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Integrated Urban Models: Design Issues for Integrated Urban Models


Models are defined as mathematical algorithms which simulate human activities, ... Travel demand forecasting models are used to predict travel on a ... – PowerPoint PPT presentation

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Title: Integrated Urban Models: Design Issues for Integrated Urban Models

Integrated Urban ModelsDesign Issues for
Integrated Urban Models
Definitions (Miller et al)
  • Models are defined as mathematical algorithms
    which simulate human activities, such as travel
    behavior. Once they are calibrated against
    existing (known) conditions, the models can be
    used to forecast these activities. For present
    purposes, however, it is appropriate also to
    include theory and data, in addition to
    algorithms, in the definition of a model.

Definitions (Miller et al)
  • Travel demand forecasting models are used to
    predict travel on a transportation network.
    Travel demand -- i.e., traveler behavior -- is a
    function of both human activities (generally
    represented in the models by land-use see below)
    and network characteristics (including the
    services provided and their costs where costs
    generally are expressed in terms of time and/or
    money expended by the user and/or the
    transporter, and possibly intangibles such as
    comfort, etc.). Therefore, travel forecasts will
    vary according to changes in human activities or
    network characteristics.

Definitions (Miller et al)
  • Land-use models generally are mathematical tools
    that forecast demographic and economic measures
    of land-based activities. These measures can be
    described as meaningful attributes of the
    population (e.g., age-group cohorts, income,
    jobs, etc.), but also can be expressed in terms
    of development (e.g., residential dwelling units,
    commercial floor space, etc.). They take into
    account trends and policies regarding demographic
    and economic development, as well as the supply
    of developable land. The shape of the
    transportation network, and its possible
    influence on the location of human activities are
    taken into account, but usually in a generalized
    manner. Among other uses, the results serve as
    input to a travel demand forecasting model.
    However, generally there is little or no feedback
    from travel demand forecasting models into
    land-use models.

Definitions (Miller et al)
  • Integrated transportation - land-use models
    simulate the interaction of land-use and
    transportation. They are intended to provide a
    feedback mechanism between land-use models and
    travel demand forecasting models. For example, a
    new or improved transportation link can influence
    the timing, type and extent of development in a
    particular area, which in turn can influence the
    need for further network improvements. Similarly,
    the evolution of land-based human activities
    influences the demand for improved transportation
    services, which in turn further influences
    development. Therefore, integrated models take
    land-use models a step further, by also
    simulating the location of human activities as a
    function of transportation accessibility.
    Similarly, integrated models do not simply
    provide land-use input to travel demand
    forecasting models.

Definitions (Miller et al)
  • Transit accessibility describes how well a given
    point in an urban area is connected by transit to
    other points within the area. "Accessibility" can
    be defined in many ways
  • the absolute existence of a physical means of
    traveling between two points, where no other link
    exists (in this case, by transit)
  • the relative level of service (speed, cost,
    comfort, etc.) of a particular transit link,
    where alternate transit links exist and which may
    offer different levels of service and
  • the relative level of service of a particular
    transit link, compared with alternate modes
    (e.g., auto) which provide the same link.

Model Development Life Cycle
  • Requirements Analysis
  • Conceptual Design
  • Model Specification
  • Data Development
  • Model Estimation
  • Application Software
  • Model System Validation
  • Model Application
  • Model and Software Documentation

Model Development Life Cycle
  • Requirements Analysis
  • What questions should the model address?
  • What level of detail is needed?
  • Who will use it, and for what applications?
  • What data are available to use in developing it?
  • What resource and time constraints apply?
  • How should trade-offs be made on design choices
  • Accuracy
  • Policy-sensitivity
  • Complexity
  • Resource requirements

Model Development Life Cycle
  • Conceptual Design
  • What modeling framework best addresses the
    requirements, under the time and resource
  • Given a particular modeling framework, what are
    the major model components that need to be
  • How should model components interact within a
    model system?
  • Given a high-level conceptual design, are there
    requirements that cannot be met? Should the
    design or the requirements be modified to be
  • How should data and resource requirements be
    modified, given this conceptual design?

Model Development Life Cycle
  • Model Specification
  • For each model component, how should the
    equations be specified?
  • What is the dependent variable? (continuous,
  • What are the independent variables? What does
    theory suggest should be included?
  • Are any effects likely to be nonlinear? What
    transformations would be appropriate? (log,
  • Are there likely to be interactions between
    variables? (if so, add interaction variables)

Model Development Life Cycle
  • Data Development
  • What data are needed to estimate the model
  • What processing needs to be done?
  • What quality control checks should be done?
  • How should missing data be handled?
  • How should outliers be handled?

Model Development Life Cycle
  • Model Estimation
  • For each specified model equation, need to
    estimate the parameters associated with each
    independent variable.
  • Depending on the nature of the model
    specification, will need specialized econometric
  • Multiple regression
  • Multinomial logit
  • Nested Logit

Model Development Life Cycle
  • Application Software
  • What kind of software is needed to implement the
    model for use in forecasting and policy analysis,
    using the estimated model?
  • What language should it be implemented in?
  • How can it be modularized to make it easy to
    maintain and evolve?
  • What kind of user-interface does it require?
  • What other performance or functional requirements
    are there?

Model Development Life Cycle
  • Model System Validation
  • Once the estimated model is implemented in
    application software, it is operational.
  • How should we test how well the model is working?
  • Are the model predictions generally reasonable?
  • Are the results sensitive to changes in model
    inputs and assumptions in ways that seem
  • Are there observed data that we can use to
    compare the model results to, in order to
    validate the model?
  • Are adjustments needed to equation constants to
    improve the overall fit of the predictions to the
    observed data?

Model Development Life Cycle
  • Model Application
  • What initial real-world projects could be used
    to apply the model to?
  • Helpful not to start with extremely large, or
    politically sensitive topics
  • Results may still reveal problems in the model
    that need further adjustment or refinement
  • Once model has been tested successfully on a few
    non-critical projects, can be applied to more
    important ones

Model Development Life Cycle
  • Model and Software Documentation
  • Last, but not least, the model needs to be
  • Good documentation of operational models often is
    provided in three levels
  • Executive Summary
  • Reference Guide
  • User Guide