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 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.
The SANDAG transportation model provides a systematic analytical platform so that different alternatives and inputs can be evaluated in an iterative and controlled environment. SANDAG uses an enhanced four-step transportation model. Four-step models have been the standard in transportation modeling since the late 1950s, and they are used by nearly every MPO in the United States for the development of transportation plans, corridor studies, Federal Transit Administration New Starts proposals, and air quality analyses. The estimates of regional transportation related emissions analyses meet the requirements established in the Transportation Conformity Rule, 40 CFR Sections 93.122(b) and 93.122(c). These requirements relate to the procedures to determine regional transportation-related emissions, including the use of network-based travel models, methods to estimate traffic speeds and delays, and the estimation of vehicle miles of travel.
The four steps of the transportation model include trip generation, trip distribution, mode choice, and network assignment.
More information on the 4-Step Transportation Model
Demographic, Economic, and Land Use Models
SANDAG 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), in conjunction with the Transportation Model described above. 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 (PECAS and ABM)
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 transitioning from its existing 4-step transportation model to a more advanced model, an activity-based model (ABM), and from its existing UDM model to an economic microsimulation model (PECAS).