Editing Grid plan (section)

===Ecological features, rainwater absorption, and pollutant generation===
[[File:Platslc.jpg|thumb|right|Surveyor's plan of Salt Lake City, circa 1870s – an example of a typical, uniform, square-grid street network]]
Typical uniform grids are unresponsive to [[topography]]. [[Priene]]'s plan, for example, is set on a hill side and most of its north–south streets are stepped, a feature that would have made them inaccessible to carts, chariots and loaded animals. Many modern cities, such as [[San Francisco]], [[Vancouver]], and [[Saint John, New Brunswick]], follow Priene's example. In a modern context, steep grades limit accessibility by car, and more so by bicycle, on foot, or wheelchair, particularly in cold climates.

The same inflexibility of the grid leads to disregarding [[Natural environment|environment]]ally sensitive areas such as small streams and [[creek (stream)|creeks]] or mature woodlots in preference for the application of the immutable geometry. It is said{{By whom|date=July 2023}} of the New York City grid plan that it flattened all obstacles in its way. By contrast, recent discontinuous street patterns follow the configuration of natural features without disrupting them. The grid represents a rationalist, [[reductionist]] solution to a multifaceted issue.

The grid's inherent high street and intersection frequencies produce large areas of [[impermeable surface]]s in the street pavement and the [[sidewalk]]s. In comparison with recent networks with discontinuous street types, grids can be up to 30% higher in impermeable surfaces attributable to roads. The emerging environmental priority of retaining as much as 90% of [[rain|rain water]] on site becomes problematic with high percentages of impermeable surfaces. And since roads constitute the largest share of the total impermeable surfaces of a development, the difficulty is compounded by the grid type of layout. For these reasons modern planners have attempted to modify the rigid, uniform, classic grid.

Some cities, notably [[Seattle]], have devised means to improve a street's retention capacity. However, frequent intersections as they occur in a regular grid would pose an obstacle to their effective application.

A street network pattern can affect the production of pollutants by the amount of car travel that it necessitates and the speed at which cars can travel. The grid plan with its frequent intersections may displace a portion of the local car trips with walking or biking due to the directness of route that it offers to [[pedestrian]]s. But, as long as cars are also allowed on those streets, it makes the same routes more direct for cars, which could be an enticement for driving. The potential car trip displacement would result in a reduction of [[pollutant]] [[exhaust gas|emissions]]. The advantage of the intersection density for pedestrians, however, can have a contrary effect for cars due to its potential for reducing speeds. Low speeds below {{cvt|20|mph|km/h}} have a significantly higher coefficient of pollutant production than above {{cvt|30|mph|km/h}}, though the coefficient after leveling off tends to increase gradually after {{cvt|50|mph|km/h}}.<ref>Final Facility Specific Speed Correction Factors:M6.SPD.002 David Brzezinski, Constance Hart, Phil Enns Assessment and Standards Division, Office of Transportation and Air Quality, U.S. Environmental Protection Agency</ref> This effect is accentuated with high traffic density in areas with commercial uses where speeds come to a crawl. Since the grid plan is non-hierarchical and intersections are frequent, all streets can be subject to this potential reduction of average speeds, leading to a high production of pollutants. Greenhouse and noxious gases can be detrimental to the environment and to resident health.