Most U.S. cities specify minimum parking requirements in local zoning codes, even in areas served by a variety of travel options. Little work has been done to understand how aggregate parking supplies compare with requirements in urban areas or to evaluate the potential impacts of parking requirements met in such places. The research reported here tracked changes in parking supplies and the built environment in six urban business districts between 1960 and 2000. The work also summarized parking requirements in each city and compared those requirements with actual supplies. This research demonstrated that parking requirements could be met only through a combination of costly parking infrastructure and considerable commitments of land. In half of the cities studied (i.e., those with the more stringent parking requirements) off-street parking supply ratios increased by 150% to 300%; the increase required comparable amounts of land but still fell short of areawide parking requirements. Through quantification of existing parking supplies in urban areas and evaluation of the potential impacts of increases in those supplies, this work offers guidance and justification for the establishment of parking standards that conform to long-term transportation and development goals.