Permeable paving is a range of sustainable materials and techniques for permeable pavements with a base and subbase that allow the movement of stormwater through the surface. In addition to reducing runoff, this effectively traps suspended solids and filters pollutants from the water. Examples include roads, paths, lawns and lots that are subject to light vehicular traffic, such as car/parking lots, cycle-paths, service or emergency access lanes, road and airport shoulders, and residential sidewalks and driveways.
Although some porous paving materials appear nearly indistinguishable from nonporous materials, their environmental effects are qualitatively different. Whether pervious concrete, porous asphalt, paving stones or concrete or plastic-based pavers, all these pervious materials allow stormwater to percolate and infiltrate the surface areas, traditionally impervious to the soil below. The goal is to control stormwater at the source, reduce runoff and improve water quality by filtering pollutants in the substrata layers.
A sett, usually referred to in the plural and known in some places as a Belgian block, is a broadly rectangular quarried stone used for paving roads, Formerly in more widespread use, it is now encountered more as a decorative stone paving in landscape architecture. Setts are often idiomatically referred to as "cobbles", although a sett is distinct from a cobblestone by being quarried or shaped to a regular form, whereas the latter is generally of a naturally occurring form.
Streets paved with setts are highlights in several cycling competitions such as the final Champs-Élysées stage of the Tour de France and the Paris–Roubaix road race. Riding upon sett is technically more challenging than riding on asphalt.
Germantown Avenue in Philadelphia, in particular its upper reaches through Germantown, Mount Airy and Chestnut Hill, is notable for being paved with Belgian blocks; repaving projects on this thoroughfare have retained or reintroduced setts to give additional historic character to these neighborhoods. Part of this character includes the tracks of the 23 trolley, though the modern tracks are encased in concrete slabs rather than setts, and the trolley line itself is currently operated by buses.