Green Parking: Lot Design Techniques That Cut Impervious Cover

Green parking refers to a suite of design techniques applied in combination to reduce the impervious cover created by parking lots. From a stormwater management perspective, parking lots are a significant source of runoff due to their large, impermeable surfaces. By implementing green parking lot design, developers and municipalities can substantially decrease a site’s overall imperviousness, which in turn reduces the volume and pollutant load of stormwater runoff and helps protect local streams.

These strategies move beyond traditional “gray” infrastructure by integrating stormwater treatment, minimizing land consumption, and improving site aesthetics. The core principle is to view parking areas not just as vehicle storage but as functional parts of a site’s landscape and drainage system. This approach is a key component of comprehensive better site design, applicable to new developments and, in many cases, to redevelopment and retrofit projects.

The Six Green Parking Techniques

Effective green parking lot design typically involves a combination of six primary techniques. These strategies address the quantity of parking provided, the physical footprint of each space, the materials used for surfacing, and the integration of landscape-based stormwater controls.

Set Parking Maximums

Many municipal codes mandate parking minimums based on peak demand estimates, such as the busiest shopping day of the year. This often results in vast, underutilized lots. A green parking approach sets parking standards as maximums, not minimums, based on studies of average, rather than peak, demand. This prevents the overbuilding of parking, directly reducing the amount of land converted to impervious surface from the outset.

Minimize Stall Dimensions

Reducing the physical dimensions of parking stalls and aisles can yield significant reductions in impervious cover. Even a small reduction in width and length, when multiplied across hundreds of stalls, can free up considerable space for landscaping or stormwater practices. This includes designating a percentage of stalls for compact cars, which require less area. While the increasing size of vehicles is a common concern, most local parking codes still specify stall widths that are much larger than even the widest consumer SUVs (CWP, 1998).

Utilize Alternative Pavers

For areas that experience infrequent use, such as overflow or spillover parking, conventional asphalt and concrete can be replaced with permeable surfaces. These alternative paving materials allow stormwater to infiltrate into the ground rather than running off. Options include grass pavers, turf blocks, gravel, pervious concrete, and porous asphalt. While effective for stormwater management, these materials often require specific installation methods and more intensive maintenance, making them best suited for low-traffic zones.

Incorporate Bioretention Areas

Bioretention areas, or rain gardens, are landscaped depressions designed to collect and treat parking lot stormwater. Runoff is directed from the paved surface into the bioretention cell, where it ponds temporarily before filtering through a specialized soil media. This process removes pollutants and reduces runoff volume through infiltration and evapotranspiration. Bioretention can be integrated into parking islands, medians, and lot perimeters, satisfying both landscaping and stormwater treatment requirements.

Encourage Shared Parking

In mixed-use developments, adjacent land uses often have different peak parking demands. For example, an office building’s parking demand peaks on weekdays, while a nearby church or restaurant may see its highest demand on weekends and evenings. A shared parking agreement allows these establishments to use the same parking facility, satisfying the needs of both with a single, smaller lot than the two would require if built separately. This strategy is highly effective at reducing the overall parking footprint in dense, mixed-use areas.

Incentivize Structured Parking

Structured parking, whether in a multi-level garage or underground, drastically reduces the land area dedicated to parking. While construction costs are higher than for surface lots, the land saved can be used for more profitable development or preserved as open space. Municipalities can incentivize structured parking through density bonuses, reduced fees, or other regulatory flexibility. The decision often depends on a direct comparison between the high cost of construction and the local price of land.

Parking Ratios: Required vs. Actual Demand

A primary driver of oversized parking lots is the reliance on conventional minimum parking ratios found in local zoning codes. These ratios are often based on outdated data or are calibrated to accommodate the highest theoretical demand, ensuring that a parking space is available for every potential visitor during the busiest hour of the year. As a result, most parking lots sit partially empty for the vast majority of the time.

The following table, with data from the Institute of Transportation Engineers and other studies, compares common parking requirements with observed average demand for several land uses. The gap between the required number of spaces and the number typically used highlights the potential for impervious cover reduction.

Note: GFA refers to the gross floor area of a building, excluding storage or utility spaces. By right-sizing parking facilities based on measured average demand, communities can unlock significant environmental and economic benefits.

Treating What Remains: Bioretention in Parking Lots

Even after reducing the overall parking footprint, the remaining impervious surface still generates contaminated runoff. Bioretention is a highly effective and aesthetically pleasing technique for managing this parking lot stormwater. Runoff is directed into shallow landscaped depressions, where it is temporarily detained on the surface. The captured water then filters through a layered bed of mulch, soil media, and sand, which removes pollutants through physical filtration and biological uptake.

Treated water either infiltrates into the underlying native soil or is collected by an underdrain system for controlled release into the storm drain network or a downstream water body. When discharging near a sensitive area, designers should also ensure adequate stream buffer zones are in place. Bioretention facilities can be seamlessly integrated into required landscaping areas like islands and medians, and their maintenance can be handled by standard commercial landscaping contractors.

While direct pollutant removal studies for parking lot bioretention are ongoing, performance is considered comparable to that of a dry swale. This equates to high removal rates for common stormwater pollutants: 91% for total suspended solids (TSS), 67% for total phosphorus, 92% for total nitrogen, and 80-90% for metals (Claytor and Schueler, 1996). The cost of bioretention is approximately $6.40 per cubic foot of water quality volume treated.

Stormwater Benefits: The Runoff Math

Parking lots are one of the most hydrologically active surfaces in urban and suburban landscapes. Composed of conventional asphalt or concrete, they are nearly 100% impervious, with a runoff coefficient (Rv) approaching 0.95. This means that for every inch of rain that falls, 0.95 inches becomes instant surface runoff, carrying with it hydrocarbons, heavy metals, sediment, and thermal pollution.

The stormwater benefit of green parking is directly tied to reducing this impervious fraction (Ia). Every square foot of asphalt that is eliminated or converted to a permeable surface lowers the composite Rv for the entire site. The relationship can be estimated using the Simple Method, where the site runoff coefficient is calculated as Rv = 0.05 + 0.9 * Ia. Reducing Ia through smaller stalls, fewer spaces, or permeable pavers directly translates to less runoff volume and a lower pollutant load. Designers can use the Simple Method runoff calculator to quantify these benefits during the planning phase.

Implementation Barriers and Local Code Notes

Despite the clear benefits, several barriers can hinder the widespread adoption of green parking techniques. Local development codes are a primary obstacle, as many still mandate rigid parking minimums, effectively functioning as floors rather than ceilings. This removes any incentive for developers to build less parking, even when demand studies support it.

Financial and market pressures also play a role. Lenders may require developers to build excess parking as a condition of financing, and developers themselves may overbuild out of fear of customer complaints about parking availability. The higher initial costs of structured parking and bioretention construction, along with the perceived maintenance burden of alternative pavers, can also be deterrents. Furthermore, shared parking is only a viable option in mixed-use areas where land uses have complementary demand patterns.

To overcome these barriers, municipalities can update their local codes. Key reforms include converting parking minimums to maximums based on local data, establishing a formal process for reducing parking requirements based on demand studies, and allowing shared parking agreements by right in designated zones. Offering credits toward landscaping or open space requirements for the use of bioretention can also incentivize its adoption.

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