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Determining the Free-Flow Speeds in a Regional Travel Demand Model based on

the Highway Capacity Manual

Chao WangJoseph Huegy

Institute for Transportation Research and Education (ITRE)at North Carolina State University

May 06, 2013Columbus, OH

14th TRB National Transportation Planning Applications Conference

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Topics

• Background

• Current practices

• Methods

• Validation

• Conclusions

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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

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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

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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

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Current Practices

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

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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

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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.

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Ideas to Use Formulas in HCM

• Simplify the formulas to fit the free-flow condition

• Develop default values for the unavailable link attributes

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Facility Types in the Triangle Regional ModelFacility Type Description HCM 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 17-20Minor Arterial Interrupted facility that is not major

arterial or collector

Collector Interrupted facility that provide accessibility

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Default Values in the Triangle Regional Model

  DescriptionAccess 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 RatioDefault 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

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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 3.3 - -Urban 2.6 - -Suburb 1.1 - -Rural 0.9 - -

Major Arterial

CBD 38 0.12 90Urban 30 0.40 120Suburb 18 0.80 150Rural 9 1.50 200

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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

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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.

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Map of the Floating Car Survey Routes

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Roadway Length (miles) Covered by the Floating Car Survey

CBD Urban Suburb Rural

Freeway 3 54 44 24

Multilane Highway   7 2  

Two-lane Highway     22 9

Major Arterial 11 123 49  

Minor Arterial 3 75 42  

Collector 1 3  

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Comparison of the Observed and the Modeled Travel Times

0.0 200.0 400.0 600.0 800.0 1000.0 1200.0 1400.00

200

400

600

800

1000

1200

1400

f(x) = 0.980133742657745 xR² = 0.993652967973904

Observed Travel Time (s)

Mod

eled

Tra

vel T

ime

(s)

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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

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Contacts

Chao Wang: cwang11@ncsu.edu

Joseph Huegy: jbhuegy@ncsu.edu