Mitigating Environmental Impacts

Large expanses of pavement can be very detrimental to the environment in a number of ways:

  • The impervious surface means that rainwater flows to storm sewers rather than infiltrating into the ground to recharge groundwater;
  • Oil and other fluids that leak from parked vehicles contaminate the stormwater; and
  • The pavement absorbs the heat of the sun and causes heat island effects.

In addition to reducing the number of surface parking spaces constructed through flexible minimum parking requirementselimination of minimum parking requirements, use of landscaped parking reserves, and/or building structured parking where possible, there are several other strategies that can help mitigate the environmental impacts of surface parking, including landscaping and Low Impact Development (LID) techniques:

  • Reducing the dimensions of parking stalls and encouraging inclusion of compact car spaces
  • Using pervious surfaces for low-volume parking areas to allow infiltration of stormwater
  • Requiring landscaping to provide shade and improve air quality
  • Using bioretention basins or rain gardens to treat and infiltrate stormwater
  • Building parking garages with green roofs to capture stormwater and mitigate heat island effects

MAPC’s Massachusetts Low Impact Development Toolkit contains more details on many of the techniques discussed below.

Reduced Stall Dimensions and Compact Car Spaces

While parking stalls that are too small can result in vehicles taking up multiple spaces, in many cases, required stall dimensions in local ordinances are larger than even the widest SUVs [1, p.22]. In establishing standards for the dimensions of parking stalls, keep them as small as is feasible, and allow smaller spaces for long-term parking (residents and employees) than for short-term parking (customers). Developers of large parking lots can also be encouraged or required to provide some compact car spaces with reduced dimensions. This can help reduce the amount of pavement required to supply the necessary number of parking spaces. Many communities in Massachusetts allow some percentage of spaces in large parking lots to have smaller dimensions and be reserved for compact cars; for example:

  • In Marlborough compact car spaces can represent up to 33% of the total spaces in lots used by residents or employees, but extra open space must be provided at least matching the area conserved with the smaller stall dimensions (see regulation).
  • In Needham up to 50% of off-street parking spaces can be designed for compact cars with reduced dimensions (see regulation).

Pervious Surfaces for Low Volume and Overflow Parking

For parking areas that are used infrequently, especially if they are primarily used during the summer, it may be possible to use a surface other than asphalt in order to reduce stormwater runoff and allow greater infiltration. Depending on the site characteristics, any of the following may be appropriate:

  • Leaving an area unpaved or using turf blocks (for overflow parking areas used rarely): Grass pavers (aka turf blocks) are a type of open-cell unit paver in which the cells are filled with soil and planted with turf. The pavers, made of concrete or synthetic, distribute the weight of traffic and prevent compression of the underlying soil.
  • Using paving stones (for areas that get light use and are not used by heavy trucks or fast-moving vehicles): Paving stones (aka unit pavers) are impermeable blocks made of brick, stone, or concrete, set on a prepared sand base. The joints between the blocks are filled with sand or stone dust to allow water to percolate downward.
  • Using pervious concrete or porous asphalt (for areas that get moderate use but are not used by heavy trucks or fast-moving vehicles): Porous asphalt and pervious concrete appear to be the same as traditional asphalt or concrete pavement. However, they are mixed with a very low content of fine sand, so that water can pass through small openings within the pavement.

For more information on parking surface alternatives, including maintenance and plowing concerns, benefits and costs, see the Permeable Paving Fact Sheet in MAPC’s Massachusetts Low Impact Development Toolkit.

One local example of pervious paving materials used for a parking lot has been constructed at Silver Lake Beach in Wilmington, where a demonstration project funded by an EPA Targeted Watershed Grant included repaving the parking lot with a combination of surfaces:

  • The parking spaces – about 8,000 square feet total – were paved with porous pavers.
  • The parking aisles – 16,600 square feet – were paved with porous asphalt.
  • A portion of the overflow parking area was paved with Gravelpave, a porous paving system consisting of interlocking plastic cells filled with gravel.
  • Another small section of the overflow parking lot was paved with a porous material called Flexi-Pave, made from recycled tires.
  • One half of the existing main parking lot was repaved with standard asphalt for comparison.

Runoff from the part of the lot with standard pavement and any overflow from the pervious portion will drain to bioretention cells in the parking lot. Groundwater quantity and quality impacts will be measured by the U.S. Geological Survey with a collection of groundwater wells installed for the project. For more information on this project, see

Landscaping for Shade and Air Quality

Most communities have landscaping requirements for larger parking facilities. Requiring, at a minimum, that large surface parking lots include landscaped islands with trees can provide some shade to reduce heat from the asphalt and can provide some air quality benefits. While any landscaping, and shade trees in particular, are an improvement over no landscaping, in communities where stormwater is an issue, it may be desirable to write any landscaping requirements such that LID techniques are allowable and encouraged.

As an example, the Town of Acton has an extensive section in the zoning bylaws specifying the landscaping requirements for parking lots of over 5 spaces, and in the East Acton Village District, consolidated bioretention areas may be substituted for interior landscaping (see regulation). Another option is to encourage LID techniques and stormwater management best management practices more generally for all parking, as the Town of Littleton does (see regulation). Several LID techniques applicable to parking areas are discussed below and can be encouraged through the parking landscaping section of the bylaws.

Rain Gardens and Bioretention Cells

Parking areas where pervious surfaces are impractical can still have a reduced impact on stormwater through the implementation of Low Impact Development techniques such as bioretention cells (shallow depressions filled with sandy soil, topped with a thick layer of mulch, and planted with dense vegetation where stormwater runoff flows in and filters through the soil and into the groundwater) and other strategies that slow and filter stormwater runoff, allowing a greater percentage to infiltrate into the ground.

The landscaped islands that many communities require in large parking lots can be used for bioretention cells that can provide pleasant landscaping and stormwater treatment. Implementing this change will generally require adjustments to the landscaping requirements for parking areas so that they can be designed as bioretention areas, which are depressed rather than raised and must not be fully surrounded by curbs, so that the runoff from the paved areas can flow into the bioretention cell.

For more information on bioretention cells, including maintenance concerns, benefits, and costs, see the Bioretention Area Fact Sheet in MAPC’s Massachusetts Low Impact Development Toolkit. There are also fact sheets on Filter Strips, Infiltration Trenches and Dry Wells, and other LID techniques.

Bioretention basins, also called rain gardens, have been implemented in several places around Massachusetts; for example:

  • In Lincoln, the Drumlin Farm Wildlife Sanctuary incorporates a a bioretention system that will intercept road runoff and filter it prior to discharge to a nearby pond. More information
  • The Town of Cohasset implemented a number of rain gardens around town to treat stormwater runoff and protect drinking water quality
  • Marla Circle development in Tyngsborough incorporated several LID techniques including a bioretention cell in the center of the cul-de-sac and rain gardens on the individual properties
  • The demonstration project at Silver Lake in Wilmington (see above) also incorporated rain gardens.
  • Other examples include Pembroke Town Hall (permeable paving, a grass infiltration trench, and rain gardens) and Kingston Elementary School (bioretention areas, vegetated swales). More information

Green Roofs on Parking Garages

Parking structures can also have reduced impact on stormwater systems by incorporating green roofs to retain and filter stormwater runoff.

A green roof is a low-maintenance vegetated roof system that stores rainwater in a lightweight engineered soil medium, where the water is taken up by plants and transpired into the air. Green roofs provide an extra layer of insulation that reduces heating and cooling costs, and they are likely to last much longer than conventional roofs, since the roofing material itself is shielded from ultraviolet light and thermal stress.

The vegetation on green roofs also improves air quality, enhances the appearance of the building, and reduces the urban “heat island” effect. Green roofs are appropriate anywhere it is desirable to reduce the overall amount of stormwater runoff. They are an excellent technique to use in dense urban areas, in areas where infiltration is difficult due to tight soils or shallow bedrock, or on sites where infiltration is undesirable due to existing soil contamination.

Because green roofs return rainwater to the atmosphere, they should not be used in situations where groundwater recharge is a priority, such as in stressed basins with chronic low-flow conditions. In these circumstances, roof runoff should be infiltrated whenever feasible. For more information on applicability, maintenance, costs, and other concerns, see the Green Roof Fact Sheet in MAPC’s Massachusetts Low Impact Development Toolkit.

Locally, there are a number of buildings with green roofs, but currently just one green roof on a parking garage, though another is proposed:

  • In 2003, Harvard University replaced the asphalt roof of a parking garage at 29 Garden Street with a 10,000 square foot green roof and courtyard.
  • The proposed CitySquare project in Worcester is slated to include a green roof over a parking garage.

Nationally, there are a number of examples of parking garages constructed with green roofs, including:

  • Blue Cross Blue Shield of Michigan’s new parking garage downtown Detroit incorporates a green roof and employee walking path
  • The 5.5 acre Millennium Park on top of the sloping roof of the parking garage at Soldier Field in Chicago