Post processing procedures in travel demand modeling
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Post Processing Procedures in Travel Demand Modeling. Why Post Process?. Models are based on many assumptions. We are dealing with the future. There is no crystal ball. No model is perfect. We can assume that if the model is wrong now, it will not magically improve itself in the future.

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Why post process
Why Post Process?

  • Models are based on many assumptions.

  • We are dealing with the future.

  • There is no crystal ball.

  • No model is perfect.

  • We can assume that if the model is wrong now, it will not magically improve itself in the future.


Base model
Base Model

  • We always start off by building a base year model.

  • This model is used to test our assumptions and compare the model output to empirical data. (i.e. Traffic Counts)

  • There will always be some amount of error.


Forecast model
Forecast Model

  • Since we know there are areas in our base model with error….

  • We need to account for that error in the forecast model.

  • This is referred to as “Post Processing”


Guarantees of omitting post processing
Guarantees Of Omitting Post Processing

  • Some day, it will predict less traffic than current levels

  • It will recommend too much construction; implications on:

    • Funding

    • Environmental

    • Design feasibility/ROW constraints

  • It will Undermine credibility of entire process – Most likely in public


2 refinement methods
2 Refinement Methods

  • Link-based refinement

  • Screenline/Cutline-based refinement

  • Both Require Judgement



Link based refinement
Link-Based Refinement

  • Adjust future assignment based on:

    • Absolute difference in base

      • (Count - Assignment)

  • Ratio difference in base

    • (Count/Assignment)

  • Apply both to future; Then Average

    • Assumes relative is ±15%

  • >15%; Use only the absolute

    • (Dampens Extremes)


  • Example link base refinement
    Example: Link-Base Refinement

    • Base:

      Count – 11,000 VPD

      Assign - 10,000 VPD

    • To bring assignment in line, increase assignment by 10% (Ratio difference)

    • Or, add 1000 VPD (Absolute Difference)

    • If Future Assignment = 15,000 VPD

    • Adjustment:

      Ratio = (15,000/10,000)*11,500 = (11,000/10,000)*15,000 = 16,500 VPD

      Absolute = (11,000-10,000)+15,000 = 16,000 VPD

      Average = 16,250 VPD


    Example link base refinement1
    Example: Link-Base Refinement

    • Base:

      Count –2,000 VPD

      Assign - 4,000 VPD

    • To bring assignment in line, decrease assignment by 50% (Ratio difference)

    • Or, subtract 2000 VPD (Absolute Difference)

    • If Future Assignment = 15,000 VPD

    • Adjustment:

      Ratio = (2000/4000)*15,000 = (15,000/4000)*2000 = 7,500 VPD

      Absolute = 13,000 VPD … use this one … why?


    Base

    Year

    Yellow shows where model

    volumes are greater than counts


    Base

    Year

    Red shows where counts are greater than model volumes


    Horizon

    or Future

    Year

    2030 Unadjusted Model Volumes


    Horizon

    or Future

    Year

    2030 Adjusted Model (ratio adjustment)


    Horizon

    or Future

    Year

    2030 Adjusted Model (additive adjustment)


    Screenline/Cutline-based

    Refinements (NCHRP 255)


    Screenline cutline refinement
    Screenline/Cutline Refinement

    • Similar to link adjustment; Considers capacity (remember, your roads must be able to handle the traffic you adjust to)

      • Question: what happens to “equilibrium” when we post process?

    • Considers multiple links in one analysis

    • Attempt to smooth highs/lows over parallel routes in the corridor (alt. to Dial’s method?)



    Screenline cutline adjustment concept
    Screenline/Cutline Adjustment Concept

    • Influencing factors:

      • Base year traffic patterns

      • Change in traffic (base to future)

      • Change in network (e.g., adding a new route)

      • Congestion

    • Influence of capacity/base count

      (See next page)


    Capacity and base count adjustments
    Capacity and Base Count Adjustments

    1.00

    0.00

    0.90

    0.10

    0.80

    0.20

    0.70

    0.30

    0.60

    0.40

    Count Adjustment Factor

    Capacity Adjustment Factor

    0.50

    0.50

    0.40

    0.60

    0.30

    0.70

    0.20

    0.80

    0.10

    0.90

    0.00

    1.00

    0.8

    1.2

    1.5

    0.0

    0.3

    0.6

    0.7

    0.9

    1.1

    1.3

    0.2

    0.1

    0.4

    0.5

    1.0

    1.4

    Future Year Screenline V/C Ratio


    • * Notes:

    • FCAP and FCOUNT from chart

    • in this example, the refined future assignment RA(f) is averaged, despite the ratio being > 15%



    Link based refinement1

    Simple application

    Reasonably defensible

    Can be applied on a regional scale

    May not balance out system highs/lows

    Only moderately responsive to congestion (Adjustments not really capacity restrained)

    Adjustment difficult without base data (new facilities/limited counts)

    In areas of high growth, adjustments based on base year calibration may not make much sense

    Link-based Refinement

    Advantages

    Disadvantages


    Screen line cutline refinement

    For at least study area - balance highs/lows - More dynamic

    Reasonably defensible

    Spreadsheet/GIS applications make more practical

    What is application area? Constantly changing-Must re-evaluate

    Adjustment less defensible without base data (new facilities/ limited counts)

    Not reasonable for regional analysis – Without GIS application

    Screen line/Cutline Refinement

    Advantages

    Disadvantages


    Thanks to bill troe aicp vice president urs corporation

    Thanks to: Bill Troe, AICPVice President, URS Corporation

    [email protected]

    515-284-5500

    and Phil Mescher, Iowa DOT

    515-239-1629


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