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Water: Best Management Practices

Green Parking

Minimum Measure: Post-Construction Stormwater Management in New Development and Redevelopment

Subcategory: Innovative BMPs for Site Plans

Green parking refers to several techniques that applied together reduce the contribution of parking lots to total impervious cover. From a stormwater perspective, green parking techniques applied in the right combination can dramatically reduce impervious cover and, consequently, reduce the amount of stormwater runoff. Green parking lot techniques include: setting maximums for the number of parking lots created; minimizing the dimensions of parking lot spaces; utilizing alternative pavers in overflow parking areas; using bioretention areas to treat stormwater; encouraging shared parking; and providing economic incentives for structured parking.


All of the green parking techniques can be applied in new developments, and some can be applied in redevelopment projects, depending on the extent and parameters of the project. In urban areas, some techniques, like encouraging shared parking and providing economic incentives for structured parking, can be practical and necessary. Commercial areas can have excessively high parking ratios.  By applying green parking techniques in various combinations, a site's impervious cover can be dramatically reduced.


Parking lot designs frequently result in far more spaces than are required. This problem is exacerbated by a common practice of setting parking ratios to accommodate the highest hourly parking during the peak season. By determining average parking demand instead, a lower maximum number of parking spaces can accommodate most of the demand. 

Table 1 provides examples of conventional parking requirements and compares them to average parking demand.

Table 1: Conventional minimum parking ratios (Source: ITE, 1987; Smith, 1984; Wells, 1994)

Land Use

Parking Requirement

Actual Average Parking Demand

Parking Ratio

Typical Range

Single family homes

2 spaces per dwelling unit


1.11 spaces per dwelling unit

Shopping center

5 spaces per 1000 ft2 GFA


3.97 per 1000 ft2 GFA

Convenience store

3.3 spaces per 1000 ft2 GFA




1 space per 1000 ft2 GFA


1.48 per 1000 ft2 GFA

Medical/ dental office

5.7 spaces per 1000 ft2 GFA


4.11 per 1000 ft2 GFA

GFA = Gross floor area of a building without storage or utility spaces.

Minimizing the dimensions of parking spaces is another green parking lot technique.  Besides reducing the length and width, parking stall dimensions can be reduced by providing compact- vehicle spaces.  While large sport utility vehicles (SUVs) are often cited as barriers to stall minimization techniques, most local parking codes require stall widths wider than the widest SUVs (CWP, 1998).

Another effective green parking technique is the use of alternative pavers.  Alternative pavers include gravel, cobbles, wood mulch, brick, grass pavers, turf blocks, natural stone, pervious concrete, and porous asphalt.  In new developments and redevelopment projects, they can replace conventional asphalt and concrete.  The effectiveness of alternative pavers in meeting stormwater quality goals can range from medium to relatively high.  Alternative pavers require proper installation, and they generally need more maintenance that conventional asphalt or concrete.  For more specific information on alternate pavers, refer to the Alternative Pavers fact sheet.

Bioretention areas can effectively treat stormwater leaving a parking lot. Stormwater is directed into a shallow, landscaped area, where it is temporarily detained. The runoff then filters down through the bed of the facility, where it is either infiltrated into the subsurface or collected into an under-drain pipe for discharge into a stream or another stormwater facility. Attractively designed bio-retention facilities can be integrated into landscaped areas and maintained by commercial landscaping firms. For detailed design specifications of bioretention areas, refer to the Bioretention (Rain Gardens) fact sheet.

In mixed use areas, shared and structured parking are green parking techniques that can reduce the conversion of land to impervious cover. A shared parking arrangement involves two parties that share one parking lot.  For example, an office that experiences peak demand during weekdays can share their parking lot with a church that experiences peak demand during weekends and evenings.  Costs may dictate the use of structured parking, but building above or below-ground structured parking garages can help minimize surface parking. 


Limitations to green parking techniques include applicability, cost, and maintenance. For example, shared parking is practical only in mixed use areas, and structured parking may be limited by the cost of land versus the cost of construction. Currently, alternative pavers are recommended only for overflow parking because of their expensive maintenance costs. Bioretention areas also increase construction costs.

The pressure to provide an excessive number of parking spaces can result from the fear of customer complaints, as well as the requirements of bank loans. These factors can pressure developers into constructing more parking than is necessary.  Together, these barriers inhibit the construction of the greenest parking lots possible.


Applied together, green parking techniques can effectively reduce the amount of impervious cover.  They can help to protect local streams, reduce stormwater management costs, and enhance a site's ascetics. Proper design of bioretention areas can help meet stormwater management and landscaping requirements while keeping maintenance costs at a minimum.

Green parking lots can dramatically reduce the creation of new impervious cover. How much is reduced depends on the combination of techniques used to achieve the greenest parking lot.  While the pollutant removal rates of bioretention areas have not been directly measured, their capability is considered comparable to a dry swale, which removes 91 percent of total suspended solids, 67 percent of total phosphorous, 92 percent of total nitrogen, and 80-90 percent of metals (Claytor and Schueler, 1996).

North Carolina's Fort Bragg vehicle maintenance facility parking lot is an excellent example of the benefits of rethinking parking lot design (NRDC, 1999).  The redesign incorporated stormwater management features, such as detention basins located within grassed islands, and an onsite drainage system that exploited existing sandy soils.  The redesign reduced impervious cover by 40 percent, increased parking by 20 percent, and saved 20 percent or $1.6 million on construction costs over the original, conventional design.     

Cost Considerations

Setting maximums for parking spaces, minimizing stall dimensions, and encouraging shared parking can result in considerable construction cost savings. At the same time, implementing green parking techniques can also reduce stormwater management costs.


Center for Watershed Protection. 1998. Better Site Design: A Handbook for Changing Development Rules in Your Community. Center for Watershed Protection, Ellicott City, MD.

Claytor, R.A., and T.R. Schueler. 1996. Design of Stormwater Filtering Systems. Center for Watershed Protection, Inc., Ellicott City, MD.

Invisible Structures.  No date.  Grasspave2: Porous Paving System. [http://www.invisiblestructures.com/GP2/grasspave.htm Exit EPA Site]. Accessed November 17, 2005.

ITE. 1987. Parking Generation, 2nd edition. Institute of Transportation Engineers, Washington, DC.

Natural Resources Defense Council (NRDC). 1999. Stormwater Strategies: Community Responses to Runoff Pollution. Natural Resources Defense Council, Washington, DC.

Smith, T. 1984. Flexible Parking Requirements. Planning Advisory Service Report No. 377. American Planning Association, Chicago, IL.

Wells, C. 1994. Impervious Surface Reduction Technical Study. Draft Report. City of Olympia Public Works Department. Washington Department of Ecology, Olympia, WA.

Information Resources

Bergman, D. 1991. Off-Street Parking Requirements. American Planning Association, Chicago, IL.

Brown, W.E., D.S. Caraco, R.A. Claytor, P.M. Hinkle, H.Y. Kwon, and T.R. Schueler. 1998. Better Site Design: A Handbook for Changing Development Rules in Your Community. Center for Watershed Protection, Inc., Ellicott City, MD.

Brown, W.E., and T.R. Schueler. 1996. The Economics of Stormwater BMPs in the Mid-Atlantic Region: Final Report. Center for Watershed Protection, Ellicott City, MD.

Diniz, E. 1993. Hydrologic and Water Quality Comparisons of Runoff from Porous and Conventional Pavements. CRC Press, Lewis Publishers, Boca Raton, FL.

Morris, M. 1989. Parking Standards¿Problems, Solutions, Examples. PAS Memo, December 1989 issue. American Society of Planning Officials, Chicago, IL.

Smith, D.R. 1981. Life Cycle and Energy Comparison of Grass Pavement and Asphalt Based on Data and Experience from the Green Parking Lot. The Heritage Conservation and Recreation Service, Washington, DC.

Smith, D.R., and D.A. Sholtis. 1981. An Experimental Installation of Grass Pavement. The Heritage Conservation and Recreation Service, Washington, DC.

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