1. Post Processing Procedures in Travel Demand Modeling

2. 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.

3. 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.

4. 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”

5. 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

6. Both Require Judgement
2 Refinement Methods
Link-based refinement
Screenline/Cutline-based refinement
Both Require Judgement

7. Link-based Refinements

8. 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)

9. Example: Link-Base Refinement
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

10. Example: Link-Base Refinement
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?

11. Base
Year
Yellow shows where model
volumes are greater than counts

12. Base
Year
Red shows where counts are greater than model volumes

13. Horizon
or Future
Year
2030 Unadjusted Model Volumes

14. Horizon
or Future
Year
2030 Adjusted Model (ratio adjustment)

15. Horizon
or Future
Year
2030 Adjusted Model (additive adjustment)

16. Screenline/Cutline-based
Refinements (NCHRP 255)

17. 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?)

18. Screenline/Cutline Refinement
Screen/Cutline

19. 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)

20. 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
0.50
Capacity Adjustment Factor
0.50
0.40
0.60
0.30
0.70
0.20
0.80
0.10
0.90
0.00
1.00
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
Future Year Screenline V/C Ratio

21. * Notes:
FCAP and FCOUNT from chart
in this example, the refined future assignment RA(f) is averaged, despite the ratio being > 15%

22. Summary/Opinions

23. Link-based Refinement
Advantages
Disadvantages
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

24. Screen line/Cutline Refinement
Advantages
Disadvantages
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

25. Thanks to: Bill Troe, AICP Vice President, URS Corporation
and Phil Mescher, Iowa DOT