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Chao Wang Joseph Huegy Institute for Transportation Research and Education (ITRE)

14 th TRB National Transportation Planning Applications Conference. Determining the Free-Flow Speeds in a Regional Travel Demand Model based on the Highway Capacity Manual. Chao Wang Joseph Huegy Institute for Transportation Research and Education (ITRE)

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Chao Wang Joseph Huegy Institute for Transportation Research and Education (ITRE)

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  1. 14th TRB National Transportation Planning Applications Conference Determining the Free-Flow Speeds in a Regional Travel Demand Model based on the Highway Capacity Manual Chao Wang Joseph Huegy Institute for Transportation Research and Education (ITRE) at North Carolina State University May 06, 2013 Columbus, OH

  2. Topics • Background • Current practices • Methods • Validation • Conclusions

  3. Background The Triangle Regional Model (TRM) • Covers 3,430 square miles • Population: 1.6 million • TRM is a trip based 4-step model • TRM Version 6 model is under development

  4. Why Free-flow Speeds are Important in Travel Demand Models • They are used to determine the network skims in the off-peak period • They are used to calculate the congested speeds

  5. Current Practices • There are two approaches • Formulas • Lookup tables • How are the formulas and the values in the lookup tables determined? • Speed limit • Formulas in research reports (NCHRP 387) • Speed survey • Professional judgment • Borrow from other models

  6. Current Practices • Factors that are considered • Facility type • Area type • Posted speed • Traffic control type (e.g. traffic signal) • Median • On-street parking

  7. Advantages to Using Formulas in HCM • Directly benefit from the enormous research efforts put into the development of HCM • To help the free-flow speeds and capacities be consistent

  8. Challenges to Using Formulas in HCM • The formulas in the HCM sometimes require traffic flow information. • The formulas in HCM require very detailed link attributes. Some of them are not usually available in a travel demand model, such as access point density and signal spacing.

  9. Ideas to Use Formulas in HCM • Simplify the formulas to fit the free-flow condition • Develop default values for the unavailable link attributes

  10. Facility Types in the Triangle Regional Model

  11. Default Values in the Triangle Regional Model

  12. Sample Default Values in the Triangle Regional Model

  13. Methods to use Formulas in HCM • Follow the facility types defined in the HCM 2010 • Simplify the formulas to fit the free-flow condition • Develop default values for the unavailable link attributes • The default values are defined by facility type and area type

  14. Validation of Free-flow Speeds in the Triangle Regional Model • Method: compare the observed travel times from a floating car survey and the calculated travel times for the same segments in the TRM model • The floating car survey was conducted in 2011. It selected 48 routes and each route had two directions. • There were 9 vehicle runs for each direction. Only the vehicle runs that were in free-flow conditions were used in this study.

  15. Map of the Floating Car Survey Routes

  16. Roadway Length (miles) Covered by the Floating Car Survey

  17. Comparison of the Observed and the Modeled Travel Times

  18. Conclusions • Using formulas in HCM can yield reasonable free-flow speeds for a regional travel demand model • Default values used in HCM formulas should be carefully determined • It is suggested to validate the free-flow speeds based on observed data

  19. ContactsChao Wang: cwang11@ncsu.eduJoseph Huegy: jbhuegy@ncsu.edu

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