Impact Methodologies

Transportation System Performance

Overview

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:

Detailed networks for multiple modes, including transit and high-occupancy vehicle as well as the standard highway network;
A nested or multinomial logit mode choice model;
Incorporation of non-motorized as well as motorized modes;
Feedback of travel times that consider congestion into earlier steps of the modeling process, including mode choice, trip distribution, and possibly trip generation;
A time-of-day modeling component; and
Modeling of automobile ownership.

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.