Roads, parking lots, and driveways account for over 60% of impervious surfaces in urban areas.  Consequently, pavement is the largest generator of stormwater runoff. Pervious pavement allows water to infiltrate into layers of rock placed below the pavement and then into surrounding soils. A perforated drain pipe is installed in the rock chamber. If surrounding soils won't allow water to percolate, water will move to the drain pipe and be slowly discharged. The movement of water through the rock and soil helps filter out pollutants, maintain stable stream flows and can reduce flood potential.