with styrofoam cups, plastic wrappers, and other debris. This might work in the suburbs where building sites are large enough you can hide them out back. But if you try to build a new Main Street today and don’t want to pay the really high price of underground stormwater detention, that means that every building on Main Street will be separated from every other building by an ugly stormwater retention pond, which would kill the Walk Appeal and therefore the vitality of the Main Street. If we look at stormwater in gallons per person instead of gallons per acre, an amazing thing happens: the more people there are living in a neighborhood, the less runoff there is per person. Once you get more than four homes per acre, the neighborhood performs better than untouched land even if you do nothing at all about the stormwater. And more people per acre means the neighborhoods and the Main Streets have much more vitality than if we spread everything out.