Because of all the uncertainties, they decided to examine the environmental impact of five different scenarios for parking. The first scenario was limited to only the 100 million metered parking spaces in the previous study. The next three scenarios examine what the group considers to be the most likely situation -- that there are somewhere around 800 million parking spaces in the United States, or nearly three official parking spaces for every car on the road.
The final case was the most extreme of the scenarios. The researchers extrapolated on a rule of thumb used by urban planners that claims eight parking spaces exist for every one car. The group says that there is little science to support this scenario, but the result would be a whopping 2 billion parking spaces. If all of those spots were consolidated into a single location they would cover an area the size of Massachusetts. The most likely scenario calls for about half that area in parking spaces.
"The environmental effects of parking are not just from encouraging the use of the automobile over public transit or walking and biking," the group stated in their paper, "but also from ... activities related to building and maintaining the infrastructure."
"There's actually a larger infrastructure for parking than for roadways," said Chester. "This speaks to the sort of hidden infrastructure components that are there to store our vehicles when they're not moving."
Once the parking estimates were completed, the researchers calculated the energy requirements as well as the emissions from creating asphalt and other things associated with constructing and maintaining those parking spaces. They then added their estimates to the emissions caused by an average vehicle.
Their results are considerable, even when compared to the environmental effects of driving a car. The group found that parking contributes to greenhouse gases like carbon dioxide, methane and nitrous oxide. In fact, the environmental cost of so many parking spaces can also raise the amount of carbon dioxide emitted per mile by as much as 10 percent for an average car. And, when calculated over the lifetime of a vehicle, the amount of other gases like sulfur dioxide can rise by as much as 25 percent and the amount of soot as much as 90 percent.
Sulfur dioxide and soot are both harmful to humans and are associated with things like acid rain and respiratory illnesses.
"We've traditionally thought about the environmental impact of parking as being limited to the heat island effect," said Chester, referring to the process by which large areas of asphalt are thought to heat cities to higher temperatures than surrounding rural areas. "The amount of parking has a rather drastic impact on the energy and emission contributions from vehicles."
Shoup said that the informal calculations he's done produced parking space estimates similar to the Berkeley teams, but adds that he thinks the impact of driving cars still dwarfs the environmental cost of parking.
"Only in the last 5-10 years have we been giving some thought to whether there should be an abundance of free parking," said Chester. "Ninety-nine percent of automobile trips end in free parking and this has a major effect on people's choice of what means of transportation to take."