U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590

Skip to content
Facebook iconYouTube iconTwitter iconFlickr iconLinkedInInstagram

Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

This report is an archived publication and may contain dated technical, contact, and link information
Back to Publication List        
Publication Number:  FHWA-HRT-17-066    Date:  April 2018
Publication Number: FHWA-HRT-17-066
Date: April 2018


Safety Evaluation of Horizontal Curve Realignment on Rural, Two-Lane Roads

PDF Version (1.71 MB)

PDF files can be viewed with the Acrobat® Reader®



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


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 No.


3 Recipient's Catalog No.


4. Title and Subtitle

Safety Evaluation of Horizontal Curve Realignment on Rural, Two-Lane Roads

5. Report Date

April 2018

6. Performing Organization Code


7. Author(s)

Raghavan Srinivasan, Daniel Carter, Craig Lyon, and Matthew Albee

8. Performing Organization Report No.


9. Performing Organization Name and Address
8300 Boone Blvd., Ste. 700
Vienna, VA 22182-2626
Highway Safety Research Center
University of North Carolina
Chapel Hill, NC 27599-3430


10. Work Unit No. (TRAIS)


11. Contract or Grant No.


12. Sponsoring Agency Name and Address

U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590

13. Type of Report and Period Covered

Safety Evaluation; August 2013–November 2016

14. Sponsoring Agency Code


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

Horizontal curve, rural two-lane roads, realignment, radius, degree of curve, lane departure crashes, safety improvements, safety evaluations, empirical Bayesian, run-off-road, crash modification factors, crash modification functions, cross-sectional studies

18. Distribution Statement

No restrictions. This document is available through the National Technical Information Service, Springfield, VA 22161.

19. Security Classification
(of this report)


20. Security Classification
(of this page)


21. No. of Pages


22. Price


Form DOT F 1700.7 (8-72) Reproduction of completed page authorized




Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000
Turner-Fairbank Highway Research Center | 6300 Georgetown Pike | McLean, VA | 22101