Why does Arlington County produce a population and jobs forecast?
Arlington County participates in the Metropolitan Washington Council of Governments (MWCOG) Cooperative Forecasting program – a joint effort with Federal and local governments of the region to produce a consistent set of long-range economic and demographic forecasts. These forecasts are the basis of functional plans in the areas of transportation, water resources, air quality, housing, land use, and energy.
The regional forecast is used in travel demand models that determine how various transportation invests affect mobility in the region. A primary purpose of the model is to determine air quality conformity through the regional air quality analysis as mandated by the federal Clean Air Act Amendments of 1990. Air quality conformity is necessary to receive federal funding for transportation projects.
What is produced in the forecasting process?
Arlington County produces a forecast for four variables: housing units, households, population and employment.
- Housing Unit: A house, an apartment, a mobile home or trailer, a group of rooms, or a single room occupied as separate living quarters, or if vacant, intended for occupancy as separate living quarters. (U.S. Census Bureau)
- Household: A household includes all the people who occupy a housing unit (such as a house or apartment) as their usual place of residence. (U.S. Census Bureau)
- Population: All people, male and female, child and adult, living in a given geographic area. (U.S. Census Bureau)
- Employment: The total number of jobs located in a given geographic area.
How is the forecast produced?
Arlington County’s Population and Employment forecast is produced by the County’s Urban Design and Research section.
Arlington’s forecasting process is development based and is informed by the land use policy guidance of Arlington’s General Land Use Plan (GLUP). The forecast uses two primary sources of development information:
- County-wide development pipeline data on recently completed projects, projects under construction, and approved projects.
- development assumptions derived from parcels with anticipated growth from approved sector plans and small area plans
Updates to Arlington County’s forecast are generally produced on an annual or 2-year basis. Forecasts are produced for a 30-year period, broken down in to 5-year increments. Each major forecasting process led by MWCOG is call a Round. In February 2016, Arlington County submitted updated forecast numbers for Round 9.0.
What assumptions are included in the forecast?
The following are assumptions applied to the forecast Round 9.0, which was submitted to MWCOG in February 2016.
The forecast includes:
- Projects that were completed, under construction, or approved as of July 1, 2015.
- Parcels with anticipated growth from approved plans.
- The model takes into account Arlington’s most current planning assumptions through documents approved by the Arlington County Board such as the GLUP, sector plans, small area plans, and the zoning ordinance.
- Residential unit occupancy and household size are based on 2010 Census rates (these rates vary by planning areas).
Office vacancy rates are based on Second Quarter 2015 CoStar data.
- Vacancy rates are adjusted for the remaining known leases in BRAC affected buildings.
- Vacancy rates vary based on Arlington submarket area and are normalized to each submarket’s 20-year average by 2045.
- Vacancy rates for existing office space in the Coordinated Redevelopment Districts (CRD) in Rosslyn and Crystal City are normalized to vacancy rates higher than the 20-year averages. This is to account for the period of time in which sizeable, vintage office buildings are taken off the market due to demolition and or redevelopment into denser mixed-use developments.
- Vacancy rates for forecasted office space are kept at a constant 10% throughout the 30-year forecast. This is to demonstrate that new office construction will be Class A space and will be occupied at different rates than existing office space.
The timing of forecasted residential and office development is informed by property ownership patterns, developer activity, and plan assumptions. This timing is then further calibrated using historic countywide and submarket residential and office absorption rates to ensure that forecasted countywide residential and commercial office growth is consistent with previously established growth rates.
Does Arlington project Students?
Yes, Arlington Public Schools projects students. Arlington’s population and forecasting process, managed by the department of Community Planning, Housing & Development (CPHD), forecasts population, households, housing units, and employment; it does not forecast students.
How accurate is Arlington County’s forecast?
Arlington County’s population forecast has proven to be accurate with a low margin of error. Below is a chart that shows the population forecast for 2010 based on the forecast version (Round 5-8). Between the forecast years of 1994-2010, the forecasts has a relatively low margin of error – an average of 3%. For each of the rounds, the percent difference between the actual 2010 Census number reported by the U.S. Census Bureau and the population forecast by the County. Every round the forecast is readjusted to reflect current economic conditions. For example, Round 7.2 was produced in 2008 (adopted by MWCOG in 2009), right before the first impacts of the recession were experienced, which lead to a higher than average margin of error. In the next year, Round 8 was produced and updated for the impacts of the recession. This lead to a much lower margin of error, 1.4%.
|Forecast Version||Year Adopted||Forecast Years||2010 Forecast Population||Difference||% Difference|
Arlington’s 5-step forecasting process
Step 1: Calculate current net new construction. Development and permitting data is used to calculate net new construction that has occurred since April 2010. Net new construction is calculated for housing units, office square footage, retail square footage, other square footage, and hotel rooms.
Step 2: Determine development potential. The General Land Use Plan (GLUP), County Board approved site plans, phased development site plans, sector plans, small area plans, and the zoning ordinance are used to determine the development potential at a parcel or combined parcel level. The primary use (hotel, office, or residential) of the potential development is determined through appropriate plan guidance, the zoning ordinance, the general land use plan, parcel configuration, and lastly, market conditions.
Step 3: Calibration. Historic absorptions rates are calculated for multifamily housing units, office square footage and retail square footage. These rates are used to calibrate the timing of development over the forecast period. In some cases, anticipated development may occur after the end of the forecasting period.
Step 4: Calculate Net New Development. Total net new development is determined for each 5-year interval for the following categories: housing units, office square footage, retail square footage, other square footage, and hotel rooms.
Step 5: Calculate Population and Employment. Population is calculated by applying occupancy and average household size factors to the net new housing units. These factors are derived by data reported by the 2010 Census. Employment is calculated by applying occupancy and an employment-space conversion factor (occupied square feet per employee) to the net new commercial space. The result is the number of jobs by the following uses: office, retail, other, and industrial.
What are the differences among counts, estimates, projections, and forecasts?
- Count: determines a total number (Decennial Census)
- Estimate: calculations of past or present conditions, utilizing counts and known statistics
- Projection: measures future growth by extrapolating current trends an applying statistical techniques
- Forecast: projections, modified by policy, work to resolve trends (past and current) with future policy