Traffic Assignment

The fourth stage of the modelling process is that of traffic assignment, its aim being to stimulate route choice through a defined transport network. It involves assigning the trips generated in the previous steps to specific routes and modes of transportation in the transportation network.

The traffic assignment process considers the capacity, speed, and travel time of the transportation network to determine the optimal routes and modes of travel for each trip. This process involves selecting the most efficient and cost-effective routes based on various factors such as travel time, distance, and congestion levels.

The traffic assignment process involves two main methods: the All-or-Nothing Assignment and the Iterative Assignment. The All-or-Nothing Assignment assumes that each trip will choose the shortest path to their destination and assigns all trips to the shortest path. The Iterative Assignment is a more complex method that involves assigning trips to different routes iteratively until an equilibrium is reached, and no further improvement can be made.

The output of the traffic assignment stage includes estimates of traffic volumes, travel speeds, and travel times for each road segment in the network. These estimates can be used to evaluate the performance of the transportation system, identify bottlenecks and congested areas, and develop transportation infrastructure and operational improvements to address these issues.

Traffic assignment may be considered in two parts.

•First, it is necessary to define the transport network and determine criteria for route choice through the network.

•Second, using the inter-zonal trip matrix as the input data, trips are assigned to this network.

The first part involves defining the transportation network and developing criteria for route choice through the network. This involves identifying the road network, public transit network, and other modes of transportation available in the study area, and defining the attributes that influence travel behavior, such as travel time, distance, and cost. The criteria for route choice can be developed based on traveler preferences and behavior, such as the desire for the shortest travel time or the preference for a particular mode of transportation. This information is used to develop mathematical models, such as the gravity model or the multinomial logit model, which help to estimate the probability of a trip choosing a particular route or mode of transportation.

The second part of traffic assignment involves using the inter-zonal trip matrix, which contains information on the number of trips between each origin-destination pair, as input data to assign trips to the network. This involves estimating the flow of traffic on each road segment or transportation mode by assigning the trips to the network in a manner that reflects the criteria for route choice developed in the first part of traffic assignment.

When future trip levels are assigned it is possible to assess deficiencies in the existing transport network and so determine a list of construction priorities. Network description refers to the process where the highway network is broken down into links and nodes. For each link, data is required on its length, road type, vehicle travel time and traffic capacity. When coding the road network, links are usually identified by the node numbers at each of its ends. In addition to such route-intersection nodes, zone-centroid nodes are also defined. In the assignment process, all traffic originating in a particular traffic zone is assumed to be loaded on to the network at this latter type of node.

The traffic assignment process can be iterative, with traffic volumes and travel times being updated and re-assigned until a traffic equilibrium is achieved, where no further improvement can be made. The output of traffic assignment includes estimates of traffic volumes, travel times, and congestion levels on each road segment or transportation mode in the network.

The traffic assignment stage is a crucial step in the four-step travel demand model as it helps to determine the optimal routes and modes of travel for each trip and provides information on the performance of the transportation network. This information can be used to improve transportation infrastructure and services and promote sustainable and efficient travel.

Traffic Impact Analysis (TIA)

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FD Planning Community Forum Discussion

• Land Use Transport Integration and Density of Urban Growth Toolkit
• Integration of Land Use and Transport Planning
• Introduction – Modelling Transport – Ortuzar Willumsen
• Mathematical Prerequisites from Modelling Transport
• data and space from Modelling Transport 
• Trip Generation Modelling from Modelling Transport
• Modal split and direct demand models from Modelling Transport
• Discrete choice models from Modelling Transport

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