While regional surveys of parking spaces exist, estimates of the number of US parking spaces are important for understanding the total cost and environmental impact of travelling by road. Mikhail Chester and colleagues created five scenarios that include: accounting for the number of metered spaces; building code requirements; home and work car spaces; parking garage structures; surface lots; and roadside parking areas. Starting with a conservative inventory of 105 million metered spaces, each subsequent scenario builds on the previous one.

Scenario one takes into account the 105 million pay-for-parking spaces reported by the International Parking Institute. Scenario two evaluates the paid spaces from scenario one, plus square foot commercial estimates, a home space, and a work space for each vehicle. Scenario two does not take into account estimates of on-street non-metered parking. Scenario three adds urban on-street parking to scenario two based on AASHTO (American Association of State Highway and Transportation Officials) roadway design specifications. Scenario four uses a 3.4 to 1 spaces per car ratio from survey data. Scenario five evaluates the extreme upper limit of 8 to 1 spaces per car ratio, which implies that both designated and non-designated parking spaces are included – so taking into account all potential parking areas.

The middle three scenarios result in 730–840 million spaces taking up 51–58 m2 of parking area space per 100 m2 of roadway paved area, and the upper bound fifth scenario at two billion spaces shows that that there are more parking areas than roadway areas.

The total amount of energy consumed and emissions produced when constructing and maintaining these infrastructures reveals the true environmental impact of parking. The Californian researchers calculated the real costs of building parking spaces by taking into account how expensive it is to mine and process the construction materials needed, such as asphalt and concrete. Each year, the US consumes 110–1800 PJ of energy to build parking, which results in 10–150 Tg carbon dioxide-equivalent (CO2e) and significant quantities of CO, SO2, NOx, VOCs and PM being emitted. Including parking within the overall "life-cycle" inventory means that light-duty vehicle energy consumption goes up from 3.1 to 4.8 MJ by 0.1–0.3 MJ and greenhouse-gas emissions increase from 230 to 380 g CO2e by 6–23 g CO2e per passenger kilometre travelled. Life-cycle automobile SO2 and PM10 emissions show some of the largest increases, by as much as 24% and 89% from the baseline inventory.

We conclude that transport planning could benefit from a more comprehensive understanding of how parking infrastructures impact the environment by calculating how much they really cost in terms of energy consumption and pollution emitted during their construction and maintenance, says team member Mikhail Chester.