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Travel Demand Scenarios

Travel Demand modeling uses a four-step process to model trip generation and trip linking, travel mode choice, and assignment to the transport network. A Travel Demand model uses different scenarios to model the different steps of the process and can also use a Four-Step Model scenario to link the outputs from one scenario to the inputs or another.

Different scenarios with different land use patterns and traveler response assumptions can be run using from within a single Aimsun document with different demand and different road network options as necessary

  • A Four-Step Model Scenario is used to connect the inputs and outputs of Travel Demand Scenarios and Static Assignment Scenarios and to connect data sources to scenario inputs. A Four-Step Model Scenario can then be used to automate the process of running some or all of the chain of scenarios.

  • Generation/Attraction Scenario: A Generation/Attraction (G/A) calculation takes socioeconomic data and the G/A factors corresponding to the G/A rates of each one of the socioeconomic values for each time period and trip purpose. Its output is a set of G/A vectors which go into a Four-Step Model scenario.

  • Distribution Scenario: The Distribution process transforms the Generation/Attraction vectors into OD matrices for each user class.

  • Modal Split Scenario: The Modal Split process transforms assigns trips to the modes available in the transport network.

  • Static Assignment Scenario: A Static Assignment scenario assigns vehicles to the available infrastructure.

  • Transit Assignment Scenario: A Transit Assignment scenario assigns passengers to the Transit Lines. The output is the loading on the Transit lines.

Other scenario types focus on non assignment tasks in the network:

  • Static OD Adjustment Scenario and Static OD Departure Adjustment Scenario: The Static OD Adjustment calculations adjust an a-priori OD matrix using observed link flow an turn count data to help calibrate a model. The Departure Adjustment calculations profile the OD matrix to distribute the OD demand over the time period to reproduce the observed data at a more detailed time scale.

  • Transit OD Adjustment Scenario: The Transit OD Adjustment calculations adjust an a-priori transit matrix using observed data on transit loading and on boarding and alighting counts to help calibrate a model.

There are other tools which assist in travel demand modeling, described elsewhere in the manual.

  • Static Traversal Generation: Used to generate a matrix for a cordoned subnetwork.
  • OD Matrix Editing and OD Matrix Balancing: Tools to manipulate and balance OD matrices.
  • Detector Location: This tool is used with an Adjustment Scenario to advise where further data collection devices should be placed to give the best coverage of the routes in the transport network and hence improve the adjustment results.
  • Supernodes: Supernodes are used in static assignments to model complex junctions with multiple road sections where the junction should be treated as a whole rather than as its constituent parts.