Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report (2021)

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    NCHRP Web-Only Document 313 Selecting Ramp Design Speeds VOL. 2: CONDUCT OF RESEARCH REPORT Darren J. Torbic MRIGlobal Kansas City, MO Marcus A. Brewer Michael P. Pratt Eun Sug Park Texas A&M Transportation Institute College Station, TX    Conduct of Research Report for NCHRP Project 15-56 Submitted June 2021 NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed, and implementable research is the most effective way to solve many problems facing state departments of transportation (DOTs) administrators and engineers. Often, highway problems are of local or regional interest and can best be studied by state DOTs individually or in cooperation with their state universities and others. However, the accelerating growth of highway transportation results in increasingly complex problems of wide interest to highway authorities. These problems are best studied through a coordinated program of cooperative research. Recognizing this need, the leadership of the American Association of State Highway and Transportation Officials (AASHTO) in 1962 initiated an objective national highway research program using modern scientific techniques—the National Cooperative Highway Research Program (NCHRP). NCHRP is supported on a continuing basis by funds from participating member states of AASHTO and receives the full cooperation and support of the Federal Highway Administration (FHWA), United States Department of Transportation, under Agreement No. 693JJ31950003. 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, FTA, GHSA, NHTSA, 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; the FHWA; 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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C O O P E R A T I V E R E S E A R C H P R O G R A M S CRP STAFF FOR NCHRP WEB-ONLY DOCUMENT 313, Vol. 2 Christopher J. Hedges, Director, Cooperative Research Programs Lori L. Sundstrom, Deputy Director, Cooperative Research Programs Camille Crichton-Sumners, Associate Program Manager, National Cooperative Highway Research Program Tyler Smith, Senior Program Assistant Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications Jennifer Correro, Assistant Editor NCHRP PROJECT 15-56 PANEL Field of Design—Area of General Design Luis Betancourt, California Department of Transportation (CALTRANS), Sacramento, CA (Chair) Leticia Sosa Estavillo, AIA Engineers, a CONSOR company, Austin, TX Theron Knouse, Virginia Department of Transportation, Richmond, VA Alejandra L. Medina, Virginia Polytechnic Institute and State University (Virginia Tech), Blacksburg, VA Hasmukh M. Patel, Massachusetts Department of Transportation, Boston, MA James Allen Rosenow, Minnesota Department of Transportation, Shoreview, MN Brian David Toombs, Burgess and Niple, Inc., Columbus, OH Mark A. Doctor, FHWA Liaison Stephen F. Maher, TRB Liaison ACKNOWLEDGMENTS The research reported herein was performed under NCHRP Project 15-56, “Guidelines for Selecting Ramp Design Speeds.” This report was prepared by Dr. Darren J. Torbic (formerly with MRIGlobal), Mr. Marcus A. Brewer, and Mr. Michael P. Pratt of the Texas A&M Transportation Institute. Mr. Daniel J. Cook of HDR, Mr. Douglas W. Harwood of Harwood Road Safety LLC, and Ms. Heidi Ouren of HQE Incorporated played key roles in this research. The authors wish to thank the State Departments of Transportation of California, Kansas, Missouri, and Texas for their assistance in this research.

iv Table of Contents Summary ..........................................................................................................................................1 Section 1. Introduction ....................................................................................................................3 1.1 Background ...........................................................................................................3 1.2 Research Objective and Scope ..............................................................................5 1.3 General Research Approach ..................................................................................6 1.4 Outline of Report ...................................................................................................7 Section 2. Literature Review and Survey of Practice .....................................................................8 2.1 Literature Review ..................................................................................................8 2.2 Survey of Practice ...............................................................................................36 2.3 Summary of Key Issues and Limitations of AASHTO’s Current Policy on Ramp Design Speed ............................................................................................56 Section 3. Observational Field Study............................................................................................60 3.1 Study Site Selection ............................................................................................60 3.2 Study Locations ...................................................................................................60 3.3 Data Collection ....................................................................................................66 Section 4. Development of Ramp Speed Prediction Models ........................................................68 4.1 Ramp Speed Prediction Models for Curves ........................................................68 4.2 Ramp Speed Prediction Models for Tangents .....................................................77 Section 5. Summary of Design Guidelines ...................................................................................87 Section 6. Project Summary and Future Research Needs .............................................................93 Section 7. References ....................................................................................................................96 Section 8. Abbreviations, Acronyms, Initialisms, and Symbols ...................................................99 Appendix A: Suggested Text for Future Edition of AASHTO Green Book ...............................104 Figures Figure 1. Primary Components of an Interchange Ramp ..............................................................4 Figure 2. Tapered and Parallel Designs for Entrance Ramps .....................................................11 Figure 3. Tapered and Parallel Designs for Exit Ramps ............................................................13 Figure 4. Aggregated Ramp Entrance CMF ................................................................................24 Figure 5. Ramp Horizontal Curve CMF for Fatal-and-Injury Crashes .......................................25 Figure 6. Example Designs—Metered Ramp ..............................................................................32 Figure 7. Elements of the Functional Area of an Intersection .....................................................58 Figure 8. Loop Ramp – Westbound Exit at I-10 and Harper Rd .................................................61 Figure 9. Loop Ramp – Eastbound Entrance at I-10 and Harper Rd ..........................................61 Figure 10. Diagonal Ramp – Eastbound Entrance at I-10 and Martinez St ..................................62 Figure 11. Diagonal Ramp – Eastbound Exit at I-10 and US-277 ................................................62 Figure 12. Outer Connection Ramp – Ramp from Westbound Missouri Route 152 to Northbound I-29...........................................................................................................63 Figure 13. Outer Connection Ramp – Ramp from Southbound I-29 to Westbound Missouri Route 152 .....................................................................................................................64

v Figure 14. Direct Connection Ramp – Ramp from Northbound Missouri Route 9 and Southbound US-69 to Northbound I-635 .....................................................................65 Figure 15. Plot of actual versus predicted values of final model to estimate vehicle speed at midpoint of curve on an entrance ramp .......................................................................71 Figure 16. Plot of actual versus predicted values of final model to estimate vehicle speed at end of curve on an entrance ramp ................................................................................73 Figure 17. Plot of actual versus predicted values of final model to estimate vehicle speed at midpoint of curve on an exit ramp ...............................................................................75 Figure 18. Plot of actual versus predicted values of final model to estimate vehicle speed at end of curve on an exit ramp ........................................................................................77 Figure 19. Plot of actual versus predicted values of final model to estimate vehicle speed at end of tangent followed by subsequent curve on an entrance ramp ............................82 Figure 20. Plot of actual versus predicted values of final model to estimate vehicle speed at end of tangent without subsequent curve on an entrance ramp ...................................83 Figure 21. Plot of actual versus predicted values of final model to estimate vehicle speed at end of tangent with subsequent curve on an exit ramp ................................................85 Figure 22. Regression plot of speed at beginning of tangent and speed at end of tangent for final model to estimate vehicle speed at end of tangent without subsequent curve on an exit ramp.............................................................................................................86 Figure 23. Acceleration of Passenger Cars, Level Condition .......................................................90 Figure 24. Deceleration Distances for Passenger Vehicles Approaching Intersections ................90 Tables Table 1. Guide Values for Ramp Design Speed as Related to Highway Design Speed ..............3 Table 2. Design Speed Guidelines for Ramps .............................................................................4 Table 3. Desirable Limiting Upgrades for Ramps Based on Ramp Design Speeds ..................10 Table 4. Minimum Acceleration Lengths for Entrance Terminals with Flat Grades of Two Percent or Less .............................................................................................................12 Table 5. SCL Adjustment Factors as a Function of Grade ........................................................12 Table 6. Minimum Deceleration Lengths for Exit Terminals with Flat Grades of Two Percent or Less .............................................................................................................14 Table 7. Ramp Design Speed from Washington DOT Design Manual Exhibit 1360-4 ............15 Table 8. Input Data for Ramp Curve Speed Prediction Procedures in ISATe ...........................16 Table 9. Loop Ramp Characteristics .........................................................................................27 Table 10. Horizontal Curve Design Criteria for Ramps as a Function of Design Speed and Functional Classification of the Mainline Highway ....................................................28 Table 11. Minimum Radii for Interchange Ramp Curves ...........................................................29 Table 12. Suggested Lengths of Spirals for Ramps .....................................................................30 Table 13. Guidelines for Maximum Grades for Design of Ramps ..............................................30 Table 14. Suggested Ramp Exit/Entrance Vertical Curve Design Speed ....................................31 Table 15. Maximum Algebraic Difference in Pavement Cross Slope for Design of Ramp Terminals at Crossover Crownline ..............................................................................31 Table 16. Summary of data collected ...........................................................................................67 Table 17. Number of curves and observations in database to model speeds at midpoint and end of curves ................................................................................................................68 Table 18. Descriptive statistics of ramp speed database for curves .............................................69

vi Table 19. Statistical parameters of final model to estimate vehicle speed at midpoint of curve on an entrance ramp ...........................................................................................70 Table 20. Statistical parameters of final model to estimate vehicle speed at end of curve on an entrance ramp ..........................................................................................................72 Table 21. Statistical parameters of final model to estimate vehicle speed at midpoint of curve on an exit ramp ...................................................................................................74 Table 22. Statistical parameters of final model to estimate vehicle speed at end of curve on an exit ramp..................................................................................................................76 Table 23. Number of tangents and observations in database to model speeds at the end of the tangent ....................................................................................................................78 Table 24. Descriptive statistics of ramp speed database for tangents ..........................................79 Table 25. Statistical parameters of final model to estimate vehicle speed at end of tangent with subsequent curve on an entrance ramp ................................................................81 Table 26. Statistical parameters of final model to estimate vehicle speed at end of tangent without subsequent curve on an entrance ramp ...........................................................83 Table 27. Statistical parameters of final model to estimate vehicle speed at end of tangent with subsequent curve on an exit ramp ........................................................................84 Table 28. Statistical parameters of final model to estimate vehicle speed at end of tangent without subsequent curve on an exit ramp ...................................................................86 Table 29. Range of Guide Values for Ramp Design Speed as Related to Highway Design Speed, Interchange Type, Ramp Configuration, and Contextual Considerations .......88 Table 30. Minimum Acceleration Lengths for Entrance Terminals with Flat Grades of Less Than 3 Percent .............................................................................................................91 Table 31. Minimum Deceleration Lengths for Exit Terminals with Flat Grades of Two Percent or Less .............................................................................................................92