Adjusted Average Rent:
To illustrate how unbundled and bundled parking can influence parking/unit ratios, rent adjustment assumptions are made to balance revenue in a hypothetical multi-family development.
  1. Adjusted Rent in the Unbundled Parking Option: Average Rent – ((Unbundled Parking Price Per Stall - Monthly Price Per Stall) * Parking/Unit Ratio)
  2. Adjusted Rent in the Bundled Parking Option: Adjusted Rent in the Unbundled Parking Option + (Unbundled Parking Price Per Stall * Parking/Unit Ratio)
Affordable Units, Percent (variable):
Units in the development that are designated affordable by the governing land use body. This does not account for relative affordability, only those designated affordable. This variable includes all units identified as affordable by any designation (i.e., 60%, 80%) as a percent of all units (regardless of occupancy). Observations show that as percent of affordable units goes up, parking/unit ratio goes down.
Average Building:
Five of the variables in the final model are specific to the building and obtained from field work. Specific data for these variables are not available for all parcels across the study area. The calculator uses a prototypical building, with average values from the field work sample for these variables, as default when you first choose a parcel. You can then edit the five input variables to create a different scenario and view their effects on the parking/unit estimate.
Bedroom Count, Average Occupied (variable):
The average number of bedrooms in all occupied units. To calculate this average, studio units were assumed to have a bedroom count of one. Observations show that as bedroom count goes up, parking/unit ratio goes up.
Concentration or Gravity:
A measure in which both the quantity and proximity of a given variable are taken into account by calculating the quantity divided by the distance squared from the center of a parcel.
Geographic Coverage of Calculator:
To ensure confidence in the model estimates, limits were established for the geographic coverage of the calculator. The sample utilized for data collection covered a wide range of built environment characteristics and land uses, but it did not cover the full spectrum found throughout the County. Therefore, the coverage for which model estimates were calculated was limited to range of built environment characteristics found in the data collection sample. In other words, areas of the County that had lower transit service, population, or job concentrations than those found in the sample were removed from the coverage area.
Intensity (population + jobs), Gravity Measure (variable):
Intensity represents the gravity of the sum of population and jobs. Therefore, understanding this as a concentration, a high value can be the result of highly concentrated residential populations, highly concentrated jobs, or some combination of the two. Observations show that this measure has a strong inverse correlation with parking use (which goes down as intensity goes up). Population and jobs calculations are both derived from 2010 data sources. Population and jobs were disassociated in the location characteristics tab to provide greater flexibility with scenario building.
Jobs, Concentration:
The number of jobs in the region, scaled by their proximity to a parcel(s). One piece of the model input variable gravity measure of intensity. Puget Sound Regional Council (PSRC) employment estimates for 2010 are utilized in this calculation.
Parking Price as a fraction of Rent (variable):
The monthly price of parking per stall charged to the tenant divided by the average monthly rent. In properties with unpaid parking, this value is zero. This value approaches one as the cost of parking nears the cost of rent. Observations show that as parking price rises relative to the cost of rent, parking/unit ratio goes down.
Parking/Unit Ratio, Estimated Average (dependent variable):
The modeled value predicting parking use per residential unit based on the parcel’s Building & Parking Specifications and Location Characteristics. The value is expressed as x parking spaces per multi-family dwelling unit.
Population, Concentration:
The count of people in the region, scaled by their proximity to a parcel(s). One piece of the model input variable gravity measure of intensity. U.S. Census 2010 data are utilized in this calculation.
Rent, Average (variable):
Obtained from property managers, average rent represents the average monthly tenant rental costs of all residential units in the building (excluding parking costs). Observations show that as average rent goes up, parking/unit ratio goes up.
Strength of Estimate:
Using the standard error of the predicted value from the model, a 95% confidence interval was calculated (standard error * 1.96). Therefore, a stronger estimate will be represented with a smaller confidence interval, or +/- a smaller number.
Transit Service, Gravity Measure (variable):
All transit stops and stations, scaled by the frequency of service, and then summed by the value to each parcel based on the distance from the parcel. Observations show that transit service has a strong correlation with parking/unit ratio (which goes down as transit service goes up). Transit data were updated and current as of May 2012, and include all public transit service within the County.
Trip Generation Reduction:
Using a model developed by the Environmental Protection Agency, this figure represents the estimated percent reduction in trip generation relative to figures published by the Institute of Transportation Engineers (ITE), Trip Generation Manual. Average trip generation reductions are calculated as a function of the selected parcel(s) location and urban design characteristics. Trip generation is the number of trips originating from a particular parcel.
Unbundled Parking:
Represents a practice to charge the cost of parking to the multi-family tenant on a monthly basis separately from the cost of their rent. The unbundled price of parking per stall is based on the location of the parcel(s) selected (Suburban = $242/month; Urban = $275/month; CBD = $344/month).* The prices were set to recover the cost of supplying a parking facility and to provide a 10% annual profit (including land, capital, construction, operations, maintenance, and 95% occupancy) over a 40 year loan at 4.5% interest. For more information on how the prices were set, see here for the technical memo. Users are encouraged to use the VTPI Calculator (see below) to adjust the assumptions, develop a custom price based on the unique characteristics of the site, and manually enter it into the Monthly Price Per Stall form field found in the Building & Parking Specifications tab.

Methods: The three prices were derived using the VTPI Parking Costs, Pricing and Revenue Calculator developed by Todd Litman at the Victoria Transport Policy Institute, with hard and soft cost inputs reflecting Marshall&Swift Valuation Service Data as of 2014 for the Greater Seattle Area, and operating and Maintenance cost data reflecting a survey of local building owners and managers. Parking facility types were chosen to reflect a type of parking commonly constructed in new developments in areas of similar densities (Suburban = 2-Level Structure; Urban = 3-Level Structure; CBD = Underground). The pricing calculator uses 2012 King County land costs by suburban, urban, and CBD location types.
Units per Residential square feet (variable):
Obtained from property managers, units per residential square feet is calculated as total residential units divided by the residential square feet of the development. Observations show that as units per residential square feet goes up (or average unit size goes down), the parking/unit ratio goes down.

* The unbundled price of parking per stall was updated July, 2014 to reflect current market prices.