Travel Demand Models (TDM) are used to forecast traffic flows on the transportation system. Although the transportation system may include other modes of travel such as walking, bikes, or trains, the models are typically used for evaluating roadway improvements or improvements to bus service. These models are used only for long range forecasts: 20 to 30 years. TDMs are used by consulting firms, metropolitan planning organizations (MPO), and state departments of transportation (DOT) to identify possible future year transportation system deficiencies that may not exist today. These agencies also use the models to evaluate the impacts of alternative transportation solutions for development of long range transportation plans. TDMs are used as the basis for “Certified Traffic” forecasts, for pollution emission estimates, and for congestion management system statistics.
A travel demand model is a program or set of computer programs and data which are assembled and usually run by professionals who specialize in travel forecasting. There are only a few developers that offer travel demand modeling software packages that run on personal computers. Some of the well known packages are QRSII, Cube Voyager, and TransCAD. For models used in Ohio, other programs developed by the DOT, consultants, or MPOs are also part of the model. Land use, demographic data, and regional trip characteristics for a region (typically one or more counties) are combined with this software for a single model.
The traffic forecasts are based on forecasted land use, demographics, and travel patterns unique to the region. Regions range in size from half a county to several counties. The largest models cover an entire state and include effects of growth of the economy from coast to coast.
Therefore, inputs to the models include the transportation network and variables such as population, employment, households, dwelling units, trip rates, transit fares, etc. Among other statistics and reports, outputs from the model are maps of the transportation system with traffic volumes for every roadway segment.