Safety Evaluation of Horizontal Curve Realignment on Rural, Two-Lane Roads PUBLICATION NO. FHWA-HRT-17-066 APRIL 2018 U.S. Department of Transportation Federal Highway Administration Research, Development, and Technology Turner-Fairbank Highway Research Center 6300 Georgetown Pike McLean, VA 22101-2296 FOREWORD The research documented in this report was conducted as part of the Federal Highway Administration’s (FHWA) Evaluation of Low-Cost Safety Improvements Pooled Fund Study (ELCSI-PFS). FHWA established this PFS in 2005 to conduct research on the effectiveness of the safety improvements identified by the National Cooperative Highway Research Program Report 500 Guides as part of the implementation of the American Association of State Highway and Transportation Officials (AASHTO) Strategic Highway Safety Plan. The ELCSI-PFS studies provide a crash modification factor and benefit–cost (B/C) economic analysis for each of the targeted safety strategies identified as priorities by the pooled fund member States. This study evaluates the safety effectiveness of horizontal curve realignment by increasing the radius of curved roadway segments on two-lane rural roads. One objective of this strategy is to reduce lane departure crashes, especially run-off-road crashes. The evaluation results showed substantial and significant reduction in crashes (total, injury and fatal crashes, run-off-road, and fixed object crashes, dark crashes, and wet-road crashes). The economic analysis revealed that increasing the radius of a horizontally curved roadway segment on two-lane, rural roads is a cost- effective safety improvement for reducing all types of crashes. This document is intended for safety engineers, highway designers, planners, and practitioners at State and local agencies involved with AASHTO Strategic Highway Safety Plan implementation. Monique R. Evans, P.E., CPM Director, Office of Safety Research and Development Notice This document is disseminated under the sponsorship of the U.S. Department of Transportation (USDOT) in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers' names appear in this report only because they are considered essential to the objective of the document. Quality Assurance Statement The Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement. TECHNICAL REPORT DOCUMENTATION PAGE 1. Report No. 2. Government Accession 3. Recipient's Catalog No. FHWA-HRT-17-066 I No. 4. Title and Subtitle 5. Report Date Safety Evaluation of Horizontal Curve Realignment on Rural, Two-Lane April 2018 Roads 6. Performing Organization Code 7. Author(s) 8. Performing Organization Report No. Raghavan Srinivasan, Daniel Carter, Craig Lyon, and Matthew Albee 9. Performing Organization Name and Address 10. Work Unit No. VHB Highway Safety Research Center 11. Contract or Grant No. 8300 Boone Blvd., Ste. 700 University of North Carolina DTFH61-13-D-0001 Vienna, VA 22182-2626 Chapel Hill, NC 27599-3430 12. Sponsoring Agency Name and Address 13. Type of Report and Period U.S. Department of Transportation Safety Evaluation; August 2013– Federal Highway Administration November 2016 1200 New Jersey Avenue, SE 14. Sponsoring Agency Code Washington, DC 20590 HRDS-20 15. Supplementary Notes The Federal Highway Administration (FHWA) Office of Safety Research and Development managed this study under the Development of Crash Modification Factors program. The FHWA Office of Safety Research and Development Program and Task Manager was Roya Amjadi (HRDS-20). 16. Abstract The Development of Crash Modification Factors program conducted safety evaluation of horizontal curve realignment on rural, two-lane roads for the Evaluation of Low Cost Safety Improvements Pooled Fund Study. The goal of this evaluation is to evaluate the safety effectiveness of this strategy. This strategy sought to reduce lane departure crashes, especially run-off-road crashes. This study determined the crash modification factors (CMFs) associated with curve realignment using the before–after empirical Bayes method and compared the results from published CMFs from cross-sectional studies. This evaluation used data from rural, two-lane roads in California, North Carolina, and Ohio. The evaluation revealed a 68-percent reduction in total crashes, a 74-percent reduction in injury and fatal crashes, a 78-percent reduction in run-off-road and fixed object crashes, a 42- percent reduction in dark crashes, and an 80-percent reduction in wet crashes, all of which were statistically significant at the 95-percent confidence level. The results pertain to a range of site characteristics, the most important of which is the range of the before and after degree of curve. The average degrees of curve in the before and after periods were 18.1 (with a minimum of 3.2 and a maximum of 52.1) and 6.9 (with a minimum of 0.0 and a maximum of 16.3), respectively. The average central angle of the curves was approximately 42 degrees (with a minimum of 1 and a maximum of 117). The average annual average daily traffic at the treated sites was about 3,500 (with a minimum of 465 and a maximum of 11,917). The average length of the realigned segments was 0.15 mi (with a minimum of 0.03 and a maximum of 0.60). This study compared the total crash CMFs with the results from previous cross-sectional studies. The CMFs from this before–after evaluation are lower compared to CMFs estimated from two previous cross-sectional studies. The economic analysis revealed a benefit–cost ratio of 3.17:1 with a range of 1.75:1 to 4.38:1. There is a need for further research with a larger sample of sites to assess the reliability of the CMFs obtained from this before–after evaluation. 17. Key Words 18. Distribution Statement Horizontal curve, rural two-lane roads, realignment, radius, degree No restrictions. This document is available of curve, lane departure crashes, safety improvements, safety through the National Technical Information evaluations, empirical Bayesian, run-off-road, crash modification Service, Springfield, VA 22161. factors, crash modification functions, cross-sectional studies http://www.ntis.gov 19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages: 22. Price Unclassified I Unclassified I 54 I Form DOT F 1700.7 (8-72) Reproduction of completed pages authorized. SI* (MODERN METRIC) CONVERSION FACTORS APPROXIMATE CONVERSIONS TO SI UNITS Symbol When You Know Multiply By To Find Symbol LENGTH in inches 25.4 millimeters mm ft feet 0.305 meters m yd yards 0.914 meters m mi miles 1.61 kilometers km AREA 2 2 in square inches 645.2 square millimeters mm ft2 square feet 0.093 square meters m2 yd2 square yard 0.836 square meters m2 ac acres 0.405 hectares ha mi2 square miles 2.59 square kilometers km2 VOLUME fl oz fluid ounces 29.57 milliliters ml gal gallons 3.785 liters l ft3 cubic feet 0.028 cubic meters m3 yd3 cubic yards 0.765 cubic meters m3 NOTE: volumes greater than 1000 l shall be shown in m3 MASS oz ounces 28.35 grams g lb pounds 0.454 kilograms kg T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or"t") TEMPERATURE (exact degrees) OF Fahrenheit 5 (F-32)/9 Celsius oc or (F-32)/1.8 ILLUMINATION fc foot-candles 10.76 lux Ix fl foot-lamberts 3.426 candela/m2 cd/m2 FORCE and PRESSURE or STRESS lbf poundforce 4.45 new1ons N lbf/in2 poundforce per square inch 6.89 kilopascals kPa APPROXIMATE CONVERSIONS FROM SI UNITS Symbol When You Know Multiply By To Find Symbol LENGTH mm millimeters 0.039 inches in m meters 3.28 feet ft m meters 1.09 yards yd km kilometers 0.621 miles mi AREA mm2 square millimeters 0.0016 square inches in2 m2 square meters 10.764 square feet ft2 m2 square meters 1.195 square yards yd2 ha hectares 2.47 acres ac km2 square kilometers 0.386 square miles mi2 VOLUME ml milliliters 0.034 fluid ounces fl oz l liters 0.264 gallons gal m3 cubic meters 35.314 cubic feet ft3 m3 cubic meters 1.307 cubic yards yd3 MASS g grams 0.035 ounces oz kg kilograms 2.202 pounds lb Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T TEMPERATURE (exact degrees) oc Celsius 1.8C+32 Fahrenheit OF ILLUMINATION Ix lux 0.0929 foot-candles fc cd/m2 candela/m2 0.2919 foot-lamberts fl FORCE and PRESSURE or STRESS N new1ons 0.225 poundforce lbf 2 kPa kilopascals 0.145 poundforce per square inch lbf/in *SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. (Revised March 2003) ii TABLE OF CONTENTS EXECUTIVE SUMMARY ........................................................................................................1 CHAPTER 1. INTRODUCTION .................................................................................................3 BACKGROUND ON STRATEGY ......................................................................................... 3 BACKGROUND ON STUDY .................................................................................................. 3 LITERATURE REVIEW .......................................................................................................