From 1980 to 1985, the PSRC implemented an early public version of the DRAM (Disaggregate Residential Allocation Model) and EMPAL (Employment Allocation Model) models, developed by Dr. Stephen Putman. These models were subsequently modified and tailored to better fit the analysis and forecasting needs, along with the data availability, of the agency. From 1980 through 2006, the Regional Council has relied on its versions of the DRAM and EMPAL to produce zonal-level forecasts of population, households, and jobs – a product known as the Small Area Forecasts.

Essentially, the DRAM and EMPAL models function as “top-down” allocation models. The process starts by inputting the total number of households, persons, and jobs forecast for the region by the agency’s regional economic forecasting model (currently that is the Puget Sound Economic Forecaster model). DRAM and EMPAL then allocates the forecasts to each of 219 Forecast Analysis Zones (FAZs) in the region. County-level forecasts are derived by summing up each county’s FAZs, as opposed to developing a county-level forecast first. This process can better account for shifts in county-level forecasts due to forecasts of key zones in the region, such as those within fast-growing regional growth centers or suburban large-scale planned developments.

After establishing FAZ-level base year estimates of employment and residential data, the EMPAL model begins by forecasting jobs by zone, for the first forecast year (currently 2010). After jobs have been allocated, the DRAM model then assigns households and population. Both routines include as input estimated travel times between zones. After the first round of forecasts are completed, the results are fed back into the travel demand model, and a revised set of travel times used to re-run the DRAM and EMPAL models, to ensure that changes in land use due to travel time changes are being accounted for in the forecasts.

Turning model output into forecasts

In an effort to better reflect some of the factors that DRAM and EMPAL do not reasonably account for, the PSRC works in conjunction with planning staff from its member jurisdictions in an extensive review process of the DRAM and EMPAL results prior to being finalized.

Replacing DRAM and EMPAL

In 2003 the PSRC began working on implementing the UrbanSim land use model as a replacement for DRAM and EMPAL. UrbanSim’s ability to use future year land use plans as an input addresses one of the primary shortcomings of the DRAM and EMPAL models, in that comprehensive plans could not be directly used as a model input. Current plans are for UrbanSim to be used instead of DRAM and EMPAL in the next Small Area Forecast update in 2011.