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Regional Models

Models are the principal tools used for alternatives analysis, and they provide planners and decision makers with information to help them equitably allocate scarce resources.

Modeling Overview

SANDAG deals with many complex mobility issues facing the San Diego region, including the development of a long-range Regional Transportation Plan (RTP). Transportation and land use models perform a very basic yet vital set of functions. Models are the principal tools used for alternatives analysis, and they provide planners and decision makers with information to help them equitably allocate scarce resources.

SANDAG has produced forecasts of demographic and economic growth in the region since 1971. Transportation forecasting at SANDAG began in 1981. The SANDAG forecasts are used by policymakers and the general public, as well as by public and private agencies throughout the region. For example, SANDAG uses the forecasts to develop the Regional Transportation Plan (RTP), the Regional Comprehensive Plan (RCP), and the Air Quality Conformity Plan. Local jurisdictions use the forecasts for general plan updates and capital facilities planning, including environmental impact reports (EIR), as well as for local transportation planning. Other agencies, such as the San Diego County Water Authority and the San Diego Regional Energy Office, use aspects of the SANDAG forecasts to develop plans for providing these essential services.

Transportation Modeling

SANDAG completed the transition from an enhanced four-step transportation model to an activity-based model (ABM1) in 2013 and applied ABM1 in the 2015 Regional Plan. SANDAG has since completed the development of ABM2 and applied it in the 2019 Federal Regional Transportation Plan. SANDAG is currently deploying ABM2+ to support the 2021 Regional Plan, also known as the 5 Big Moves.

More information about Transportation Modeling and the Activity Based Model

Demographic, Economic, and Land Use Models

The currently adopted Regional Growth Forecast (referred to as the Series 13 Forecast) uses four integrated models in its demographic, economic, and land use forecasts: (1) the Demographic and Economic Forecasting Model (DEFM), (2) the Interregional Commute Model (IRCM), (3) the Urban Development Model (UDM) and (4) the Population Age, Sex, and Ethnicity Forecast (PASEF). The Series 13 Forecast was used in conjunction with the 4-Step Transportation Model. A noteworthy feature of the forecasting process is the feedback of information from one model to another (See Figure 1).

For example, information from DEFM is used in the IRCM and then the output from the IRCM is used to modify the output from DEFM. DEFM then provides the regionwide projections that serve as the basis for UDM and PASEF. Similarly, data from UDM and PASEF are major inputs to the transportation model, and then transportation model data are used in subsequent UDM calculations. A key feature of the modeling system is the central role that land use and transportation policies play in determining future travel patterns and the associated location of people, houses, and jobs.

These interrelated models satisfy the federal requirements specified in the Clean Air Act Amendments of 1990 and the Safe, Accountable, Flexible, Efficient, Transportation Equity Act: A Legacy for Users (SAFETEA-LU). These legislative acts mandate that transportation plans consider the long-range effects of the interaction between land uses and the transportation system.

More information on the Demographic, Economic, and Land Use Models

Looking Ahead: New Models in Development

Before deciding whether to implement a project, policy, or plan, SANDAG first analyzes how the transportation system will respond to the change and what other potential impacts may occur. Transportation models are a central part of the toolset used to undertake such analysis. Because it is important to use the best available tools to inform regional transportation decision-making, SANDAG is currently updating the suite of regionwide and subregional forecasting models. This updated model structure is referred to as the Integrated Land Use, Demographic and Economic Model (I-LUDEM). An important feature of I-LUDEM is that it integrates the yearly population and housing estimates with the regional and subregional forecast models. This streamlines SANDAG modeling efforts because all data products are in alignment with each other. It also allows updates to be done more frequently and as a part of one process.