With all this pavement connected, there is much less opportunity for runoff to soak into the ground. In short, conventional development carries with it a subtle but powerful bias toward maximizing both the quantity and speed of runoff. However, it allows developers to put the houses on smaller lots and requires the preservation of large areas of a site as open space where houses can never be built. These two provisions give local governments and developers the flexibility needed for good design and modern storm water management. Smaller, narrower lots also help reduce the need for pavement in driveways and walks, as in Figure 3.
Clustering enables a better relationship between impervious surfaces and natural drainageways, too.
Runoff from impervious surfaces now flows slowly over pervious, vegetation-covered areas, soaking into the soil, which filters out some of the pollutants before the storm water reaches lakes, rivers and streams. Greater use is made of drainage devices such as ditches and swales. Costs are lowered because curbs and storm sewers are no longer needed. More rainfall is directed toward the replenishment of ground water. The development generates a smaller volume of runoff moving more slowly toward the bodies of water receiving it.