Safety Evaluation of Cable Median Barriers in Combination With Rumble Strips on Divided Roads

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FOREWORD

The research documented in this report was conducted as part of the Federal Highway Administration (FHWA) Evaluation of Low-Cost Safety Improvements Pooled Fund Study (ELCSI-PFS). FHWA established this pooled fund study 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 implementation of the American Association of State Highway and Transportation Officials Strategic Highway Safety Plan. The ELCSI-PFS research provides 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 cable median barriers in combination with rumble strips on the inside shoulder of divided roads. This strategy is intended to reduce the frequency of cross-median crashes, which tend to be very severe. Data were obtained in Illinois, Kentucky, and Missouri. In Illinois and Kentucky, cable median barriers were introduced many years after the inside shoulder rumble strips were installed; therefore, the evaluation determined the safety effect of implementing cable barriers along sections that already had rumble strips. Conversely, in Missouri, the inside shoulder rumble strips and cable barrier were implemented around the same time. The evaluation in Missouri determined the combined safety effect of both strategies. The combined Illinois and Kentucky results indicate an increase in total crashes but a decrease in injury and fatal crashes and head-on plus opposite-direction sideswipe crashes (used as a proxy for cross-median crashes). The results from Missouri for total and injury and fatal crashes were very similar to the combined Illinois and Kentucky results. However, the reduction in cross-median crashes in Missouri was much more dramatic. The economic analysis for B/C ratios shows that this strategy is cost beneficial.

Monique R. Evans
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
  • Safety Effects of Cable Median Barriers
  • Safety Effects of Shoulder Rumble Strips
  • Safety Effects of Cable Median Barriers with Shoulder Rumble Strips
  • Summary of Literature Review

CHAPTER 2. OBJECTIVE

CHAPTER 3. STUDY DESIGN

• CASE 1: SAMPLE SIZE REQUIRED TO DETECT AN EXPECTED CHANGE IN SAFETY
• CASE 2: CHANGE IN SAFETY THAT CAN BE DETECTED WITH AVAILABLE SAMPLE SIZES

CHAPTER 4. METHODOLOGY

CHAPTER 5. DATA COLLECTION

• ILLINOIS
  • Installation Data
  • Reference Sites
  • Roadway Data
  • Traffic Data
  • Crash Data
  • Treatment Cost Data
    
    
• KENTUCKY
  • Installation Data
  • Reference Sites
  • Roadway Data
  • Traffic Data
  • Crash Data
  • Treatment Cost Data
    
    
• MISSOURI
  • Installation Data
  • Reference Sites
  • Roadway Data
  • Traffic Data
  • Crash Data
  • Treatment Cost Data
    
    
• DATA CHARACTERISTICS AND SUMMARY

CHAPTER 6. DEVELOPMENT OF SPFS

• ILLINOIS AND KENTUCKY SPFS
• MISSOURI SPFS

CHAPTER 7. BEFORE–AFTER EVALUATION RESULTS

• AGGREGATE ANALYSIS
• DISAGGREGATE ANALYSIS

CHAPTER 8. ECONOMIC ANALYSIS

CHAPTER 9. SUMMARY AND CONCLUSIONS

APPENDIX A. ADDITIONAL INSTALLATION DETAILS FROM ILLINOIS

APPENDIX B. ADDITIONAL INSTALLATION DETAILS FROM KENTUCKY

APPENDIX C. ADDITIONAL INSTALLATION DETAILS FROM MISSOURI

APPENDIX D. METHODOLOGY FOR CALCULATING CRASH COSTS

ACKNOWLEDGMENTS

REFERENCES

List of Figures

• Figure 1. Graph. Cross-median fatalities in Missouri.⁽³⁾
• Figure 2. Graph. Miles of cable barrier installation in Missouri.⁽³⁾
• 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. Functional form for AADT.
• Figure 9. Equation. Functional form for SPFs.
• Figure 10. Equation. Determining annual cost.

List of Tables

• Table 1. Summarized results of studies that calculated the effect of shoulder rumble strips on crashes (adapted from Torbic et al.).⁽⁷⁾
• Table 2. Before-period crash rate assumptions.
• Table 3. Minimum required before-period mile-years.
• Table 4. Sample analysis for crash effects.
• Table 5. Illustration of alternate approach.
• Table 6. Illinois treatment cost and service life data.
• Table 7. Kentucky treatment cost and service life data.
• Table 8. Missouri cost and service life data.
• Table 9. Data summary for treatment sites in Illinois and Kentucky.
• Table 10. Data summary for treatment sites in Missouri.
• Table 11. Data summary for reference sites.
• Table 12. Illinois SPFs.
• Table 13. Kentucky SPFs.
• Table 14. Missouri before-period SPFs.
• Table 15. CMFs from 27 urban sites in Illinois.
• Table 16. CMFs from 13 rural sites in Illinois.
• Table 17. CMFs from urban and rural sites in Illinois.
• Table 18. CMFs from seven urban sites in Kentucky.
• Table 19. CMFs from 32 rural sites in Kentucky.
• Table 20. CMFs from urban and rural sites in Kentucky.
• Table 21. CMFs from urban sites in Illinois and Kentucky combined.
• Table 22. CMFs from rural sites in Illinois and Kentucky combined.
• Table 23. CMFs from urban and rural sites in Illinois and Kentucky combined.
• Table 24. CMFs from 310 urban sites in Missouri.
• Table 25. CMFs from 729 rural sites in Missouri.
• Table 26. CMFs from urban and rural sites in Missouri.
• Table 27. CMFs for the combination of cable median barriers and rumble strips when the before condition included inside shoulder rumble strips.
• Table 28. CMFs for the combination of cable median barriers and rumble strips when the before condition had neither inside shoulder rumble strips nor cable median barrier.

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 broad applications of new strategies for safety improvements. Forty State transportation departments provide technical feedback on safety improvements to the DCMF program and implement 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 cable median barriers in combination with rumble strips on the inside shoulders of divided roads. This strategy is intended to reduce the frequency of crashes by reducing the frequency and severity of cross-median crashes.

The research team obtained geometric, traffic, and crash data at treated multilane divided roads in Illinois, Kentucky, and Missouri. To account for potential selection bias and regression- to-the-mean, the research team conducted an empirical Bayes (EB) before–after analysis using reference groups of untreated multilane lane roads with characteristics similar to those of the treated sites. The analysis of the treatment sites in Missouri required a slightly different approach because Missouri installed cable median barriers and inside shoulder rumble strips on a systemwide basis on certain types of roads. As a result, there was no suitable reference group without the treatment for this road type. The analysis also controlled for changes in traffic volumes over time and time trends in crash counts unrelated to the treatment.

Illinois and Kentucky introduced cable median barriers many years after the inside shoulder rumble strips were installed; therefore, the evaluation determined the safety effect of implementing cable barrier along sections that already had rumble strips. Missouri implemented the inside shoulder rumble strips and cable barriers around the same time; as a result, the Missouri evaluation determined the combined safety effect of inside shoulder rumble strips and cable barriers. The combined Illinois and Kentucky results indicate about a 27-percent increase in total crashes, a 22- to 24-percent decrease in injury and fatal crashes (depending on whether injury crashes were defined as KAB or KABC), and a 48-percent decrease in cross-median crashes (defined as head-on plus opposite-direction sideswipe crashes).^[1] The results from Missouri for total and injury and fatal crashes were very similar to the combined Illinois and Kentucky results. However, the reduction in cross-median crashes in Missouri was more dramatic, with a 96-percent reduction (based on cross-median indicator only) and an 88-percent reduction (based on cross-median indicator plus head-on). The research team estimates the B/C ratios to be about 8.28 for the treatment in Illinois and Kentucky and 4.14 for the treatment in Missouri.

^[1] The KABCO Scale is used to represent injury severity in crash reporting (K is fatal injury, A is incapacitating injury, B is non-incapacitating injury, C is possible injury, and O is property damage only).