access the individual lots than the conventional approach. The LID concept also uses permeable pavement that allows stormwater to pass through and recharge groundwater, as opposed to being diverted to costly subsurface infrastructure. Additional methods to reduce imper vious surfaces include building vertically and using vegetated roofs. In some cases, municipal ordinances may prevent the minimization of imper vious surfaces by requiring roads and driveways of a certain width or that a minimum number of parking spaces be provided in parking lots ser ving commercial and institutional properties. Such ordinances may result in oversized parking lots, drives, and roads, and may not permit shared parking between adjacent uses with peak parking demands at different times of day or days of a week.
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Municipal zoning ordinances establish standards to which parking lots must be designed. These standards vary between municipalities and are driven by the project’s land use designation. Elements commonly found in parking lot design ordinances include a minimum number of parking spaces as a ratio to building area (minimum spaces per square-feet), parking space and driveway dimensions, and specifications for landscape islands and/or shade tree size and intervals. These elements affect the size of the overall parking lot, and thus significantly affect site hydrology.
A survey of regional parking ordinances for commercial uses indicates that most municipalities require a minimum of four to five spaces per 1,000 square-feet of building space. Low impact parking codes are beginning to set minimum parking space requirements as low as three spaces per 1,000 square-feet, in addition to space maximums, shared parking allowances, and reduced stall and aisle dimensions to minimize the amount of impervious surfaces. The image on the facing page compares the minimum number of parking spaces as would be required by historic and some current parking codes in the PlanET Region against LID design standards. Under LID standards, spaces desired by the business that exceed the ordinances’ stipulated maximum must be built using pervious pavements. Shade trees with minimum caliper and spread requirements, plus minimum island dimensions, enhance the likelihood that a tree will survive and establish a healthy canopy, providing shade that reduces ambient air temperature, stormwater runoff quantity, and stormwater runoff temperatures following a storm event. By reducing impervious surfaces, minimizing topsoil removal and soil compaction, and preserving existing tree canopy, the amount of stormwater runoff and peak flow rate is also minimized. Reduced stormwater runoff requires less stormwater management infrastructure and may allow for significant infrastructure cost savings.