The hITE of absurdity: Minimum Parking in an era of Declining Traffic

The Institute of Transportation Engineers, founded in 1931, is the main professional association for traffic engineers in the United States. The ITE Trip Generation Manual (currently on the 9th Edition) aggregates a set of studies and outputs the expected number of trips coming or going to a site, by mode, as a function of the type of site. The book Parking Generation does something similar, and guides municipalities on the number of parking spaces a particular new development should have.

Municipalities generally adopt the rates (influenced by the ITE books) as a set of Minimum Parking Requirements, though there is great variation (aptly illustrated by the website Graphing Parking). New development must have parking spaces in excess of the minimum number in order to be approved. Most municipalities, using logic-copying, simply adopt the statues of neighboring jurisdictions without much deep thought. Some do customize them though. For instance, this is the code of the well-endowed (from a parking perspective), City of Maple Grove, Minnesota.

If the world remained the same, and operated at the average level it has for the past 10 or 20 or 30 or 40 years of data that are represented in the Manuals, the Manual might be producing the number that which presumably most municipalities seem to want … the number of parking spaces required so that no customer any day of the year would have to park off-site.

But this requirement itself is odd, and leads to the construction of excess off-street parking, since at least some of that parking is vacant 300, 350, 360, or even 364 days per year depending on how tight you set the threshold and how flat the peak demand is seasonally. Is it really worth vacant paved impervious surface 364 days so that 1 day there is no spillover to nearby streets?

Yet we know that the next 40 years will differ significant from the past 40, just as the past 40 differed from the 40 before that. Our best estimates are that traffic per capita (and perhaps overall) has peaked. This blog has discussed this issue multiple times.

More significantly, peak-season (Christmas) retail travel has fallen off a cliff, as shown in this Wall Street Journal article. Total retail foot traffic for November and December 2013 is at 17.6 billion trips, down from about 33 billion in 2010, just 4 years ago, according to data the WSJ obtained from ShopperTrak. Clearly the rise of the tablet has facilitated shopping via couch rather than car. Does anyone doubt this will continue to rise with tablet adoption?

Graphic from Wall Street Journal
We have further anecdotal evidence. On Black Friday, reportedly the busiest shopping day of the year, the community around Strong Towns went to major shopping areas and photographed mostly empty (over-built) parking lots. Photos were posted to Twitter using the hashtag #BlackFridayParking

Paved Surface Area in the US

To the best of my knowledge, no one really knows the answer to the question of how much total paved surface there is in the US. The best estimate I have seen is 43,000 mi^2 (111,369 km^2).

As a check, we can make some back of the envelope estimates. Feel free to improve these with data

2,605,331 miles (4,192,874 km) of paved roads in the US x 30 feet = 412684430400 ft^2 (38,259,975,250 m^2 or 38,359 km^2). This is about the size of Maryland plus Delaware.

This is back of the envelope. Most roads are 2 lanes (1 in each direction), some are wider. I am taking lane width as 12 feet, and assuming an additional 1/2 lane for each centerline mile to account for multiple lanes.

Parking is less well documented.
250,000,000 vehicles in the US x 3 spaces per vehicle x 180 ft^2 per space = 135,000,000,000 ft^2 (12,541,910,400 m^2 or 12,542 km^2).

As a point of comparison, Connecticut is 14,357 km^2, so we have almost paved over the equivalent of Connecticut to store vehicles off street in the US. This number is probably an underestimate, as some space must be allocated to accessing parking spaces.

Parking plus roads gives us an estimate (rounded to 2 significant digits) of 51,000 km^2, 80% of the size of West Virginia.

So my estimate of paved area is a smaller than above by a factor of 2. It is still quite large.

All else equal, we would definitely prefer to reduce this, as paved area has environmental implications (less pervious surface to filter water, more pavement to absorb heat), direct costs (paving roads and parking cost money – asphalt and concrete production and construction have further environmental costs), and opportunity costs (land that is paved for roads and parking cannot be easily used for something else, money spent paving that land cannot be used for something better).


Democratic municipalities should do what they want. They should want to reduce Minimum Parking Requirements, or instead impose Maximums where the Minimums used to be.

The reasons they don’t of course have something to do with just unthinkingly using Standard Operating Procedure and something to do with neighbors who do not want spillover parking in front of their own property. The mix of which depends on the location. The neighbor problem can be addressed with on-street parking enforcement and parking charges returned to the neighborhood, but is not the main problem in most of the US.

Unfortunately, these changes won’t affect much in the short run. Land owners are not going to suddenly roll up under-used parking like an old rug. There is not all that much new development taking place (especially retail) which can take advantage of the lessened requirements. Many land owners build in excess of the Minimum anyway.

However as opportunities for re-development arise, better laws will lead incrementally, to better land use.