Transcript Using Sub-Zone Assignment In An Otherwise Traditional Four

Using Sub-Zone Assignment In An
Otherwise Traditional Four-Step Model
Aichong Sun
Thomas Cooney
Pima Association of Governments
Most Relevant Studies
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Williams Mann, VDOT
B-Node Model: New Subarea Traffic Assignment Model &
Application
Presented at 8th TRB conference on the Application of Transportation
Planning Methods, 2002
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Alan Horowitz, U of Wisconsin-Milwaukee
Computational Issues in Increasing the Spatial Precision of
Traffic Assignments
Transportation Research Record No. 1777, 2001
TAZ & Network of Traditional Four-Step Model
Issues With Typical Four-Step Model
Unloaded Centroid Connector
Loaded Centroid Connector
Minor Arterial
Minor Arterial
Issue 1: unloaded centroid connectors
Issues With Typical Four-Step Model
Average Travel Distance
― Model may report 10 miles
― Actually 11.2 miles, 12% higher
In a large urban area, even 5%
increase in total VMT could mean
serious traffic problem and many
air quality issues.
Issue 2: underestimate VMT
Issues With Typical Four-Step Model
Why issues?
― Local traffic is represented by intra-zonal trips
― Traffic assignment algorithm (UE)
Common practice is to,
― Reduce traffic counts by 10 percent for model calibration
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Add 10 percent more VMT for air quality analysis
Possible Other Solutions
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Re-Calibrate Model ????
Raise trip generation rates
Distribution
Mode Choice
TAZ’s !!!!
Input data development costs
Computational burden
Smaller
Example: PAG’s Model
Currently,
System Scale: 870 TAZ’s; 12,700 links; 4,700 nodes
Model-Run Time:
6 hrs on one fast PC
2.5 hrs by distributing process (DP) to 2 duo-core computers
But, if
expand to include 2000 TAZ’s, 20 hrs without DP and over 8 hrs with
Chance
to make mistakes and need to rerun model - 20%-30%
Easy Solution – Sub-Zone Assignment
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Purpose?
Ensure centroid connectors all get used because
they are actually used by traffic
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When to apply?
Easy Solution – Sub-Zone Assignment
Trip Generation
Matrix skimming
Trip Distribution
Mode Choice
Traffic & Transit
Assignment
Final Traffic Assignment
Feedback
Loop
Easy Solution – Sub-Zone Assignment
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What it does?
Create sub-zone centroids for each TAZ
How many? – as many as TAZ centroid connectors
• Location - wherever you like
For best looking, middle point of TAZ centroid
connector
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Connect sub-zone centroid to where TAZ centroid
connects to network to form sub-zone centroid
connector
Easy Solution – Sub-zone Assignment
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What it does? (Cont’)
Assign factors to each sub-zone centroid connector to
represent level of usage
• Sending Factor
• Receiving Factor
• How to do? – Give each TAZ’s centroid connectors
identical sending and receiving factors unless you
know they are different
Easy Solution – Sub-zone Assignment
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What it does? (Cont’)
Split trip table based on the sending and receiving
factors assigned to sub-zone centroid connectors
Traffic assignment with expanded trip table and
modified network
Innovative Solution ?
Then, you will have ……
Plus a much bigger trip table
Innovative Solution ?
Also, you will have ……
Application in PAG Model
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Sub-Zone assignment approach has been officially adopted
in PAG model
Add 5-10 more minutes to total model-run time
Better - captured local traffic
5% - 10% increase on VMT
Possibly better conformity of assigned traffic volume and
traffic counts
What should be done next?
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Apply sub-zone approach at different stages of
four-step model and compare the results
Give better idea how to achieve trade-off between
computational cost and precision
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Model Validation
Confirm Sub-Zone approach brings better model
validation results based on traffic counts
Questions?