Reduced and More Accurate Parking Requirements

Motor vehicle parking is costly [pdf]. Considering land, construction, and operations a parking space typically costs $500 to $4,000 annually. Many vehicles are worth less than the parking space they occupy, and since typical communities have two to six off-street parking spaces per vehicle, most vehicles are worth less than the total value of parking spaces provided for their use. For each dollar motorists spend on their vehicles, somebody spends more than a dollar to park it.

This parking is never actually free, the choice is really between paying directly or indirectly. Paying indirectly, with parking costs incorporated into taxes and the prices of other goods, is inefficient and unfair; it underprices automobile travel, which increases traffic problems, and it forces households that own fewer than average vehicles to subsidize the parking costs of their more vehicle-owning neighbors.

A growing body of literature criticizes these subsidies, including Donald Shoup’s wonderful book, The High Cost of Free Parking [pdf], and a recent Economist article, "Parkageddon: How Not to Create Traffic Jams, Pollution And Urban Sprawl, Don’t Let People Park For Free." Many recent housing affordability strategies highlight the importance of reducing parking requirements.

These reflect a paradigm shift: the old paradigm assumed that parking should be abundant and free; the new paradigm recognizes that too much parking is as harmful as too little, and that parking should be managed and priced for efficiency. In response, some jurisdictions are eliminating minimum parking requirements, many are significantly reducing those requirements, and most jurisdictions are starting to encourage more efficient parking management. Implementing these changes is a job for planners. If you want job security, become a parking management expert!

Eliminating minimum parking requirements is not as radical as it sounds; in fact, many jurisdictions have a few areas, such as downtowns and heritage districts, that have no parking requirements. Removing zoning requirements does not eliminate parking supply, it simply allows developers to decide how many spaces to build based on market demand. This may lead to spillover parking problems (motorists parking where they shouldn't) and so requires more regulation and enforcement, but it leads to a more efficient and equitable parking market in which households only pay for the number of parking spaces they need, and so allows households to save thousands of dollars annually by reducing their vehicle ownership. However, pubic officials are often reluctant to eliminate parking regulations, so other approaches may be needed. By incorporating parking requirement adjustment factors that reflect demand it is possible to significantly reduce costs and increase efficiency and fairness. Many jurisdictions apply a few adjustment factors, such as modest reductions for housing located near a transit stations or carshare service, but the potential is much larger. 

Various studies identify how various demographic, geographic and management factors affect vehicle ownership and use, and therefore parking demands. In general, reductions in vehicle ownership reduce residential parking demand, and reductions in vehicle trips reduce parking demand at other destinations. Several recent studies indicate that households in smart growth or transit-oriented developments own about half as many vehicles and generate about half as many trips as predicted by conventional models. Some targeted studies measure parking occupancy rates in specific areas and building types to guide parking supply adjustment factors. 

Several data sources can be used to determine how demographic and geographic factors affect vehicle ownership and use, and therefore parking demands. For example, the U.S. Consumer Expenditure Survey Tables provide vehicle ownership data by income (quintile [pdf] and decile [pdf]), housing tenure [pdf] and location [pdf]. These indicate that:

• The lowest income quintile households own on average 0.9 vehicles, compared with 2.7 for the highest income households.
• Renter households own on average 1.2 vehicles, compared with 2.3 for homeowners.
• Central city households own on average 1.5 vehicles, compared with 2.4 in rural areas.

This information can be used to adjust parking requirements to more accurately reflect demands. You can find similar information from the American Community Survey and other Census data sets, and in local travel surveys. Of course, some of these factors overlap, for example, renters tend to have lower incomes and live in more urban areas than home owners, so it is important to use judgement when evaluating the total vehicle reduction impacts of multiple adjustment factors.

The table below summarizes my recommended parking supply adjustment factors, which can be used as default values if more specific information is unavailable. This table was originally published in my book, Parking Management Best Practices [pdf], and has been widely reproduced since, including in the new ITE Transportation Planning Handbook. 

Parking Requirement Adjustment Factors

Special care is needed when predicting the impacts of a program that includes multiple parking management strategies. Total impacts are multiplicative not additive since each additional factor applies to a base level reduced by previous factors. For example, shared parking might reduce parking needs by 20%, to 80% of the base level; efficient parking pricing might reduce demand 15%, to 85% of the base; and carsharing might reduce demand by 10%, to 90% of the base. If implemented together the total reduction of these three strategies is calculated by multiplying 80% x 85% x 90%, which equals 61%, a 39% reduction, which is somewhat less than the 45% reduction that would be calculated by adding 20% + 15% + 10%.

On the other hand, some combinations of strategies have synergistic effects (total impacts are greater than the sum of their individual impacts), and so become more effective if implemented together. For example, sharing parking and walkability improvements may each reduce parking requirements just 10% if implemented alone, but 25% if implemented together because they are complementary; improving local walkability increases the range of parking spaces that can serve a destination, making sharing more feasible.

Of course, other factors can affect the number of optimal parking spaces that should be supplied at a particular location, including the costs of increasing supply, the severity of problems that will occur if there is a shortage, and the ability of local governments to establish and enforce parking regulations to address spillover problems. Planners must apply professional judgement when developing adjustment factors and parking management programs, which is job security for planners who specialize in parking planning and management.

For More information

G.B. Arrington, et al. (2008), Effects of TOD on Housing, Parking, and Travel, Report 128, Transit Cooperative Research Program.

Joshua Engel-Yan and Dylan Passmore (2010), “Assessing Alternative Approaches to Setting Parking Requirements,” ITE Journal, Vo. 80, No. 12, December, 30-25.

GreenTRIP Connect. This free tool allowing users to easily calculate how accessible, multi-modal location and traffic reduction strategies can reduce driving from residential development throughout California, and calculates how much money and space can be saved from right-sized parking.

Adam Millard-Ball (2015), “Phantom Trips: Overestimating the Traffic Impacts of New Development,” Journal of Transportation and Land Use.

Daniel Rowe, et al. (2013), “Do Land Use, Transit and Walk Access Affect Residential Parking Demand?” ITE Journal, Vol. 83. No. 2, February, pp. 24-28. This article summarizes the results of King County’s Right Size Parking Project.

Robert J. Schneider, Susan L. Handy and Kevan Shafizadeh (2014), “Trip Generation for Smart Growth Projects,” ACCESS 45, pp. 10-15; at. Also see the Smart Growth Trip-Generation Adjustment Tool. 

Rachel Weinberger and Joshua Karlin-Resnick (2015), Parking In Mixed-Use U.S. Districts: Oversupplied No Matter How You Slice The Pie, Transportation Research Board Annual Meeting 

Michael Kodransky and Gabrielle Hermann (2011), Europe’s Parking U-Turn: From Accommodation to Regulation, Institute for Transportation and Development Policy.

Todd Litman (2006), Parking Management Best Practices, Planners Press. 

Todd Litman (2016), Parking Management: Comprehensive Implementation Guide, Victoria Transport Policy Institute.

Todd Litman (2017), Reforming Municipal Parking Policies to Align With Strategic Community Goals, Victoria Transport Policy Institute.

Reinventing Parking is a website the provides ideas for parking policy reforms.

Shoupistas Facebook Page honors the insights of Donald Shoup. 

Richard Willson (2015), Parking Management for Smart Growth, Island Press.

Reduce requirements 10-30% for cost-recovery pricing (i.e. fees that pay the full cost of parking facilities), and 10-20% for unbundling (parking rented separate from building space).