Safety Evaluation of Edge-Line Rumble Stripes on Rural Two-Lane Horizontal Curves

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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 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 evaluated application of edge-line rumble stripes (ELRSs) on rural two-lane horizontal curves. ELRSs are a variation of common shoulder rumble strips used to alert drowsy or distracted drivers when they are leaving the travel lane to the right. ELRSs are installed with the edge-line pavement marking placed directly over the rumble strip. Data were obtained at treated rural two-lane horizontal curves in Kentucky and Ohio. The results for Kentucky indicate statistically significant reductions for total, injury, run-off-road (ROR), and nighttime crashes. The results for Ohio indicate statistically significant reductions for all crash types (i.e., total, injury, ROR, nighttime, and nighttime ROR). The B/C analysis results suggest that this treatment can be highly cost-effective. This report is intended for State departments of transportation, transportation agencies, academics, researchers, and other practitioners.

Jonathan Porter, Ph.D.
Acting Director, Office of Safety
Research and Development

Technical Report Documentation Page

SI* (Modern Metric) Conversion Factors

TABLE OF CONTENTS

Executive Summary

Chapter 1. Introduction

• Background on Strategy
• Background on Study
• Literature Review

Chapter 2. Objective

Chapter 3. Study Design

Chapter 4. Methodology

Chapter 5. Data Collection

• Kentucky
  • Installation Data
  • Reference Sites
  • Roadway Data
  • Traffic Data
  • Crash Data
  • Treatment Cost Data
• Ohio
  • Installation Data
  • Reference Sites
  • Roadway Data
  • Traffic Data
  • Crash Data
  • Treatment Cost Data
  • Data Characteristics and Summary

Chapter 6. Development of Safety Performance Functions

• Crash Spillover and Migration
• Safety Performance Functions

Chapter 7. Before–After Evaluation Results

• Aggregate Analysis
• Disaggregate Analysis of Ohio Data

Chapter 8. Economic Analysis

Chapter 9. Summary and Conclusions

Appendix: Additional Installation Details

• Responses from Kentucky
• Responses from Ohio

Acknowledgements

References

LIST OF FIGURES

• Figure 1. Illustration. Examples of longitudinal rumble strip markings
• Figure 2. Illustration. Overview of rumble strip dimensions
• Figure 3. Equation. Estimated change in safety
• Figure 4. Equation. EB estimate of expected crashes
• Figure 5. Equation. EB weight
• Figure 6. Equation. Index of effectiveness
• Figure 7. Equation. Standard deviation of index of effectiveness
• Figure 8. Equation. SPF model form for crash migration or spillover effects
• Figure 9. Graph. Yearly multiplier by year for reference groups in Kentucky
• Figure 10. Graph. Yearly multiplier by year for reference groups in Ohio
• Figure 11. Equation. SPF model form for Kentucky and Ohio

LIST OF TABLES

• Table 1. Summary of studies on changes in target collision frequency from application of SRSs
• Table 2. Rumble strip dimensions from Lyon et al
• Table 3. Before-period crash rate assumptions
• Table 4. Minimum required before-period mile-years for treated sites—total crashes
• Table 5. Minimum required before-period mile-years for treated sites—injury crashes
• Table 6. Minimum required before-period mile-years for treated sites—ROR crashes
• Table 7. Minimum required before-period mile-years for treated sites—nighttime crashes
• Table 8. Minimum required before-period mile-years for treated sites—nighttime ROR crashes
• Table 9. Analysis for crash effects in Kentucky
• Table 10. Analysis for crash effects in Ohio
• Table 11. Kentucky treatment cost and service life data
• Table 12. Ohio treatment cost data
• Table 13. Definitions of crash types
• Table 14. Data summary for treatment sites
• Table 15. Data summary for Kentucky reference sites
• Table 16. Data summary for Ohio reference sites
• Table 17. Reference group SPF for total crashes—parameter estimates
• Table 18. Observed and predicted crashes for reference groups in Kentucky
• Table 19. Observed and predicted crashes for reference groups in Ohio
• Table 20. SPFs by crash type
• Table 21. Time trend factors for predicted crashes
• Table 22. Aggregate analysis results for Kentucky
• Table 23. Aggregate analysis results for Ohio
• Table 24. Ohio results by AADT
• Table 25. Ohio results by before-period expected crash frequency
• Table 26. Recommended CMFs for ELRSs based on Kentucky data
• Table 27. Recommended CMFs for ELRSs and curve signage based on Ohio data

LIST OF ABBREVIATIONS

Executive Summary

The Federal Highway Administration (FHWA) established the Development of Crash Modification Factors (DCMF) program in 2012 to address highway safety research needs for evaluating new and innovative safety strategies (improvements) by developing reliable quantitative estimates of their effectiveness in reducing crashes. The ultimate goal of the DCMF program is to save lives by identifying new safety strategies that effectively reduce crashes and promote those strategies for nationwide implementation by providing measures of their safety effectiveness and benefit–cost (B/C) ratios through research. State transportation departments and other transportation agencies need to have objective measures for safety effectiveness and B/C ratios before investing in new strategies for statewide safety improvements. Forty State transportation departments have provided technical feedback on safety improvements to the DCMF program and have implemented new safety improvements to facilitate evaluations. These States are members of the Evaluation of Low-Cost Safety Improvements-Pooled Fund Study, which functions under the DCMF program.

This study evaluated the application of edge-line rumble stripes (ELRSs) on rural two-lane horizontal curves. ELRSs are a variation of common shoulder rumble strips (SRSs) used to alert drowsy or distracted drivers when they leave the travel lane to the right. ELRSs are installed where edge-line pavement markings would normally be placed, and the pavement marking is installed directly over the rumble strip. In this way, the ELRSs are installed closer to the travel lane than common SRSs. In addition, the vertical faces that are created within the milled rumble strip to which pavement markings are applied have the effect of enhancing the visibility of the edge line during nighttime and wet-weather conditions.

The project team obtained geometric, traffic, and crash data at treated rural horizontal curve locations in Kentucky and Ohio. To account for potential selection bias and regression-to-the-mean, the project team conducted an empirical Bayes (EB) before–after analysis using reference groups of untreated rural horizontal curves with similar characteristics to the treated sites. The analysis also controlled for changes in traffic volumes over time and time trends in crash counts unrelated to the treatment. While the analysis focused on the safety effectiveness on horizontal curves, the treatment applications were not limited only to horizontal curves. The treatment was a corridor treatment applied to segments consisting of both tangents and curves; however, the analysis removed horizontal tangents and only considered the effectiveness on curves.

The results for Kentucky indicated statistically significant reductions for total, injury, run-off-road (ROR), and nighttime crashes at the 95-percent confidence level. Nighttime crashes had the smallest crash modification factor (CMF), or the greatest reduction, with a value of 0.63. Total, injury, and ROR crashes had CMFs of 0.75, 0.64, and 0.74, respectively. The CMF for nighttime ROR crashes was 0.75 and was consistent with the same CMF from Ohio; however, it was significant only at the 80-percent level, suggesting that sample size was the reason for the lack of statistical significance at the 95-percent level. The results for Ohio indicated statistically significant reductions for all crash types. Nighttime ROR crashes had the smallest CMF of 0.71. Total, injury, ROR, and nighttime crashes had CMFs of 0.79, 0.79, 0.78, and 0.75, respectively. The resulting Ohio installation CMFs reflected the installation of ELRSs on horizontal curves as well as the impact of the statewide signing program. It is also important to note that all crash types considered in this research excluded intersection-related and animal crashes.

A disaggregate analysis of the results indicated larger safety benefits for horizontal curves with average annual daily traffic (AADT) greater than 4,000 for all crash types; however, the differences by AADT were not statistically significant at the 95-percent confidence level. The disaggregate analysis further indicated larger safety benefits for horizontal curves with a higher before-period expected crash frequency. The results suggested no benefit for curves with low before-period expected crash frequencies for all crash types. The difference in CMFs between low before-period expected crash frequency and high before-period expected crash frequency was statistically significant for total and ROR crashes. Due to correlation between variables, caution should be used in interpreting and applying these disaggregate results; however, the disaggregate analysis CMFs may be used to inform the process of prioritizing treatment sites for ELRSs.

Estimated B/C ratios range from 189:1 to 467:1 for Kentucky and from 272:1 to 672:1 for Ohio. On first inspection, the B/C ratios were larger than expected for an installation of this type. However, the installations took place on corridors, while the analysis only looked at the safety effects on horizontal curves. Horizontal curves have higher crash rates than overall corridors, and the cost per mile of installation would not be representative for installations only on horizontal curves. As a curve-specific treatment, the B/C ratio would likely be reduced owing to the higher deployment cost for spot-specific installations. Regardless, these results suggest that the treatment, even in its most expensive variation, can be highly cost effective.