Dynamic Traffic Assignment: A Primer

Posted by Content Coordinator on Wednesday, July 6th, 2011


Why Dynamic Traffic Assignment?

From a Transportation Planning Perspective
Travel forecasting models are used in transportation planning to evaluate the impact of future changes in demographics, land use, or transportation facilities on the performance of a region’s transportation system. Traveler behavior is introduced into these forecasting models through sequences of modeling steps. The traditional four-step process, for example, results in travel choices made by groups of homogeneous travelers in aggregate trip-based models. More advanced activity-based processes seek to represent travel choices made by individual travelers.

Cost and time of travel are key components of all travel models throughout the entire sequence of model steps. For example, a household’s choice of the number of personal vehicles to own is often forecast subject to aggregate measures of the accessibility of the household. The less accessible a household is, the more likely it is to own automobiles. An accessibility measure is then some representation of the travel time and cost from the residence to work places or shopping places. Likewise, time and cost are clearly significant factors in other choices made, including residential, workplace, and discretionary activity locations, as well as factors in deciding which transportation services to use and which routes to follow when engaging in travel.

From a travel forecasting perspective, the time and cost of travel are critical factors. Those measures are also critical in quantifying impacts on a regional scale for the purpose of informing policy decisions. Travel time and cost measures determined using static network analysis (assignment) procedures use variables of interest that are time-invariant. It has become increasingly evident that these procedures are inadequate as explanations of influences on travel choices and as measures used to evaluate impacts when deciding how to develop policies for managing transportation systems, how to fund transportation system improvements, and how to measure environmental impacts related to system-wide travel.

Dynamic network analysis models seek to provide another, more detailed means to represent the interaction between travel choices, traffic flows, and time and cost measures in a temporally coherent manner (e.g., further improve upon the existing time-of-day static assignment approach). More specifically, dynamic traffic assignment (DTA) models aim to describe such time-varying network and demand interaction using a behaviorally sound approach. The DTA model analysis results can be used to evaluate many meaningful measures related to individual travel time and cost, as well as system-wide network measures for regional planning purposes.

Figure 9

Download full version (PDF): Dynamic Traffic Assignment

About Transportation Research Board
“The mission of the Transportation Research Board is to provide leadership in transportation innovation and progress through research and information exchange, conducted within a setting that is objective, interdisciplinary, and multimodal.”

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