1 / 18

Using Sub-Zone Assignment In An Otherwise Traditional Four-Step Model

Using Sub-Zone Assignment In An Otherwise Traditional Four-Step Model. Aichong Sun Thomas Cooney Pima Association of Governments. Most Relevant Studies. Williams Mann, VDOT B-Node Model: New Subarea Traffic Assignment Model & Application

phil
Download Presentation

Using Sub-Zone Assignment In An Otherwise Traditional Four-Step Model

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Using Sub-Zone Assignment In An Otherwise Traditional Four-Step Model Aichong Sun Thomas Cooney Pima Association of Governments

  2. Most Relevant Studies • 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 • Alan Horowitz, U of Wisconsin-Milwaukee Computational Issues in Increasing the Spatial Precision of Traffic Assignments Transportation Research Record No. 1777, 2001

  3. TAZ & Network of Traditional Four-Step Model

  4. Issues With Typical Four-Step Model Unloaded Centroid Connector Loaded Centroid Connector Minor Arterial Minor Arterial Issue 1: unloaded centroid connectors

  5. 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

  6. 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 • Add 10 percent more VMT for air quality analysis

  7. Possible Other Solutions • Re-Calibrate Model ???? Raise trip generation rates Distribution Mode Choice • Smaller TAZ’s !!!! Input data development costs Computational burden

  8. 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%

  9. Easy Solution –Sub-Zone Assignment • Purpose? Ensure centroid connectors all get used because they are actually used by traffic • When to apply?

  10. Easy Solution –Sub-Zone Assignment Trip Generation Matrix skimming Trip Distribution Mode Choice Feedback Loop Traffic & Transit Assignment Final Traffic Assignment

  11. Easy Solution –Sub-Zone Assignment • What it does? Create sub-zone centroids for each TAZ Connect sub-zone centroid to where TAZ centroid connects to network to form sub-zone centroid connector • How many? – as many as TAZ centroid connectors • Location - wherever you like • For best looking, middle point of TAZ centroid connector

  12. Easy Solution –Sub-zone Assignment • 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

  13. Easy Solution –Sub-zone Assignment • 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

  14. Innovative Solution ? Then, you will have …… Plus a much bigger trip table

  15. Innovative Solution ? Also, you will have ……

  16. Application in PAG Model • 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

  17. What should be done next? • Apply sub-zone approach at different stages of four-step model and compare the results • Model Validation Give better idea how to achieve trade-off between computational cost and precision Confirm Sub-Zone approach brings better model validation results based on traffic counts

  18. Questions?

More Related