Measures of transportation system performance may include vehicle-miles of travel (VMT), speed, congestion, or delay, travel times, throughput, or other parameters. System performance measures such as travel time savings and VMT are commonly used in transportation planning at the project or regional level. Various aspects of system performance also serve as key inputs for forecasting other impacts. For example, vehicle-trips, VMT, and vehicular speeds are use to forecast emissions of criteria pollutants. Transportation performance also affects accessibility, which in turn affects land use patterns as well as economic development.
Transportation system performance is typically forecast using travel demand models, although other approaches such as sketch-planning methods may also be used. Improvements to the state-of-the-practice in travel modeling are discussed widely elsewhere, notably through the Travel Model Improvement Program (TMIP), and are therefore not a focus of this toolbox. The use of a high-quality travel model, however, is often critical to the accurate forecasting of other impacts discussed here. Otherwise, the model will not be fully sensitive to the range of transportation and land use policy alternatives that are tested. Desirable components not included in all travel models include:
Advanced techniques such as microsimulation and activity-based modeling are also helping to overcome many of the limitations of the current generation of travel models.
The travel model is modified to incorporate a walk/bike travel mode. Mode choice is determined in part based on land use-related variables.
A state-of-the-practice travel demand model and an integrated transportation-land use model are both used to measure the travel and emissions impacts of transportation and land use policies. The results from the two models are compared.
Other case studies including Albany, Montgomery County, and SPARTACUS also highlight measures of transportation system performance, although specific advances to travel modeling techniques are not described.
The Travel Model Improvement Program web site includes reports, papers, and conference proceedings describing short-term improvements that can be made to existing travel models, as well as longer-term advances in the practice of travel modeling.
Douglas (2000) describes modifications that can be made to travel models to better reflect neighborhood and urban design factors as well as non-motorized travel.