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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2018. Improved Analysis of Two-Lane Highway Capacity and Operational Performance. Washington, DC: The National Academies Press. doi: 10.17226/25179.
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NCHRP Web-Only Document 255: Improved Analysis of Two-Lane Highway Capacity and Operational Performance Scott S. Washburn Donald Watson Zilin Bian University of Florida Gainesville, FL Ahmed Al-Kaisy Amirhossein Jafari Montana State University Bozeman, MT Tapio Luttinen Aalto University Helsinki, Finland Richard Dowling Aaron Elias Kittelson and Associates, Inc. Oakland, California Final Report for NCHRP Project 17-65 Submitted March 2018 ACKNOWLEDGMENT This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program (NCHRP), which is administered by the Transportation Research Board (TRB) of the National Academies of Sciences, Engineering, and Medicine. COPYRIGHT INFORMATION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FMCSA, FRA, FTA, Office of the Assistant Secretary for Research and Technology, PHMSA, or TDC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. DISCLAIMER The opinions and conclusions expressed or implied in this report are those of the researchers who performed the research. They are not necessarily those of the Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; or the program sponsors. The information contained in this document was taken directly from the submission of the author(s). This material has not been edited by TRB.

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NCHRP PROJECT 17-65 PANEL AREA SEVENTEEN : TRAFFIC--SAFETY John L. Sanford, , Springfield, IL (Chair) Dirk B. Gross, Ohio DOT, Delaware, OH Mandar Khanal, Boise State University, Boise, ID Alejandra L. Medina-Flintsch, Virginia Polytechnic Institute & State University, Blacksburg, VA Steven C. Strength, Louisiana DOTD, Baton Rouge, LA R. Scott Zeller, Washington State DOT, Olympia, WA Clayton Chen, FHWA Liaison Bernardo Kleiner, TRB Liaison

NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report v Contributing Authors Scott S. Washburn, University of Florida Ahmed Al-Kaisy, Montana State University Tapio Luttinen, Aalto University Richard Dowling, Kittelson and Associates, Inc. Donald Watson, University of Florida Amirhossein Jafari, Montana State University Zilin Bian, University of Florida Aaron Elias, Kittelson and Associates, Inc. Acknowledgments The research team would like to thank the people listed below. They were instrumental in providing us with field data to use for this project. Brian Dunn, Oregon Department of Transportation Doug Norval, Oregon Department of Transportation David Keilson, North Carolina Department of Transportation Kent Taylor, North Carolina Department of Transportation Becky Duke, Montana Department of Transportation Glenda Fuller, Idaho Transportation Department In Memory of Michael Dixon Dr. Michael Dixon was one of the research team members and was instrumental in developing the proposal for this project. Mike tragically suffered a heart attack and passed away on May 7, 2014, about five months before this project started. He was a well-respected professional, a great friend, and most of all, a great father and husband. Before his passing, Mike had made many outstanding contributions to the field of transportation engineering, particularly with respect to two-lane highways. Mike’s presence on this project was certainly missed, but we hope that he would be proud of what we accomplished. This project is dedicated to Mike’s memory.

NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report vi Table of Contents 1. INTRODUCTION ............................................................................................................................. 1 1.1. BACKGROUND .................................................................................................................................. 1 1.2. COMPUTATIONAL DEFICIENCIES IN, AND KEY GAPS IN COVERAGE OF, HCM ANALYSIS METHODOLOGY ............................................................................................................................... 1 1.3. SIMULATION TOOLS ....................................................................................................................... 10 1.4. RESEARCH OBJECTIVES AND SCOPE .............................................................................................. 10 2. RESEARCH APPROACH .............................................................................................................. 11 2.1. REVIEW OF ALTERNATIVE ANALYSIS METHODOLOGIES .............................................................. 11 2.2. FIELD DATA ANALYSIS SUMMARY ................................................................................................ 21 2.3. SERVICE MEASURE EVALUATION .................................................................................................. 28 2.4. IDENTIFICATION OF VIABLE SIMULATION TOOLS FOR THE ANALYSIS OF TWO-LANE HIGHWAYS ..................................................................................................................................... 37 2.5. APPROACH FOR ESTIMATING IMPACTS OF LARGE TRUCKS ON TWO-LANE HIGHWAY OPERATIONS ................................................................................................................................... 38 2.6. APPROACH FOR IDENTIFYING FOLLOWER STATUS ........................................................................ 58 3. ANALYSIS METHODOLOGY DEVELOPMENT ..................................................................... 63 3.1. SEGMENTATION ............................................................................................................................. 63 3.2. ESTIMATION OF FREE-FLOW SPEED ............................................................................................... 65 3.3. MODELS FOR ESTIMATION OF AVERAGE SPEED ............................................................................ 67 3.4. MODELS FOR ESTIMATION OF PERCENT FOLLOWERS.................................................................... 73 3.5. PASSING LANES .............................................................................................................................. 76 3.6. LEVEL OF SERVICE ......................................................................................................................... 85 3.7. FACILITY LEVEL ANALYSIS FRAMEWORK ..................................................................................... 88 4. SUMMARY AND RECOMMENDATIONS ................................................................................. 91 4.1. SUMMARY ...................................................................................................................................... 91 4.2. RECOMMENDATIONS ...................................................................................................................... 93 5. REFERENCES................................................................................................................................. 94 APPENDICES ......................................................................................................................................... 106 A: CURRENT AGENCY PRACTICES AND PREFERENCES ....................................................................... 115 B: SERVICE MEASURE EVALUATION ..................................................................................................... 135 C: IDENTIFICATION OF VIABLE SIMULATION TOOLS FOR THE ANALYSIS OF TWO-LANE HIGHWAYS ............................................................................................. 175 D: MODELING SPEEDS OF HEAVY VEHICLES FOR SIMULATION ......................................................... 183 E: APPROACH FOR ESTIMATING FOLLOWER STATUS ......................................................................... 237 F: ANALYSIS METHODOLOGY MODEL DEVELOPMENT ....................................................................... 259 H: FIELD DATA COLLECTION SUPPLEMENTAL MATERIAL ................................................................ 334 I: SIMULATION TOOLS SUPPLEMENTAL MATERIAL ............................................................................ 532

NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report vii List of Tables Table 2-1. Follower density ranges for measuring LOS .............................................................. 13 Table 2-2. Acceptable LOS on two-lane highways ..................................................................... 13 Table 2-3. Density thresholds for measuring level of service ..................................................... 14 Table 2-4. Level of service thresholds based on freedom of flow ............................................... 16 Table 2-5. Comparison of LOS boundaries in HCM 2000 and the proposed approach .............. 19 Table 2-6. Preliminary assessment matrix of performance measures on two-lane highways ..... 36 Table 2-7. HBS Classifications for Horizontal Alignment .......................................................... 45 Table 2-8. HBS Classifications for Vertical Alignment .............................................................. 46 Table 2-9. Network Information for Scenarios Used in PCE Evaluation .................................... 50 Table 2-10. Passenger Car Equivalent Values for Intermediate Semi-Trailer Trucks ................ 53 Table 2-11. ATS Estimations for Scenario 4 Using PCE Methodology...................................... 53 Table 2-12. Reductions in Heavy Vehicle FFS Used to Classify Horizontal and Vertical Alignment.................................................................................................................. 55 Table 2-13. Classifications for Horizontal Alignment. ................................................................ 56 Table 2-14. Classifications for Vertical Alignment (Downgrades in Parentheses). .................... 57 Table 3-1. Coefficients for FFS-HV% Slope Model (Used in Equation (3-3) ............................ 66 Table 3-2. Coefficients Used in Speed-Flow Slope Model (Equation (3-6)) for Passing Zone and Passing Constrained Segments............................................................. 69 Table 3-3. Coefficients Used in Speed-Flow Slope Model (Equation (3-6)) for Passing Lane Segments ..................................................................................................... 69 Table 3-4. Coefficients Used to Calculate b3 (Equation (3-7)) for Passing Zone and Passing Constrained Segments ............................................................................... 70 Table 3-5. Coefficients Used to Calculate b3 (Equation (3-7)) for Passing Lane Segments ....... 70 Table 3-6. Coefficients Used to Calculate b4 (Equation (3-8)) for Passing Zone and Passing Constrained Segments ............................................................................... 70 Table 3-7. Coefficients Used to Calculate b4 (Equation (3-8)) for Passing Lane Segments ....... 70 Table 3-8. Coefficients Used in Speed-Flow Slope Model (Equation (3-10)) for Passing Zone and Passing Constrained Segments............................................................. 72 Table 3-9. Coefficients Used in Speed-Flow Slope Model (Equation (3-10)) for Passing Lane Segments ..................................................................................................... 72 Table 3-10. Coefficient Values for Equation (3-12) .................................................................... 73 Table 3-11. Coefficient Values for Equation (3-13) .................................................................... 74 Table 3-12. Coefficient Values for Equation (3-14) .................................................................... 74 Table 3-13. Coefficient Values for Equation (3-15) .................................................................... 75 Table 3-14. Coefficient Values for Equation (3-16) .................................................................... 75 Table 3-15. Coefficient Values for Equation (3-17) .................................................................... 75 Table 3-16. Optimum Length of Passing Lane (Proposed Approach).......................................... 78 Table 3-17. Upgrade Speed Model Coefficients for a Single-Unit Truck. ................................... 81 Table 3-18. Upgrade Speed Model Coefficients for an Intermediate Semi-Trailer Truck. .......... 81 Table 3-19. Upgrade Speed Model Coefficients for an Interstate Semi-Trailer Truck. ............... 82 Table 3-20. HCM 2010 Analysis Methodology LOS Threshold Values..................................... 85 Table 3-21. HCM 2010 Methodology Experimental Design Results .......................................... 87 Table 3-22. NCHRP 17-65 Methodology Experimental Design Results .................................... 87 Table 3-23. Follower Density Thresholds ................................................................................... 87

NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report viii List of Figures Figure 1-1. HCM 2010 speed-flow relationship ............................................................................ 2 Figure 1-2. Speed-flow relationship for German two-lane highways (under ideal roadway design conditions)* ................................................................................ 2 Figure 1-3. HCM speed-flow curves (dashed lines) versus speed-flow curves with slope as a function of FFS (solid lines) ................................................................. 3 Figure 1-4. Divergence of passenger car and mixed flow speed-flow patterns on freeway upgrade ...................................................................................................................................... 7 Figure 1-5. Speed-flow values on Finnish arterial highway 4 (grade separated intersections and passing lanes) to the north of Lahti ............................................... 9 Figure 2-1. Theoretical speed distribution with probability Pi represented ................................. 17 Figure 2-2. Non-linear regression analysis 15-, 30- and 60-min aggregation intervals at NC Site 2 .............................................................................................................. 22 Figure 2-3. Speed-flow data in passing lane of passing lane segment (Oregon site) .................. 23 Figure 2-4. Example functional form for speed-flow data in passing lane segment ................... 23 Figure 2-5. Percent followers-flow data ...................................................................................... 24 Figure 2-6. Example functional form for percent followers-flow data ........................................ 24 Figure 2-7. Follower density-flow data ....................................................................................... 25 Figure 2-8. Example functional form for follower density-flow data ......................................... 26 Figure 2-9. High flow site in North Carolina (Site 2) .................................................................. 26 Figure 2-10. High flow site in California (SR-37/Sears Point Rd) .............................................. 27 Figure 2-11. Discrepancy between speed-flow patterns for passenger car only and mixed flows ........................................................................................................ 43 Figure 2-12. HBS 2015 speed-flow curves for subsegment with bendiness class 1 and grade class 1 ......................................................................................................... 46 Figure 2-13. Travel time-versus-distance curves for a single-unit truck with an initial speed of 70 mi/h............................................................................................... 48 Figure 2-14. Speed-flow curves for PCE evaluation. A) Scenario 1. B) Scenario 2. C) Scenario 3. D) Scenario 4 .......................................................................... 51 Figure 2-15. Flow interpolation issues with scenario 2 in PCE evaluation ................................. 52 Figure 2-16. Different headway states between successive vehicles ........................................... 61 Figure 3-1. Follower density along the highway (%NP = 50) ..................................................... 76 Figure 3-2. Percent reduction in PF vs length of passing lane. ..................................................... 78 Figure 3-3. Upgrade speed versus distance curves for a single-unit truck .................................. 79 Figure 3-4. Upgrade speed versus distance curves for an intermediate semi-trailer truck .......... 80 Figure 3-5. Upgrade speed versus distance curves for an interstate semi-trailer truck ............... 80 Figure 3-6. Schematic of example 2+1 configuration (passing vehicles use center lane) ........... 83 Figure 3-7. Schematic of typical transition area design for European 2+1 configurations ......... 83 Figure 3-8. Batch Processing Utility for HCM-CALC ................................................................ 86

NCHRP 17-65 Improved Analysis of Two-Lane Highway Capacity and Operational Performance Final Report ix Abstract Two-lane highways account for a very significant portion of the national highway system and serve an essential function for the movement of people and goods. The Highway Capacity Manual (HCM) contains a chapter that provides an analysis methodology for two-lane highways. Unfortunately, the current HCM analysis procedure has been criticized on several issues, such as the speed-flow relationship, appropriate service measures, treatment of large trucks, guidance on base free-flow speed estimation, accuracy of passing lane adjustments, and limitations in analysis scope. This project sought to address these limitations and gaps. Accomplishments from this project include the following: the development of a more realistic speed-flow relationship; the introduction of a new service measure—follower density; a new headway threshold value to better identify follower status; development of a percent followers- flow relationship; elimination of passenger car equivalent (PCE) values and direct use of percentage of heavy vehicles in the models for performance measure estimation; the inclusion of a quantitative adjustment based on posted speed limit for the estimation of base free-flow speed (BFFS); the development of new functions for passing lanes—effective and optimal lengths and performance measure improvements for 2+1 sections; and the development of a method for combining the analysis of multiple contiguous segments into a facility-level analysis. This project also introduced features to improve the ease of use of the methodology in the HCM, such as elimination of tables requiring interpolation, treating trucks explicitly instead of through PCE values, using a single service measure, and eliminating the PTSF measure. And finally, two modern microsimulation tools were identified that are capable of accurately modeling two-lane highways: SwashSim and TransModeler.

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TRB's National Cooperative Highway Research Program (NCHRP) Web-Only Document 255: Improved Analysis of Two-Lane Highway Capacity and Operational Performance supplements the sixth edition of the Highway Capacity Manual (HCM). Specifically, this project includes the following updates:

  • the development of a more realistic speed-flow relationship
  • the introduction of a new service measure—follower density
  • a new headway threshold value to better identify follower status
  • development of a percent-followers flow relationship
  • elimination of passenger car equivalent (PCE) values and direct use of percentage of heavy vehicles in the models for performance measure estimation
  • the inclusion of a quantitative adjustment based on posted speed limit for the estimation of base free-flow speed (BFFS)
  • the development of new functions for passing lanes—effective and optimal lengths and performance measure improvements for 2+1 sections
  • the development of a method for combining the analysis of multiple contiguous segments into a facility-level analysis

This project also introduced features to improve the ease of use of the methodology in the HCM, such as the elimination of tables requiring interpolation, treating trucks explicitly instead of through PCE values, using a single service measure and eliminating the PTSF measure.

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