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 14 th TRB National Transportation Planning Applications Conference 1

Topics Background Current practices Methods Validation Conclusions 2

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 3

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 4

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 5

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

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 7

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. 8

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

Facility Types in the Triangle Regional Model Facility TypeDescriptionHCM 2010 Freeway Uninterrupted facility with full control of access Chapter 11 Multilane Highway Uninterrupted facility without full control of access (>1 lane in each direction) Chapter 14 Two-lane Highway Uninterrupted facility without full control of access (1 lane in each direction) Chapter 15 Major Arterial Interrupted facility that provide high- speed movement Chapters Minor Arterial Interrupted facility that is not major arterial or collector Collector Interrupted facility that provide accessibility 10

Default Values in the Triangle Regional Model Description Access Point Density Default values for ramp density on freeways or intersection/driveway density on other facilities Signal Spacing Default values for average signal spacing (distance between two adjacent signals) on interrupted facilities Cycle Length Default values for the signal cycle length on interrupted facilities GC Ratio Default values for the ratio of effective green time and signal cycle length on interrupted facilities Arrival Type Default values for the vehicle arrival type (1 to 6) on interrupted facilities 11

Sample Default Values in the Triangle Regional Model Facility Type Area Type Access Point Density (per mile) Signal Spacing (mile) Cycle Length (second) Freeway CBD Urban Suburb Rural Major Arterial CBD Urban Suburb Rural

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 13

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 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. 14

Map of the Floating Car Survey Routes 15

Roadway Length (miles) Covered by the Floating Car Survey CBDUrbanSuburbRural Freeway Multilane Highway 72 Two-lane Highway 229 Major Arterial Minor Arterial37542 Collector13 16

Comparison of the Observed and the Modeled Travel Times 17

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 18

Contacts Chao Wang: Joseph Huegy: 19