Travel demand forecasting is a major step in the transportation planning process.  By simulating the current roadway conditions and the travel demand on those roadways, deficiencies in the system are identified.  This is an important tool in planning future network enhancements and analyzing currently proposed projects.
Travel demand models are developed to simulate actual travel patterns and existing demand conditions.  Networks are constructed using current roadway inventory files containing data for each roadway within the network.  Travel demand is generated using socioeconomic data such as household size, automobile availability and employment data.  Once the existing conditions are evaluated and adjusted to satisfactorily replicate actual travel patterns and vehicle roadway volumes, the model inputs are then altered to project future year conditions.
There are four basic steps in the traditional travel demand forecasting process: trip generation, trip distribution, modal choice, and trip assignment.  There is also a preliminary step of network and zone development and a subsequent step of forecasting future conditions.  The Pioneer Valley Planning Commission (PVPC) uses the TransCAD software to perform a 3-step process for forecasting near and future conditions including trip generation, trip distribution and trip assignment.