Fundamentals of Transportation/Planning

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All forecasts are wrong, some forecasts are more wrong than others. - anonymous

Contents

[edit] Why Model?

Models may be undertaken for a variety of reasons, these include:

  • Forecasting traffic
  • Testing scenarios (alternative land uses, networks, policies)
  • Planning projects/corridor studies
  • Regulating land use: Growth management/public facility adequacy
  • Managing complexity, when eyeballs are insufficient, (different people have different intuitions)
  • Understanding travel behavior
  • Influencing decisions
  • Estimating traffic in the absence of data

[edit] Developing Models

As an engineer or planner you may be given a model to use. But that model was not spontaneously generated, it came from other engineers or planners who undertook a series of steps to translate raw data into a tool that could be used to generate useful information.

The first step, specification, tells us what depends on what (the number of trips leaving a zone may depend on the number of households). The estimation step tells us how strong are these relationships mathematically, (each household generates two trips in the peak hour). Implementation takes those relationships and puts them into a computer program. Calibration looks at the output of the computer program and compares it to other data, if they don't match exactly, adjustments to the models may be made. Validation compares the results to data at another point in time. Finally the model is applied to look at a project (how much traffic will use Granary Road after Washington Avenue is closed to traffic).

  • Specification:
    • y=f(X)\,\!
  • Estimation:
    • y=mX+b\,\!; m=1, b=2
  • Implementation
    • If Z > W, Then y=mX+b\,\!
  • Calibration
    • y_{predicted}+k=y_{observed}\,\!
  • Validation
    • y_{predicted1990}+k=y_{observed1990}\,\!
  • Application

[edit] “Four-Step” Urban Transportation Planning Models

We want to answer a number of related questions (who, what, where, when, why, and how):

  • Who is traveling or what is being shipped?
  • Where are the origin and destination of those trips, shipments?
  • When do those trips begin and end (how long do they take, how far away are the trip ends)?
  • Why are the trips being made, what is their purpose?
  • How are the trips getting there, what routes or modes are they taking?

If we know the answers to those questions, we want to know what are the answers to those questions a function of?

  • Cost: Money, Time spent on the trip,
  • Cost: Money and Time of alternatives.
  • Benefit (utility) of trip (e.g. the activity at the destination)
  • Benefit of alternatives

The reason for this is to understand what will happen under various circumstances:

  • How much “induced demand” will be generated if a roadway is expanded?
  • How many passengers will be lost if bus services are cut back?
  • How many people will be “priced off” if tolls are implemented?
  • How much traffic will a new development generate?
  • How much demand will I lose if I raise shipping costs to my customers?

In short, for urban passenger travel, we are trying to predict the number of trips by:

  • Origin Activity,
  • Destination Activity,
  • Origin Zone,
  • Destination Zone,
  • Mode,
  • Time of Day, and
  • Route.

This is clearly a multidimensional problem.

In practice, the mechanism to do this is to employ a "four-step" urban transportation planning model, each step will be detailed in subsequent modules. These steps are executed in turn, though there may be feedback between steps:

  • Trip Generation - How many trips are entering or leaving zone i or j
  • Trip Distribution or Destination Choice - How many trips are going from zone i to zone j
  • Mode Choice - How many trips from i to j are using mode m
  • Route Choice - Which links are trips from i to j by mode m using route r


[edit] Thought Questions

  • Is past behavior reflective of future behavior?
  • Can the future be predicted?
  • Is the future independent of decisions, or are prophesies self-fulfilling?
  • How do we know if forecasts were successful?
  • Against what standard are they to be judged?
  • What values are embedded in the planning process?
  • What happens when values change?

[edit] Variables

  • msXX - scalar matrix
  • moXX - origin vector matrix
  • mdXX - destination vector matrix
  • mfXX - full vector matrix
  • Tij - Total number of trips between origin i and destination j
  • Ki - Calibration parameter for origin i
  • Kj - Calibration parameter for destination j
  • Ti - Trips leaving origin i
  • Tj - Trips arriving at destination j
  • T'j - Effective Trips arriving at destination j, computed as a result for calibration to the next iteration
  • f(Cij) - Cost function between origin i and destination j
  • TC - Total Cost
  • Q - Total flow

[edit] Additional Problems

[edit] Abbreviations

  • SOV - single occupant vehicle
  • HOV - high occupancy vehicle (2+, 3+, etc.)
  • TAZ - transportation analysis zone or traffic analysis zone

[edit] Key Terms

  • Rational Planning
  • Transportation planning model
  • Matrix, Full Matrix, Vector Matrix, Scalar Matrix
  • Trip table
  • Travel time matrix
  • Origin, Destination
  • Purpose
  • Network
  • Zone (Traffic Analysis Zone or Transportation Analysis Zone, or TAZ)
  • External station or external zone
  • Centroid
  • Node
  • Link
  • Turn
  • Route
  • Path
  • Mode
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