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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?

slide11

Base

Year

Yellow shows where model

volumes are greater than counts

slide12

Base

Year

Red shows where counts are greater than model volumes

slide13

Horizon

or Future

Year

2030 Unadjusted Model Volumes

slide14

Horizon

or Future

Year

2030 Adjusted Model (ratio adjustment)

slide15

Horizon

or Future

Year

2030 Adjusted Model (additive adjustment)

slide16

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

slide21

* 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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