ABSTRACT
FINDLEY, DANIEL JONATHAN. A Comprehensive Two-Lane, Rural Road Horizontal Curve Study Procedure. (Under the direction of Joseph E. Hummer and William Rasdorf.)
Horizontal curves are relatively dangerous features, with collision rates typically three times that of comparable tangent sections on average. To help make these segments safer, this research developed a comprehensive study procedure for rural, two-lane horizontal curves. To provide the basis for a comprehensive procedure, this research includes an examination of curve crash characteristics, an investigation of study methods for geometric characteristics, and recommendations for potential countermeasures. A complete and accurate data set on a horizontal curve is important for a transportation agency to make a well-informed decision on possible improvements that could enhance the safety of the roadway. However, many agencies do not know curve radii or lengths because drawings do not exist and inventories are not available.
Typically, the analysis of a horizontal curve or set of curves for safety purposes by a highway agency is based on field visits and the judgment of experienced personnel. Many agencies rely on a drive-through by an engineer or technician and a small set of countermeasures that have proven themselves through the years. Analytical tools for curves have existed for a number of years; however, such tools have not been widely implemented due to the large number of competing highway safety objectives, real or perceived difficulties in collecting the necessary data, and calibrating models for local conditions, among other reasons. The publication of the Highway Safety Manual (HSM) offers the chance to overcome this impasse and get agencies to use crash models routinely. The HSM contains a crash prediction model for horizontal curves and estimates of crash modification factors (CMFs) for the most popular curve countermeasures. The model and CMFs have been approved by a committee of leading safety researchers and practitioners, which provides credibility to the tools. Application of the HSM is expected to be an appropriate methodology to identify curves with higher than normal crash potential, to be used to complement collision-based
methods for curve safety analysis. This research developed a CMF to account for the effect of nearby curves on safety that can supplement HSM procedures.
Several contributions to the practice of transportation engineering have resulted from this research. This research presents a new horizontal curve study method procedure to ensure a systematic approach for identifying curves, studying and measuring their characteristics, and improving hazardous locations. This research quantifies the collision characteristics of horizontal curves and created a linking of common horizontal curve collision types and effective countermeasures, which provides an engineer with the necessary information to identify and correct hazardous curves. This research recommends study methods for geometric characteristics which allows an engineer to most effectively and efficiently measure, define, and analyze horizontal curves to determine their predicted safety performance. The focus of these study methods is office procedures for collecting horizontal curve data, which are generally more efficient than field methods. This research establishes a set of parameters to which safety can be related through spatial and geometrical features. Safety was related geometrically through the establishment of guidance for horizontal curves for the implementation of the nationally accepted prediction model for roadways, presented in the Highway Safety Manual. Safety was related spatially through the impact of spatial relationships on horizontal curve safety using the predictive methodology of the Highway Safety Manual as a foundation for incorporating spatial considerations into horizontal curve safety prediction.
The comprehensive curve study method procedure includes: horizontal curve identification, investigation and inventory, analysis, evaluation, and a recommendation of appropriate countermeasures. A systematic approach for identifying, investigating, analyzing, and evaluating horizontal curves can lead to the selection and evaluation of promising curves, and a recommendation of appropriate countermeasures.
© Copyright 2011 by Daniel Jonathan Findley
All Rights Reserved
A Comprehensive Two-Lane, Rural Road Horizontal Curve Study Procedure
by Daniel Jonathan Findley
A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Doctor of Philosophy
Civil Engineering
Raleigh, North Carolina
2011
APPROVED BY:
______Dr. Joseph Hummer, Co-Chair Dr. William Rasdorf, Co-Chair
______Dr. Nagui Rouphail Dr. Hugh Devine
DEDICATION
This dissertation is dedicated to my family and the educators I have had the privilege of learning from throughout my life. My entire family has supported me in all of my academic aspirations, for which I am eternally grateful.
To my wife, Rachel, who provided unwavering support throughout my graduate education and the initial encouragement to start this endeavor.
To my daughter, Sophia, who provided inspiration and an often needed diversion from my studies.
To my parents, Frank and Jan, who provided immeasurable experiences and guidance that instilled a passion for learning and exploration.
To my grandmothers, Jane and Barbara, whose kindness and praise always encouraged me and my grandfathers, whose academic and professional accomplishments have inspired me.
To my sisters, Amy and Julie, who willingly supported my engineering curiosity with countless toys to reverse engineer.
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BIOGRAPHY
Mr. Daniel J. Findley has a broad range of transportation experiences and skills. He is currently a Senior Research Associate at the Institute for Transportation Research and Education (ITRE) who has played a major role in the several projects relating to many fields of transportation including: pedestrian, highway, ferry, aviation, and bicycle. Mr. Findley also possesses a Professional Engineer’s (PE) license in the state of North Carolina. Mr. Findley is the son of Frank and Jan Findley. He was raised in Robbinsville, NC and graduated as the valedictorian of Robbinsville High School in 2001. After high school, he attended North Carolina State University and graduated Summa Cum Laude with a Bachelor of Science in Civil Engineering degree in 2005 and a Master of Science in Civil Engineering degree in 2006.
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ACKNOWLEDGEMENTS
I would like to acknowledge the help and support of numerous individuals who assisted in the completion of my research. Dr. Joseph Hummer and Dr. William Rasdorf served as co- chairs for my committee and were instrumental in supporting and guiding my research efforts. The experience through this research in the realm of highway design and highway safety have been immense and allowed for my growth as a transportation professional. Dr. Nagui Rouphail and Dr. Hugh Devine were committee members and also provided meaningful feedback on my research. I appreciate the time and effort each of my committee members have demonstrated. I’m truly grateful for my experience as a student at NC State, where I have the opportunity to work closely with some of the top transportation researchers in the United States. I would also like to express my sincere gratitude for the opportunity to work with and learn from Charles Zegeer at the UNC Highway Safety Research Center.
I am appreciative for the support I have had at the Institute of Transportation Research and Education (ITRE) from my colleagues and supervisors. My supervisor, Robert Foyle, has provided me with valuable experience which was important for my growth as an engineer and researcher. I would also like to thank Christopher Cunningham and Dr. Bastian Schroeder for their collaboration on many research projects which has enabled me to gain much appreciated experience.
I would also like to thank the North Carolina Department of Transportation who funded the research project that formed this basis of this dissertation work. I had the privilege to work with many individuals at the North Carolina Department of Transportation who provided excellent support and direction, including: Brian Mayhew, Brian Murphy, and Shawn Troy with the Traffic Safety Systems Section; Jay Bennett and the Roadway Design Unit; Jennifer Brandenburg and the State Road Management Unit; and Charlie Brown and Betsy Pope of the Locations and Surveys Unit.
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TABLE OF CONTENTS
List of Tables ...... xi
List of Figures ...... xiii
1.0 Introduction ...... 1
1.1 Step 1: Site Identification ...... 2
1.2 Step 2: Office Data Acquisition ...... 3
1.3 Step 3: Collision Data Analysis ...... 3
1.4 Step 4: Curve Characteristics Analysis ...... 4
1.5 Step 5: Collision Data and Curve Characteristics Findings ...... 5
1.6 Step 6: Field Data Acquisition and Confirmation ...... 5
1.7 Step 7: Recommendations ...... 5
1.8 Research Objectives ...... 6
1.9 Significance of the Research ...... 7
1.10 Research Scope and Limitations ...... 8
1.11 Dissertation Organization...... 9
2.0 Literature Review ...... 10
2.1 Introduction ...... 10
2.2 Collision Characteristics and Geometric Design Features ...... 11
2.3 Horizontal Curve Study Methods ...... 14
2.3.1 Ball-Bank Indicator Method ...... 14
2.3.2 Compass Method ...... 15
2.3.3 Direct Method ...... 15
2.3.4 Lateral Acceleration Method ...... 16
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2.3.5 Yaw Rate Transducer Method ...... 16
2.3.6 GPS Method ...... 16
2.3.7 Mobile Vehicle Method ...... 17
2.3.8 Study Method Comparison ...... 20
2.4 Highway Safety Manual Analysis ...... 20
2.5 Crash Modification Factor Clearinghouse ...... 23
2.6 GIS and Remote-Sensing Procedures ...... 25
2.7 General Guides for TCDs for Horizontal Curves ...... 28
2.8 TCDs Effects on Horizontal Curves ...... 29
2.8.1 TCD Application and Safety Effects for Horizontal Curves ...... 30
2.8.2 Modeling for Determining Hazardous Curve Category ...... 33
2.9 Spatial Relationships ...... 34
2.10 Summary ...... 40
3.0 Curve Crash Characteristics ...... 42
3.1 Introduction ...... 42
3.2 Methodology ...... 43
3.3 Collision Data Analysis ...... 45
3.3.1 Road Characteristics ...... 45
3.3.2 Collision Characteristics ...... 47
3.4 Results ...... 60
3.5 Conclusions ...... 63
4.0 Manual Field Investigation Procedure ...... 64
5.0 Individual Curve Analysis GIS Process ...... 65
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5.1 Introduction ...... 65
5.1.1 Scope ...... 66
5.1.2 Objective ...... 66
5.2 Methodology ...... 67
5.2.1 Field Method ...... 69
5.2.2 Curve Calculator ...... 71
5.2.3 Curvature Extension...... 72
5.2.4 Curve Finder ...... 73
5.3 Analysis ...... 75
5.3.1 GIS-Derived Curve Analysis ...... 76
5.3.2 Field Measured Curve Analysis ...... 78
5.3.3 Safety Analysis ...... 85
5.4 Results ...... 87
5.5 Conclusions ...... 88
6.0 Network Curve Analysis GIS Process ...... 92
6.1 Introduction ...... 92
6.2 Methodology ...... 93
6.2.1 Curve Identification ...... 94
6.2.2 Geometric Characterization ...... 95
6.2.3 Collision Data ...... 95
6.3 Analysis ...... 96
6.3.1 Curve Finder Tolerance Sensitivity Analysis ...... 96
6.3.2 Route Analysis ...... 98
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6.3.3 Hazardous Curve Analysis ...... 104
6.3.4 Safety Analysis ...... 105
6.4 Results ...... 108
6.5 Conclusions ...... 109
7.0 Mobile Vehicle Comparison ...... 112
7.1 Introduction ...... 112
7.2 Methodology ...... 113
7.2.1 Chord Method ...... 116
7.2.2 GIS Method ...... 117
7.2.3 Vendor Data ...... 120
7.2.4 Survey Data ...... 120
7.2.5 Design Data ...... 122
7.2.6 Comparison ...... 122
7.3 Results ...... 124
7.4 Conclusions ...... 132
8.0 Highway Safety Manual Analysis ...... 135
8.1 Introduction ...... 135
8.2 Methodology ...... 138
8.2.1 HSM predictive method calibration ...... 138
8.2.2 Step 9: Select and apply SPF ...... 139
8.2.3 Step 10: Apply the appropriate CMFs to SPF to account for the difference in base and site specific conditions ...... 139
8.2.4 Step 11: Apply a calibration factor to the result of Step 10 ...... 140
8.2.5 Data Collection ...... 141
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8.3 Analysis ...... 143
8.3.1 Calibration Factor Analysis ...... 143
8.3.2 Sensitivity Analysis ...... 146
8.3.3 Calibration Factor Validation ...... 151
8.4 Conclusions ...... 152
9.0 Spatial Relationships ...... 153
9.1 Introduction ...... 153
9.2 Methodology ...... 154
9.2.1 Horizontal Curve Data Collection ...... 155
9.2.2 HSM Safety Prediction ...... 156
9.2.3 Spatial Relationship Analysis ...... 158
9.3 Results ...... 162
9.3.1 Model Selection ...... 163
9.3.2 Model Validation ...... 167
9.4 Conclusions ...... 168
10.0 Comprehenisve Process ...... 171
10.1 Introduction ...... 171
10.2 Methodology ...... 173
10.2.1 Step 1: Site Identification ...... 175
10.2.2 Step 2: Office Data Acquisition ...... 175
10.2.3 Step 3: Collision Data Analysis ...... 178
10.2.4 Step 4: Curve Characteristics Analysis ...... 180
10.2.5 Step 5: Curve & Collision Analysis Findings ...... 181
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10.2.6 Step 6: Field Data Acquisition and Confirmation ...... 181
10.2.7 Step 7: Develop Recommendations ...... 182
10.3 Case Studies ...... 184
10.3.1 Case Study Site # 1 – Agency Safety Improvement Program Curve ...... 184
10.3.2 Case Study Site # 2 – Collision History and Geometric Deficiencies ...... 187
10.3.3 Case Study Site # 3 – Not A High Priority Site for Improvements ...... 189
11.0 Recommendations ...... 192
11.1 Curve Crash Characteristics ...... 193
11.2 Highway Safety Manual Analysis ...... 193
11.3 Spatial Relationships ...... 194
11.4 Individual Curve Analysis GIS Process ...... 195
11.5 Network Curve Analysis GIS Process ...... 196
11.6 Mobile Vehicle Horizontal Curve Data Collection ...... 197
11.7 Horizontal Curve Procedure ...... 198
12.0 References ...... 200
Appendix ...... 217
Appendix A: Manual Field Investigation Procedure ...... 218
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LIST OF TABLES
Table 1. Horizontal Curve Collision Geometric Roadway Characteristics ...... 46 Table 2. Horizontal Curve Collision Urban vs Rural Characteristics ...... 46 Table 3. Horizontal Curve Collision Severity Characteristics ...... 48 Table 4. Rural Horizontal Curve Collision Frequency Characteristics ...... 50 Table 5. Horizontal Curve Collision Type Characteristics (Most Harmful Event) ...... 52 Table 6. Horizontal Curve Collision Most Harmful Event Characteristics ...... 54 Table 7. Horizontal Curve Collision Roadway Surface Characteristics ...... 60 Table 8. Potential Countermeasures to Reduce the Frequency and/or Severity of Horizontal Curve Collisions...... 62 Table 9. Radius in Meters (feet) Comparison of GIS Methods ...... 77 Table 10. Sensitivity Analysis for 304.8 Meter (1,000’) Radius and 152.4 Meter (500’) Length Curve ...... 78 Table 11. Curve Radius: Field Measured vs GIS Calculated ...... 80 Table 12. Curve Radius Differences from Field Measured Values ...... 81 Table 13. Correlation Coefficients and P-values for Radius Values ...... 82 Table 14. Curve Length: Field Measured vs GIS Calculated ...... 83 Table 15. Curve Length Differences from Field Measured Length Values ...... 84 Table 16. Correlation Coefficients and P-values for Length Values ...... 84 Table 17. Safety Ranking of Top 10 Most Hazardous Field Measured vs GIS Calculated Curves ...... 86 Table 18. Spearman Correlation Coefficients for Safety Rankings ...... 87 Table 19. Radius Differences with Descriptive Parameters ...... 101 Table 20. Length Differences with Descriptive Parameters ...... 103 Table 21. Radius Differences with Descriptive Parameters ...... 105 Table 22. Length Differences with Descriptive Parameters ...... 105 Table 23. Collision Data (5 years – 2005 to 2009): Curve Related and All Crashes on NC42 and NC96 ...... 107
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Table 24. Radius Inaccuracies from Locational Errors at Horizontal Curves ...... 115 Table 25. Radius Comparison of All Techniques ...... 126 Table 26. Length Comparison of All Techniques ...... 129 Table 27. Comparison of Radius and Length Values for Vendor and Chord Method Data . 131 Table 28. Comparison of Range of Radius and Length Values ...... 132 Table 29. Field Data Collection Elements ...... 142 Table 30. HSM Calibration Factors Calculated ...... 145 Table 31. Annual Calibration Factors (All Segments, Random Segments, Non-Random Segments) ...... 146 Table 32. Input Values for HSM (Minimum, Maximum, Average, and Median) ...... 147 Table 33. Output Values from HSM (Predicted Collisions Per Year) ...... 148 Table 34. Predicted Collisions (over 5 years) for Two-Lane Road Horizontal Curves ...... 150 Table 35. Variables of the Model ...... 165 Table 36. Model Predicted Collisions (collisions per year) by Adjacent Curve Distance ... 167 Table 37. Case Study Site # 1 Comprehensive Study Procedure Summary ...... 187 Table 38. Case Study Site # 2 Comprehensive Study Procedure Summary ...... 189 Table 39. Case Study Site # 3 Comprehensive Study Procedure Summary ...... 191
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LIST OF FIGURES
Figure 1. Horizontal Curve Collision Frequency Distribution ...... 49 Figure 2. Horizontal Curve Collision Time of Day Characteristics ...... 57 Figure 3. Horizontal Curve Collision Day of Week Characteristics ...... 58 Figure 4. Horizontal Curve Collision Month of Year Characteristics ...... 59 Figure 5. GIS Line Work – Horizontal Curve with 7 Points [ESRI 2009] ...... 68 Figure 6. GIS Line Work – Horizontal Curve with 3 Points [ESRI 2009] ...... 69 Figure 7. Horizontal Curve Layout [Findley and Foyle 2009] ...... 70 Figure 8. Curve Calculator User Input Screen ...... 71 Figure 9. Curvature Extension User Input Screen ...... 73 Figure 10. Curve Finder User Input Screen ...... 74 Figure 11. Tolerance Example ...... 75 Figure 12. Example of Manual Horizontal Curve Identification ...... 94 Figure 13. Percentage of Curves Reported and Matched by Curve Finder Tolerance – NC96 & NC42 ...... 97 Figure 14. Percentage of Curves Reported and Matched by Curve Finder Tolerance – I40 .. 98 Figure 15. Curve Finder Error Quotient Diagram ...... 100 Figure 16. Radii Differences Between Curve Finder and Curvature Extension ...... 102 Figure 17. Length Differences Between Curve Finder and Curvature Extension ...... 104 Figure 18. Curve Influence Area (0.1, 0.2, and 0.5 miles) ...... 108 Figure 19. Horizontal Curve Layout with PT Offset Error ...... 116 Figure 20. Curve Radius (Macroscopic and Microscopic) Using Curvature Extension ...... 118 Figure 21. Roadway Curvature Data (Chord, GIS, Vendor, and Survey) ...... 123 Figure 22. Aerial Visualization of Roadway Curvature from Various Methods (Geofiny 2007) ...... 124 Figure 23. Radius Comparison of Vendor Data...... 127 Figure 24. Length Comparison of Vendor Data ...... 130 Figure 25. Collision Interpolation Milepost Example ...... 160
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Figure 26. Spatial Curve Example Layout ...... 161 Figure 27. Validation of Model with Difference in Collisions from Reported Collisions ... 168 Figure 28. Comprehensive Curve Safety Study Procedure ...... 174 Figure 29. Case Study Site #1 ...... 186 Figure 30. Case Study Site #2 ...... 188 Figure 31. Case Study Site #3 ...... 190
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1.0 INTRODUCTION
Horizontal curves, providing the transition between straight roadway segments, are particularly hazardous locations which deserve attention from researchers and transportation agencies. The collision rate for horizontal curves is approximately three times greater than straight (also known as tangent) section collision rates (Torbic 2004). Fatal collisions on horizontal curves comprise approximately 25% of all fatal collisions. The majority (76%) of fatal horizontal curve collisions occur from a single vehicle leaving the roadway and either striking a fixed object or overturning. In a study of collisions in North Carolina, Hummer et al (2011) found that 1.9% of two-lane road curve collisions are fatal, which was more than three times the fatal rate of collisions on all roads statewide (0.6%). The prevalence of collisions involving vehicles leaving the roadway makes horizontal curves a key focus for transportation agency improvements because these types of collisions have numerous countermeasures, or types of improvements, for collision reduction.
A complete understanding of horizontal curves is critical to enable an agency to systematically and efficiently improve the safety performance of its roads. A comprehensive safety study procedure for horizontal curves on rural, two-lane roadways was developed through this research as an improvement to existing safety procedures. Current agency practices for identifying hazardous locations focus on collision history, which could lead to inefficiently selecting and funding sites for improvements. Many collisions occur at random locations that are unlikely to be improved by engineering practices, such as those primarily caused by driver inattention or intoxication (although other broader practices in education, enforcement, or emergency response could reduce the occurrence or severity of collisions).
This proposed comprehensive safety study procedure is proactive by incorporating geometric features and roadway characteristics that influence the safety of a curve that might not be identified by methods solely based on collision history. This process is efficient because of the ability to collect a significant amount of data from an office, saving agency resources and
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providing an opportunity to review rural sites that might not otherwise be considered for improvements. This research will show that a comprehensive, proactive safety process for rural, two-lane curves is feasible and furthermore, that it is possible to conduct a significant amount of work before a field visit is necessary, if a field visit is even necessary to complete the process.
For a complete understanding of horizontal curves and their potential for improvement, the proposed curve study procedure consists of the following seven steps, as detailed in the following sections:
1. Site identification 2. Office data acquisition 3. Collision data analysis 4. Curve characteristics analysis 5. Collision data and curve characteristics findings 6. Field data acquisition and confirmation 7. Recommendations
1.1 Step 1: Site Identification
Identifying a site (Step 1) for further study can come from a systematic process of examining all horizontal curves or through targeted identification of specific curve locations. An inventory, meaning a database of curve information, is the primary method of identifying every curve in a roadway network; however, many agencies do not have curve inventories because of the cost and time required to create and maintain an inventory. Other sources for identifying curves include: collision reports, a citizen, a transportation agency employee, a police officer, through routine maintenance activities or reconstruction projects, or through a formal hazardous location identification process utilized by the agency.
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1.2 Step 2: Office Data Acquisition
Office techniques for data acquisition (Step 2) can involve collecting information by examining plans, collision history, GIS data, lidar data, satellite photographs, online mapping programs, video or photo logs, or asset inventories. Important elements to collect during this step include curve geometric features, spatial relationships with other roadway elements, the number, type, and location of signs, markings, and driveways, and other relevant factors. New techniques and technologies allow horizontal curves to be studied, at least preliminary investigations, remotely from an office. The use of office study techniques can greatly increase the efficiency of a transportation agency by avoiding resource intensive data collection visits to sites, thereby saving time, preserving equipment, and avoiding staff exposure to traffic. Many states maintain GIS data of their roadway system, which can allow office study techniques to leverage this existing resource to provide horizontal curve data while avoiding the pitfalls of field data collection.
1.3 Step 3: Collision Data Analysis
Police reports supply the necessary information to conduct a collision analysis (Step 3) of the curve. The objective of the collision data analysis is to determine if there is an overrepresentation of collisions at the curve. The Highway Safety Manual (HSM) is an invaluable tool with which to conduct a collision data analysis on a curve (AASHTO 2010). The HSM presents a model to predict the safety of a two-lane horizontal curve based on its characteristics, including: traffic volume, lane width, shoulder width, length, radius, superelevation, grade, driveway density, roadside hazard rating, spiral transition, passing lanes, roadway lighting, centerline rumble strips, two-way left-turn lanes, and automated speed enforcement. An overrepresentation of collisions at a curve signifies a potential curve of interest for improvements, particularly if the collisions appear to be correctable with available countermeasures.
Multiple methods could be used to determine overrepresentation, but three types of analyses will apply in most cases: 1) when no collision data is available or when the available
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collision data is thought to be unreliable, agencies should analyze the roadway section with the HSM to generate a collision prediction and compare the result to other sites or to a standard collision frequency (that could vary by geographic location, site type, etc.), 2) when reliable reported collision data are available, a comparison can be made to the number of collisions predicted by the HSM model for the specific location, and 3) when reliable reported collision data are available, it is possible to employ a Bayesian process which combines the predictive capabilities of the HSM with observed collisions, where the resulting value can then be compared to a standardized value for similar locations. A safety analysis which found an over-representation of collisions on a curve could signify a potential deficiency (geometric, signage, pavement condition, truck restrictions, drainage, etc.) that could be addressed by an appropriate countermeasure.
1.4 Step 4: Curve Characteristics Analysis
The analysis of a horizontal curve (Step 4) should include an examination of both its characteristics and its geometric features. An analysis and evaluation of a horizontal curve’s characteristics could focus on identifying potential deficiencies through an examination of HSM crash modification factors (CMFs) for lane width, shoulder width and type, curve length, curve radius, spiral transition presence, superelevation, grade, and driveway density (AASHTO 2010). CMFs are multiplicative factors that estimate the change in collisions after a countermeasure is implemented under specific conditions. Other relevant CMFs that are not currently incorporated into the HSM can be found through the CMF Clearinghouse (www.cmfclearinghouse.org) established by the Federal Highway Administration (FHWA) as a centralized location for CMFs. A Policy on Geometric Design of Highways and Streets provides the geometric highway design guidance most US agencies use and provides a framework for conducting an analysis of a horizontal curve by comparing to minimum levels for key curve design features (AASHTO 2004). A deviation of the geometric characteristics of the curve from the minimum guidance from AASHTO would signify a geometric deficiency and elements of interest for improvements.
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1.5 Step 5: Collision Data and Curve Characteristics Findings
Step 5 is intended to identify if the curve is of interest for improvement, based on the results of Steps 3 and 4. If Steps 3 and 4 show that there are no serious physical deficiencies and no overrepresentation of reported collisions, the site should not be considered a high priority for safety improvements. If deficiencies were found, collisions were overrepresented, or if both collision overrepresentation and physical deficiencies are identified, the process should be continued to explore the possibility of improvement through countermeasures.
1.6 Step 6: Field Data Acquisition and Confirmation
A field investigation (Step 6) can be used to acquire additional data or confirm assumptions made during prior steps. Field investigations include the direct measurement of attributes at the curve in question and inventorying of relevant roadway features not typically available with office methods during Step 2, such as the grade, superelevation, shoulder conditions (high or low points), and other condition related aspects that change over time. If the elements of concern in the HSM analysis correspond to the types of collisions examined in the collision data analysis, this should provide analysts with a high level of confidence that the source of the safety concern has been identified, and a site visit might not be necessary (users could progress directly to Step 7). An example of the value and importance of a field visit is a crash analysis which shows crashes in wet pavement conditions, followed by a site visit which reveals high shoulders which could cause hydroplaning. If significant deviations exist between the field observations and the office data collection methods or assumed values used in Steps 2-4, the process should be restarted at the curve criteria decision in Step 2.
1.7 Step 7: Recommendations
If a curve is determined to have deficiencies, appropriate countermeasures should be identified (Step 7). The analysis and evaluation of the horizontal curve should lead to the selection of appropriate countermeasures, which could include operational, pavement surface, geometric, or signage improvements. Research by Hummer et al (2010) matched common horizontal curve collisions with countermeasures. This combination of curve
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collision characteristics and potential countermeasures is essential to provide transportation agencies with effective improvements for curve collisions. Each potentially appropriate countermeasure should be evaluated for its potential effectiveness from a collision reduction (fatal, injury, and property damage only collisions) and cost effectiveness standpoint. Transportation agencies have a variety of funding mechanisms to complete safety improvement projects, such as spot safety funding; large construction projects; resurfacing, rehabilitation, and reconstruction projects; and routine maintenance. The variety of possible funding sources for safety improvements should be considered to save costs when possible by combining safety enhancements with other projects. A final recommendation of an appropriate countermeasure(s) can be made at the conclusion of this step.
1.8 Research Objectives
This research provides an engineer or agency with the ability to more efficiently and effectively study and improve the safety of horizontal curves. A comprehensive horizontal curve study procedure was developed with the following objectives in mind. These objectives supplement existing literature on horizontal curve studies and form the foundation of the comprehensive procedure.
1. This research will develop and utilize a new horizontal curve study method procedure to ensure a systematic approach for identifying curves, studying and measuring their characteristics, and improving hazardous locations. a. This research will quantify the collision characteristics of horizontal curves, which will provide an engineer with the necessary information to identify and correct hazardous curves. b. This research will recommend potential countermeasures for horizontal curves based on a linking of common horizontal curve collision types and effective countermeasures. c. This research will recommend study methods for geometric characteristics which will allow an engineer to most effectively and efficiently measure,
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define, and analyze horizontal curves to determine their predicted safety performance. d. This research will establish a set of parameters to which safety can be related through spatial and geometrical features. i. Safety will be related geometrically through the establishment of guidance for horizontal curves for the implementation of the nationally accepted prediction model for roadways, presented in the Highway Safety Manual. ii. Safety will be related spatially through the impact of spatial relationships on horizontal curve safety using the predictive methodology of the Highway Safety Manual as a foundation for incorporating spatial considerations into horizontal curve safety prediction.
1.9 Significance of the Research
This research provides important information for transportation agencies and researchers. Current procedures for performing horizontal curve investigations and implementing improvements are inconsistent and incomplete. The available guidance on studying curves and applying traffic control devices to curves is general and includes a myriad of choices, leaving much discretion to field personnel. Tables or formulas are not available to field personnel to quickly determine the optimum devices because the factors are too complex to distill into simple, quick-reference documents. Inconsistencies across and throughout agencies, regions, divisions, and personnel have resulted from this lack of procedural guidance and can result in inefficient spending for studying, analyzing and improving curves with safety concerns. The inconsistencies can also contribute to collisions because motorists expectations are not met, thereby making an agency vulnerable to legal action. A consistent process, such as the one detailed in this dissertation, provides a reasonable defense against legal actions.
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The proposed consistent and repeatable procedure furthers the transition away from opinion or intuition and towards fact-based decision making for highway safety. Below is a relevant quote from Ezra Hauer (2007) about the importance of factual knowledge in highway safety, which is promoted by this procedure:
“A change from a system of road-safety delivery rooted in opinion, intuition, and folklore to one that is founded in science and based on factual knowledge is underway. Change, as always, faces obstacles. The main obstacle is the near absence of professionals who can be the carriers and providers of factual road-safety knowledge. The second important obstacle is the weakness of the knowledge in which these professionals would have to be trained. Both obstacles stem from the same source; in a society in which it is acceptable to deliver road safety on the basis of opinion, intuition, and folklore, there is little demand for factual knowledge and for carriers thereof. Therefore, the most urgently needed change of road- safety culture is to make intuition-based road-safety delivery socially unacceptable.”
1.10 Research Scope and Limitations
Data used in this research was derived from North Carolina locations; however, findings are summarized in a manner which can lead to the implementation of recommendations beyond the state. The focus of this research was horizontal curves, primarily those located on paved, rural, two-lane roads with no sidewalks, curbs, spiral transitions, paved shoulders, and roadway lighting. Two-lane roads were the focus of this research due to their prevalence and the severe nature of collisions that occur on these roadways. However, the study method procedure developed as part of this research is applicable to many collision and roadway types through modification or elimination of curve specific elements. The procedure was designed for individual curves, but could be applied to an entire roadway or network. The principles described in this procedure are also applicable to any region, state, or nation. Specific elements herein, as described in this dissertation, should be customized for a given locality (such as, relevant CMFs for the driver population and site characteristics and HSM calibration efforts).
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Many of the recommendations made are likely to be applicable for many years, while other recommendations might need periodic updates as better techniques or more data become available for analysis. The framework provided in this research allows for periodic updates to be accomplished in a consistent and accurate manner.
1.11 Dissertation Organization
The organization of this report leads the reader through a literature review of horizontal curve collision characteristics, study methods, safety analysis, traffic control devices, and crash modification factors in Chapter 2. Curve crash characteristics, which could influence the procedure of safety investigations, are presented in Chapter 3. The manual field investigation procedure, which was developed for the collection of related data and for other uses within this report, is described in Chapter 4. Chapters 5 and 6 present potential GIS methods that can be utilized for horizontal curve data collection on an individual curve and network of curves basis. The comparison of mobile data collection vehicle data and data from other methods is detailed in Chapter 7. The Highway Safety Manual analysis, with two-lane road calibration factors, is presented in Chapter 8. Chapter 9 contains an analysis of the impacts of nearby curves on curve safety. Chapter 10 describes the comprehensive process that utilizes information from published literature and the earlier chapters. The recommendations from each analysis chapter are presented in Chapter 11. Taken together, these chapters build upon themselves and relate to each other in a way that the reader can gain valuable insight from each analysis.
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2.0 LITERATURE REVIEW
2.1 Introduction
In North Carolina, there exist a great number of highway horizontal curves. As of 2006, the State of North Carolina had about 74,000 miles of two lane roads of a total of approximately 79,000 miles of roads (NCDOT 2007). The main function of horizontal curves is to provide a smooth transition between two tangent sections of roadway. Unfortunately, horizontal curves are relatively dangerous features, with collision rates about 3 times that of comparable tangent sections on average (Lyles and Taylor 2006). According to the statistics in the Fatality Analysis Reporting System (FARS) in 2002, about 42,800 people were killed in 38,300 fatal crashes on U.S. highways and 25 percent of the fatal crashes occurred on horizontal curves on two-lane rural highways.
The North Carolina Department of Transportation (NCDOT) has used several guidelines and handbooks for dealing with horizontal curve safety, including the Manual on Uniform Traffic Control Devices (USDOT 2003), AASHTO Green Book (AASHTO 2004), North Carolina Supplement to the Manual on Uniform Traffic Control Devices (NCDOT 2005), 3R Guide (NCDOT 2004), Traffic Control Devices Handbook (ITE 2001), and Traffic Engineering Policies, Practices and Legal Authority Resources (TEPPL) (NCDOT 2010) for designing safe horizontal curves. These guidelines deal with various important horizontal design elements, such as selecting the adequate advisory speed, designing shoulder widths, and placing traffic control devices (TCDs).
There is a wide variety of traffic control devices available to assist motorists with operating safely on horizontal curves. Such measures include pavement markings, various types of warning signs (with and without flashers), chevron signs, advisory speed signs, raised pavement markers, pavement and post-mounted delineators, and others. The reference that provides the most commonly used guidance for the application of those devices is the Manual on Uniform Traffic Control Devices (MUTCD). Other manuals are also available,
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including NC’s own version of the MUTCD and the TEPPL. The guidance for traffic control devices on curves is quite general, however. Much discretion is left to transportation engineers and technicians in the field, as the factors that matter in optimum device choices are too complex to distill into simple formulas or tables.
2.2 Collision Characteristics and Geometric Design Features
There are many studies identifying collision characteristics and geometric design features that have an impact on collisions. The following studies all address horizontal curve collisions. They also identify horizontal curves as causal factors in highway collisions and indicate that curves have a significantly higher collision rate than tangent sections. The purpose here is to see what curve characteristics and agency countermeasures have been identified and are most prevalent. The literature review encompassed crash rates, roadway characteristics at curves, causal factors, and numerous potential treatments.
Garber and Kassebaum (2008) studied nearly 10,000 collisions on urban and rural two-lane highways in Virginia finding the predominate type of collision to be run-off-the-road collisions. The significant causal factors of these run-off-the-road collisions included roadway curvature and traffic volume as determined through a fault tree analysis. The countermeasures identified to mitigate run-off-the-road collisions include widening the roadway, adding advisory signs or chevrons to sharp curves, and adding or improving shoulders. However, this study did not specifically address curve collisions nor did it indicate how many of the collisions were on curves.
McGee and Hanscom (2006) provide a publication on low-cost countermeasures that can be applied to horizontal curves to address identified or potential safety problems. These countermeasures included: basic traffic signs and markings from the MUTCD, enhanced TCDs, other TCDs not mentioned in the MUTCD, rumble strips, minor roadway improvements, and innovative and experimental countermeasures. For every countermeasure, the authors concisely identified a description of the countermeasure, an application guideline,
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design elements, its effectiveness, cost, and maintenance, and additional sources of information.
In Volume 7 of NCHRP Report 500, Torbic et al. (2004) provided strategies to improve the safety of horizontal curves. The study had two primary purposes. The first was to reduce the likelihood of a vehicle leaving its lane and either crossing the roadway centerline or leaving the roadway at a horizontal curve. The other purpose was to minimize the adverse consequences of leaving the roadway at a horizontal curve. To accomplish these research objectives, twenty detailed strategies were described as countermeasures for reducing curve- related collisions. Each strategy included a general description, an estimate of the effectiveness of each countermeasure, and special issues pertaining to horizontal curves. These countermeasures addressed traffic control devices, markings, sight distances, and horizontal alignments.
Another study that investigated the relationship between roadway design attributes and collision activity was performed by Strathman et al (2001). This study investigated the statistical relationship between collision activity and roadway design attributes on Oregon highways. Using collision data from a two-year period (1997-1998), the highways were divided into variable length homogenous highway segments, yielding a set of over 11,000 segments. For non-freeway segments, maximum curve length and right shoulder width were found to be among the design attributes related to curves that were statistically related to collision activity. Maximum curve angle (a surrogate for degree of curvature) was not found to be related to collision activity in this study.
Souleyrette et al. (2001) evaluated roadway and collision characteristics for all highways in Iowa through integrating databases with digital imagery, roadway characteristics, and collision data. This project studied five collision types including collisions on horizontal curves and made use of GIS technology to collect roadway characteristics that were not identified by collision records. Curves were found by using GIS to identify a 5º or more
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change in azimuth between tangents. The analysis of high collision locations on horizontal curves found that the degree of curvature had a direct impact on the collision rate. The model also indicated that the collision rate on shorter curve lengths was significantly higher than on longer curves. In addition, this study produced a curve database for Iowa with radii and length attributes and a procedure for identifying horizontal curves with high collision occurrences statewide.
Zegeer et al. (1991) analyzed over 13,000 horizontal curves, primarily in Washington, to evaluate the relationship between curve features and collisions. To meet the study objective, the horizontal curve features which affected traffic safety and operation were first identified. A collision prediction model (consisting of variables relating to collisions and curve features) was developed through a variety of statistical methods. These variables were: curve length, volume of vehicles, degree of curve, presence of spiral transitions, and roadway width. From these identified variables, existing countermeasures for enhancing safety and operations at particular curve sections were determined and the model developed an effectiveness of collision reduction for each of these countermeasures. This study also provided general safety guidelines for curve design including signing, marking, and delineation as recommended cost-effective countermeasures.
Many other research efforts have examined specific curve collision countermeasures. However, to this point, as mentioned previously, no past study has characterized curve collisions on a large scale and matched the results of such a characterization with countermeasures directed at specific collision causes. For instance, overturn and rollover collisions were found to be primary type of horizontal curve collision. To prevent overturn and rollover collisions, a recommendation would be to avoid drop-offs at the edge of the pavement. This combination of over-represented collision types and countermeasure suggestions is useful for transportation professionals as quick and simple guidance to preventing curve collisions.
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2.3 Horizontal Curve Study Methods
This section summarizes study methods used to estimate advisory speeds and curve radii for horizontal curves. Six methods were reviewed from the several references. The criteria for the evaluation of each method included precision, cost, utility (ease to use), and safety.
2.3.1 Ball-Bank Indicator Method
Normally, advisory speeds for horizontal curve are determined through several direct runs of a test vehicle in the field. In general, the ball-bank indicator is the most commonly used method to select an advisory speed on horizontal curves (18). This method is initially based on experiments conducted in 1930s. Although the MUTCD provides general guidelines for several TCDs, there still exist a variety of difficulties in practical field implementation due to the subjectivity and variability in traffic engineer’s opinions. Although there have been a lot of mechanical improvements in vehicle characteristics for the last 50 years, the criteria for setting advisory speeds on curves still use the old method.
Chowdhury et. al. (1998) assessed the validity in ball-bank indicator criteria for determining advisory speeds on horizontal curves. To accomplish the study objective, the authors collected the data on curve geometry, spot speeds, and ball-bank readings on 28 two-lane highways in Virginia, Maryland, and West Virginia. Data were analyzed to consider various factors including posted advisory speed, driver’s compliance, and friction factors. The authors compared the existing posted speed with the speed recommended by ball-bank indicator, a standard formula, and the 85th percentile. The authors suggested that the existing criteria of ball-bank indicator reading (10°, 12°, and 14°) should be revised upward to 12°, 16°, and 20° to better reflect average curve speeds.
Carlson et. al. (2004) estimated the curve radius using ball-bank indicator method and curve speed. The curve radius was calculated by a point-mass equation from AASHTO Green Book (2001). Finally, the estimated radius was compared to true curve radius and the relative error was larger than other methods.
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2.3.2 Compass Method
The compass method is based on an advisory speed equation for a curve of specified radius and superelevation rate. Basically, this method needs curve radius and superelevation rate information.
Bonneson et. al. (2007a, 2007b) provided traffic engineers with technical guidelines of TCDs application and procedure for rural horizontal curves in the “Horizontal Curve Signing Handbook” using a compass method. This reference described detailed processes and methods for establishing advisory speed on horizontal curves.
Currently, the ball-bank indicator method is a widely used method to establish various TCDs. As an alternative method for determining the advisory speed, compass method was developed in this project which is based on measurement of curve geometry. To evaluate the developed compass method, it was compared with traditional ball-bank indicator method with respect to speed variability. The result indicated that the compass method is more stable than the ball-bank indicator method for curves having similar geometries. This means that the compass method provides more uniform and consistent advisory speeds for horizontal curves. In addition, it was found that ball-bank indicator method does not consider tangent section speed although the speed affects the advisory speed. However, the compass method has safety problems since the field personnel leave their test vehicle to collect data on the roadside (Carlson et al., 2005).
2.3.3 Direct Method
The direct method is based on the measurement of vehicle speed at the curve mid-point using a radar gun, laser gun, or traffic classifier. The Horizontal Curve Signing Handbook (Bonneson et al., 2007b) describes the three steps of the direct method: 1) field measurements of speed, 2) Determination of advisory speed, and 3) confirmation of speed for conditions. The direct method has the advantage of being able to directly measure the speed preferences of driver population (car and truck) as they have an interaction with the subject
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curve. However, this method also has the disadvantage of taking more resources to determine adequate advisory speed comparing to ball-bank indicator method and compass method.
2.3.4 Lateral Acceleration Method
The lateral acceleration method is similar to the ball-bank indicator method except that the unbiased lateral acceleration rate is substituted in the point-mass equation of BBI to determine the curve radius. The data measured by a lateral acceleration device are stored with traveled distance and vehicle speed. The error of this method is relatively low compared to ball-bank indicator method and compass method (Carlson et al., 2005). Also, just one field technician is required to collect needed data. However, the measuring device is expensive and, like the ball-bank indicator method, it is essential to drive the curve several times to obtain a good lateral acceleration.
2.3.5 Yaw Rate Transducer Method
The yaw rate transducer method uses a lateral acceleration device. Additionally, it provides not only traveled curve distance and vehicle speed but also the deflection angle of the curve. Therefore, this method can calculate the final curve radius using a simple equation like the following: