Safety Evaluation of Cable Median

Barriers in Combination with Rumble 

Strips on the Inside Shoulder of 

Divided Roads

PUBLICATION NO. FHWA-HRT-17-070 AUGUST 2017

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 (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

Notice

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

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.

FHWA-HRT-17-070

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

Safety Evaluation of Cable Median Barriers in Combination with Rumble 

Strips on Divided Roads

5. Report Date

August 2017

6. Performing Organization Code

7. Author(s)

Raghavan Srinivasan, Bo Lan, Daniel Carter, Bhagwant Persaud, and 

Kimberly Eccles

8. Performing Organization Report 

No.

9. Performing Organization Name and Address

VHB Persaud & Lyon, Inc.

8300 Boone Boulevard, Suite 700 87 Elmcrest Road

Vienna, VA 22182-2626 Toronto, ON M9C 3R7

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

DTFH61-13-D-00001

12. Sponsoring Agency Name and Address

U.S. Department of Transportation

Federal Highway Administration

1200 New Jersey Avenue, SE 

Washington, DC 20590-3660

13. Type of Report and Period 

Covered

Safety Evaluation

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.

16. Abstract

The Development of Crash Modification Factors program conducted the safety evaluation of cable median barriers 

in combination with rumble strips on the inside shoulder of divided roads for the Evaluation of Low Cost Safety 

Improvements Pooled Fund Study. This study evaluated safety effectiveness 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 

cross-median crashes, which tend to be very severe. Geometric, traffic, and crash data were obtained for divided 

roads in Illinois, Kentucky, and Missouri. To account for potential selection bias and regression-to-the-mean, an 

empirical Bayes before–after analysis was conducted using reference groups of untreated roads with characteristics 

similar to those of the treated sites. The analysis also controlled for changes in traffic volumes over time and time 

trends in crash counts unrelated to the treatment. 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. Hence, the evaluation in 

Missouri 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 24-percent decrease in fatal, 

incapacitating, non-incapacitating, and possible injury crashes; a 22-percent decrease in fatal, incapacitating, and 

non-incapacitating injury crashes; and a 48-percent decrease in 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, showing a 96-percent reduction (based on cross-median indicator only) and an 88-percent 

reduction (based on cross-median indicator plus head-on). The economic analysis for benefit-cost ratios shows that 

this strategy is cost beneficial.

17. Key Words

Rumble strips, cross-median crashes, low-cost, safety 

improvements, safety evaluations, empirical Bayesian, 

cross-median, head-on, sideswipe, injury, cable barrier, 

median barrier

18. Distribution Statement.

No restrictions. This document is available through the 

National Technical Information Service, 

Springfield, VA 22161.

http://www.ntis.gov

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

71

22. Price

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

TABLE OF CONTENTS

EXECUTIVE SUMMARY...........................................................................................................1

CHAPTER 1. INTRODUCTION ................................................................................................3

BACKGROUND ON STRATEGY .........................................................................................3

BACKGROUND ON STUDY..................................................................................................4

LITERATURE REVIEW.........................................................................................................4

Safety Effects of Cable Median Barriers.................................................................................4

Safety Effects of Shoulder Rumble Strips...............................................................................6

Safety Effects of Cable Median Barriers with Shoulder Rumble Strips...............................10

Summary of Literature Review .............................................................................................11

CHAPTER 2. OBJECTIVE .......................................................................................................13

CHAPTER 3. STUDY DESIGN ................................................................................................15

CASE 1: SAMPLE SIZE REQUIRED TO DETECT AN EXPECTED CHANGE IN

SAFETY.............................................................................................................................15

CASE 2: CHANGE IN SAFETY THAT CAN BE DETECTED WITH AVAILABLE

SAMPLE SIZES................................................................................................................17

CHAPTER 4. METHODOLOGY .............................................................................................19

CHAPTER 5. DATA COLLECTION.......................................................................................23

ILLINOIS.................................................................................................................................23

Installation Data ....................................................................................................................23

Reference Sites......................................................................................................................23

Roadway Data .......................................................................................................................24

Traffic Data ...........................................................................................................................24

Crash Data .............................................................................................................................25

Treatment Cost Data..............................................................................................................25

KENTUCKY............................................................................................................................25

Installation Data ....................................................................................................................25

Reference Sites......................................................................................................................26

Roadway Data .......................................................................................................................26

Traffic Data ...........................................................................................................................26

Crash Data .............................................................................................................................26

Treatment Cost Data..............................................................................................................27

MISSOURI...............................................................................................................................27

Installation Data ....................................................................................................................27

Reference Sites......................................................................................................................27

Roadway Data .......................................................................................................................28

Traffic Data ...........................................................................................................................28

Crash Data .............................................................................................................................28

Treatment Cost Data..............................................................................................................28

DATA CHARACTERISTICS AND SUMMARY................................................................29

CHAPTER 6. DEVELOPMENT OF SPFS ..............................................................................33

ILLINOIS AND KENTUCKY SPFS ....................................................................................33
iv

MISSOURI SPFS ....................................................................................................................35

CHAPTER 7. BEFORE–AFTER EVALUATION RESULTS...............................................37

AGGREGATE ANALYSIS....................................................................................................37

DISAGGREGATE ANALYSIS.............................................................................................41

CHAPTER 8. ECONOMIC ANALYSIS ..................................................................................43

CHAPTER 9. SUMMARY AND CONCLUSIONS.................................................................45

APPENDIX A. ADDITIONAL INSTALLATION DETAILS FROM ILLINOIS................47

APPENDIX B. ADDITIONAL INSTALLATION DETAILS FROM KENTUCKY...........51

APPENDIX C. ADDITIONAL INSTALLATION DETAILS FROM MISSOURI..............53

APPENDIX D. METHODOLOGY FOR CALCULATING CRASH COSTS......................57

ACKNOWLEDGMENTS ..........................................................................................................59

REFERENCES............................................................................................................................61
v

LIST OF FIGURES

Figure 1. Graph. Cross-median fatalities in Missouri ......................................................................5

Figure 2. Graph. Miles of cable barrier installation in Missouri......................................................6

Figure 3. Equation. Estimated change in safety.............................................................................19

Figure 4. Equation. EB estimate of expected crashes....................................................................20

Figure 5. Equation. EB weight.......................................................................................................20

Figure 6. Equation. Index of effectiveness.....................................................................................20

Figure 7. Equation. Standard deviation of index of effectiveness .................................................20

Figure 8. Equation. Functional form for AADT ............................................................................33

Figure 9. Equation. Functional form for SPFs...............................................................................33

Figure 10. Equation. Determining annual cost...............................................................................43
vi

LIST OF TABLES

Table 1. Summarized results of studies that calculated the effect of shoulder rumble strips 

on crashes (adapted from Torbic et al.)................................................................................8

Table 2. Before-period crash rate assumptions..............................................................................15

Table 3. Minimum required before-period mile-years...................................................................16

Table 4. Sample analysis for crash effects.....................................................................................17

Table 5. Illustration of alternate approach .....................................................................................22

Table 6. Illinois treatment cost and service life data......................................................................25

Table 7. Kentucky treatment cost and service life data..................................................................27

Table 8. Missouri cost and service life data ...................................................................................28

Table 9. Data summary for treatment sites in Illinois and Kentucky.............................................30

Table 10. Data summary for treatment sites in Missouri ...............................................................31

Table 11. Data summary for reference sites...................................................................................31

Table 12. Illinois SPFs...................................................................................................................34

Table 13. Kentucky SPFs...............................................................................................................35

Table 14. Missouri before-period SPF ...........................................................................................36

Table 15. CMFs from 27 urban sites in Illinois .............................................................................37

Table 16. CMFs from 13 rural sites in Illinois...............................................................................37

Table 17. CMFs from urban and rural sites Illinois.......................................................................38

Table 18. CMFs from seven urban sites in Kentucky....................................................................38

Table 19. CMFs from 32 rural sites in Kentucky...........................................................................38

Table 20. CMFs from urban and rural sites in Kentucky...............................................................39

Table 21. CMFs from urban sites in Illinois and Kentucky combined ..........................................39

Table 22. CMFs from rural sites in Illinois and Kentucky combine..............................................39

Table 23. CMFs from urban and rural sites in Illinois and Kentucky combine.............................40

Table 24. CMFs from 310 urban sites in Missour..........................................................................40

Table 25. CMFs from 729 rural sites in Missouri..........................................................................40

Table 26. CMFs from urban and rural sites in Missouri ................................................................41

Table 27. CMFs for the combination of cable median barriers and rumble strips when the 

before condition included inside shoulder rumble strips ...................................................45

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 .................................................................................................................................45
vii

LIST OF ABBREVIATIONS

AADT annual average daily traffic

B/C benefit-cost

CMF crash modification factor 

EB empirical Bayes 

ELCSI-PFS Evaluation of Low-Cost Safety Improvements Pooled Fund Study

FARS Fatality Analysis Reporting System

FHWA Federal Highway Administration 

GIS geographic information system

HSIS Highway Safety Information System

IDOT Illinois Department of Transportation

KABCO Scale 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)

KYTC Kentucky Transportation Cabinet

MoDOT Missouri Department of Transportation

NCHRP National Cooperative Highway Research Program

PDO property damage only

ROR run-off-road

SE standard error

SPF safety performance function 

SVROR single-vehicle run-off-road

USDOT U.S. Department of Transportation

VSL value of a statistical life

1

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 crossmedian 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-themean, 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 

 

 

1The 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).
2

ratios to be about 8.28 for the treatment in Illinois and Kentucky and 4.14 for the treatment in 

Missouri.
3

CHAPTER 1. INTRODUCTION

This chapter presents background information on the strategy of using cable median barriers and 

shoulder rumble strips, the goals of the study, and a review of the existing literature on the safety 

effects of cable median barriers, shoulder rumble strips, and the combination of the two 

treatments.

BACKGROUND ON STRATEGY

The United States began using cable median barriers in the 1960s as a treatment for preventing 

cross-median crashes. Initially, the design of the cable barriers was a low-tension type. Later, it 

became more common for States to use high-tension cable barriers. Villwock et al. reported the 

primary advantage of high-tension cable barrier as the ability to withstand multiple hits before 

requiring repair.(1) Many States have begun to use cable median barriers in place of more rigid 

(e.g., concrete) barriers. They have cited the following reasons:

• Contain vehicles instead of redirecting them into traffic; up to 95 percent of vehicles are 

contained.

• Are less expensive to install and maintain except for the danger of snow plow damage.

• Eliminate snow accumulation.

• Are environmentally non-intrusive and aesthetically pleasing and blend in with the 

terrain.

Although there are significant advantages to cable barriers, States have cited some disadvantages, 

including the following:

• Multiple studies indicate an increase in crash frequency. One study reported an increase in 

the injury crash rate, although the period for analysis was limited to 16 months. 

• Penetrating the barrier is possible by under-riding or over-riding. The identified causes of 

under-riding include location of the barrier on a slope and having too large an opening 

between the cable and the ground caused by the presence of a ditch. One of the causes of 

over-riding is hitting a post. 

• There are conflicting results regarding whether a cable barriers can stop a heavy truck. 

• Considerable deflection upon impact prohibits the use of cable barrier in narrow medians. 

This concern is particularly justified on curves, where the deflection can be larger than on 

straight segments.(1)

The purpose of introducing shoulder rumble strips is to reduce the frequency of run-off-road 

(ROR) crashes. Although agencies have conducted research into the performance of cable median 

barriers and shoulder rumble strips (specifically, rumble strips on the outside shoulder) 
4

separately, very few studies have looked at the combination of cable median barriers with inside 

shoulder rumble strips.

BACKGROUND ON STUDY

In 1997, the American Association of State Highway and Transportation Officials Standing 

Committee on Highway Traffic Safety—with the assistance of FHWA, the National Highway 

Traffic Safety Administration, and the Transportation Research Board Committee on 

Transportation Safety Management—met with safety experts in the field of driver, vehicle, and 

highway issues from various organizations to develop a strategic plan for highway safety. These 

participants developed 22 key areas that affected highway safety. 

The National Cooperative Highway Research Program (NCHRP) then published a series of 

guides to advance the implementation of countermeasures targeted to reduce crashes and injuries. 

Each guide addresses 1 of the 22 emphasis areas and includes an introduction to the problem, a 

list of objectives for improving safety in that emphasis area, and strategies for each objective. 

Each strategy is designated as proven, tried, or experimental. Many of the strategies discussed in 

these guides have not been rigorously evaluated; approximately 80 percent of the strategies are 

considered tried or experimental.

FHWA organized a pooled fund study of 40 States to evaluate low-cost safety strategies as part 

of this strategic highway safety effort. The purpose of the FHWA Evaluation of Low-Cost Safety 

Improvements Pooled Fund Study (ELSCI-PFS) is to evaluate the safety effectiveness of several 

tried and experimental, low-cost safety strategies through scientifically rigorous crash-based 

studies. The ELSCI-PFS selected the use of cable median barriers in combination with rumble 

strips on the inside shoulder on divided roads as a strategy to evaluate as part of this effort. 

LITERATURE REVIEW

This review is summarized in three sections: (1) safety effects of cable median barriers, (2) safety 

effects of shoulder rumble strips, and (3) safety effects of cable median barriers along with 

shoulder rumble strips.

Safety Effects of Cable Median Barriers

Villwock, Blond, and Tarko studied the effects of cable barriers on rural interstate highways.

(1)

They used data from eight States that had installed both low- and high-tension cable median 

barriers. Their data included 70 mi of treated roadway and 360 mi of control roadway (i.e., 

sections that did not receive cable barriers). They used a combination of before–after EB analysis 

with negative binomial and logistic regression to determine the effect on three types of crashes—

single-vehicle, multiple-vehicle same-direction, and multiple-vehicle opposite-direction (crossmedian or crossover crashes). Their results showed that the cable barriers affected the crash types 

differently. The overall crash modification factor (CMF) for single-vehicle crashes was 1.83, 

suggesting an increase in this crash type. The CMF for multiple-vehicle opposite-direction 

crashes was 0.06, suggesting an effective reduction of this crash type. They did not see a 

significant effect on multiple-vehicle same-direction crashes.(1)
5

Cooner et al. evaluated installations of cable median barriers in Texas.(2) Cross-median crashes 

resulted in a substantial percentage of interstate highway fatalities, which led to widespread 

installation of cable barriers. This research team had difficulty conducting a rigorous safety 

evaluation of the cable barrier; owing to a changeover in the management of crash data, the data 

were unavailable for the years preceding and following the installation. They relied on a simple 

before–after examination by the Texas Department of Transportation Traffic Operations Division 

of 407 mi that received cable barriers. This examination, which did not account for the potential 

effects due to regression-to-the-mean, examined at 1 year before and 1 year after installation at 

the treated sites only. They observed that head-on fatal crashes decreased from 14 to 1 during the 

study period.(2)

Chandler reported on the benefits of cable median barriers in Missouri.(3) In 2002, the Missouri 

Department of Transportation (MoDOT) began to install cable barriers on freeways with median 

widths of less than 60 ft, focusing on two main interstate highways. From 2002 to 2006, a total of 

179 mi of cable barriers were installed, with most of the installations occurring in 2005. From an 

examination of fatalities on one interstate highway, Chandler concluded that the installation of 

179 mi of cable median on freeways nearly eliminated cross-median fatalities. Figure 1 and 

Figure 2 show a comparison between miles of cable barriers installed and cross-median fatalities. 

The cross-median fatalities decreased from a high of 24 per year to 2 per year by 2006, a decrease 

of 92 percent. Chandler concluded that cable median barriers were an effective safety tool in 

Missouri.(3)

Figure 1. Graph. Cross-median fatalities in Missouri.

(3)

0

5

10

15

20

25

30

1999 2000 2001 2002 2003 2004 2005 2006

Fatalities

Cross-Median Fatalities
6

Figure 2. Graph. Miles of cable barrier installation in Missouri.(3)

Hunter et al. evaluated the safety of three-strand cable median barriers using data from a section 

of I-40 outside Raleigh, NC.

(4) The research team used data from 1990 to 1997 to conduct the 

analysis. The treatment population was the 8.5-mi section of Interstate 40, and the reference 

population was identified as the entire North Carolina Interstate System (except for those sections 

treated with cable median barriers). The research team also identified a secondary reference 

population to account for discrepancies in traffic volumes between the entire reference population 

and the treatment segment. They identified this subpopulation as a subset of sites where the 

annual average daily traffic (AADT) exceeded 50,000 vehicles per day. They developed several 

regression models, assuming a negative binomial error structure for many of the models, to 

estimate the effects of the installation of the cable median barriers. The models accounted for 

year-to-year variations (e.g., AADT and weather conditions).

Results showed that the installation of the three-strand cable median barriers was followed by a 

reduction in fatal and serious injury crashes. ROR left, hit-fixed-object, and rear-end crashes 

increased with the installation of the cable median barriers. The models predicted that ROR headon crashes (i.e., the primary target for the treatment) would decrease with installation of the cable 

median barriers. However, the research team was not able to confirm this hypothesis because this 

particular crash type was very rare.(4)

Milton and Albin reported on the experience with using cable median barriers in Washington 

State.(5) The authors calculated the total societal benefit of cable barriers as $420,000/mi 

annually. Their examination of the Washington State Department of Transportation bid history 

indicated a cost of cable median barriers at $44,000/mi, with an annual maintenance cost of 

$2,750/mi.

(5)

Safety Effects of Shoulder Rumble Strips

Griffith studied the safety effects of rumble strips on freeway shoulders in California and 

Illinois.(6) These States installed shoulder rumble strips on the inside and outside shoulders of the 

0

20

40

60

80

100

120

140

160

180

200

1999 2000 2001 2002 2003 2004 2005 2006

Miles

Miles of Cable Barrier Installation
7

highway in both directions. Griffith examined crash data for 280 mi of freeway in Illinois and 

120 mi of freeway in California with rumble strip installations. The author conducted a before–

after analysis to determine the effect on crashes. Because the author selected the installation sites 

based on their listing in the resurfacing schedule rather than based on high crash frequency, there 

was no need to account for regression-to-the-mean bias. Thus, Griffith used a before–after design 

with a comparison group to account for trends and found that shoulder rumble strips reduced 

single-vehicle run-off-road (SVROR) crashes by 18.3 percent on all freeways and 21.1 percent on 

rural freeways. The author did not conduct a separate analysis to determine the effect of installing 

rumble strips on only the inside shoulder.(6)

Torbic at al. conducted one of the most recent studies on rumble strips under NCHRP Project 

17-32.

(7) The authors of the report included a review of prior research on the safety impacts of 

shoulder rumble strips. Table 1 shows an adaptation of table 4 of their report, which summarizes 

20 studies that calculated the effect of shoulder rumble strips on crashes (including the study by 

Griffith previously discussed).(6,7) Although there was no specific breakout of inside versus 

outside shoulder rumble strip effects, most studies in the list used roadway facility types that 

were divided (i.e., interstate highways, freeways and other divided highways). Most studies used 

SVROR crashes as the target crash type. The most common study design was before–after, 

including naive before–after and before–after with a comparison group.
8

Table 1. Summarized results of studies that calculated the effect of shoulder rumble strips on crashes (adapted from Torbic 

et al.).

(7)

State Type of Facility

Type of Collision 

Targeted

Percent Change in 

Target Collision 

Frequency from 

Application of SRS

(Standard 

Deviation) Type of Analysis

Arizona Interstate SVROR −80 Cross-sectional comparison

California Interstate SVROR −49 Before–after with comparison sites

California Interstate Total −19 Before–after with comparison sites

Connecticut Limited-access 

roadways

SVROR −32 Before–after with comparison sites

Florida Not provided Fixed object −41 Naive before–after

Florida Not provided Ran-into-water −31 Naive before–after

Illinois and 

California

Freeways SVROR (total) −18 (± 6.8) Before–after with marked comparison sites and a 

comparison group

Illinois and 

California

Freeways SVROR (injury) −13 (± 11.7) Before–after with marked comparison sites and a 

comparison group

Illinois and 

California

Rural freeways SVROR (total) −21.1 (± 10.2) Before–after with marked comparison sites and a 

comparison group

Illinois and 

California

Rural freeways SVROR (injury) −7.3 (±15.5) Before–after with marked comparison sites and a 

comparison group

Kansas Freeways SVROR −34 Unknown

Maine Rural freeways Total Inconclusive Before–after with comparison sites

Massachusetts Not provided SVROR −42 Unknown

Michigan Not provided SVROR −39 Cross-sectional comparison

Minnesota Rural multilane 

divided highways

Total −16 Naive before–after

Minnesota Rural multilane 

divided highways

Injury −17 Naive before–after

8
9

State Type of Facility

Type of Collision 

Targeted

Percent Change in 

Target Collision 

Frequency from 

Application of SRS

(Standard 

Deviation) Type of Analysis

Minnesota Rural multilane 

divided highways

SVROR (total) −10 Naive before–after

Minnesota Rural multilane 

divided highways

SVROR (injury) −22 Naive before–after

Minnesota Rural multilane 

divided highways

Total −21 Before–after with comparison sites

Minnesota Rural multilane 

divided highways

Injury −26 Before–after with comparison sites

Minnesota Rural multilane 

divided highways

SVROR (total) −22 Before–after with comparison sites

Minnesota Rural multilane 

divided highways

SVROR (injury) −51 Before–after with comparison sites

Minnesota Rural two-lane roads SVROR (total) −13 (8) Before–after EB analysis with reference group

Minnesota Rural two-lane roads SVROR (injury) −18 (12) Before–after EB analysis with reference group

Montana Interstate and primary 

highways

SVROR −14 Before–after with comparison sites

New Jersey Not provided SVROR −34 Unknown

New York Interstate parkway SVROR −65 to 70 Naive before–after

Pennsylvania Interstate SVROR −60 Naive before–after

Tennessee Interstate SVROR −31 Unknown

Utah Interstate SVROR −27 Before–after with comparison sites

Utah Interstate Total −33 Before–after with comparison sites

Virginia Rural freeways SVROR −52 Before–after with comparison sites

Washington Not provided Total −18 Naive before–after

Multiple Rural freeways SVROR −20 Before–after with comparison sites

Note: For information on the original sources, please see Torbic et al.

(7)

SRS = Shoulder rumble strips.

9
10

Torbic et al. also conducted a survey on the rumble strip installation practices of transportation 

agencies. The results indicated that 93 percent of agencies installed shoulder rumble strips on both 

the inside and outside shoulder when installing on a median-divided roadway. Thirty-five percent 

of agencies indicated that they used different policies for inside shoulders versus outside shoulders. 

These policies included smaller offsets on the inside (median) shoulder and continuous rumble 

strips on the inside (median) shoulder compared with intermittent gaps on the outside shoulder.(7)

Torbic et al. conducted an evaluation of the safety effect of shoulder rumble strips according to 

roadway type, crash severity, placement of the rumble strip (i.e., on edge line versus not on edge

line), and offset distance. The analysis did not attempt to separate the effects of inside shoulder 

versus outside shoulder rumble strips. When developing the dataset, the authors assumed the 

rumble strips on the inside shoulder were installed at the same time as rumble strips on the 

outside shoulder. Although SVROR crashes were a target crash type, the analysis of SVROR 

crashes did not distinguish between those to the left and right of the road.(7)

Torbic et al. conducted the safety evaluation using both the EB before–after study method and a 

generalized linear modeling cross-sectional approach. Ultimately, they used the EB results 

because EB was the preferred analysis method in resources such as the Highway Safety 

Manual.

(8) They combined the results of their safety evaluation with two other reliable studies to 

create CMFs for shoulder rumble strips. For rural freeways, the authors estimated that shoulder 

rumble strips led to an 11-percent reduction in SVROR crashes (standard error (SE) 6) and a 

16-percent reduction in SVROR fatal and injury crashes (SE 8). Their analysis did not indicate 

whether the sites that were treated with shoulder rumble strips had cable median barriers.

(7)

Safety Effects of Cable Median Barriers with Shoulder Rumble Strips

Monsere et al. investigated the effects of more than 21.9 mi of continuous three-strand cable

median barriers installed on Interstate 5 in Oregon in December 1996 (approximately 7 mi) and 

April 1998 (approximately 18 mi).(9)In addition, the Oregon Department of Transportation 

installed milled shoulder rumble strips in fall 1998 in the same area. The crash analysis used two 

primary datasets: (1) reported crashes (recorded in the State’s computerized crash record system) 

and (2) maintenance logs that documented cable barrier strikes.

For the analysis using State-reported crash data, the authors could identify no suitable reference 

group, so a simple before–after study was conducted. They used 3 years of before and after data 

(with the exception of the transition period identified as January 1997 through April 1998). 

Results showed a decrease in fatal and serious injury cross-median crashes between the before 

and after time periods—three crashes in the before period and none in the after period and an 

increase in reported crashes striking a barrier. Although the analysis could not identify left and 

right side separately, there were 7 crashes in the before period and 60 in the after period. For the 

analysis using maintenance logs, the authors estimated the effectiveness of the barriers by 

examining the type of damage the cable median barriers sustained during the crash. Results 

showed that 105 potential crossovers (of 231 barrier impacts) were contained by the barriers

between December 1996 and April 2002.(9)
11

The authors acknowledged that the State transportation department installed milled shoulder 

rumble strips around the same time as the second cable median barrier installation, which could 

have contributed to some of the results from the analysis.

(9)

Summary of Literature Review

Overall, the studies seemed to indicate a reduction in cross-median crashes and an increase in 

crashes involving the striking of a barrier following the installation of cable median barriers. 

Regarding the effect of shoulder rumble strips, it was evident that they were effective in reducing

SVROR crashes. The study by Monsere et al. was the only one that looked at the combination of 

cable barriers and shoulder rumble strips.

(9) However, that study was a simple before–after 

evaluation and therefore did not account for possible bias due to regression-to-the-mean and the 

effect of changes in traffic volume. In addition, the sample of crashes was much too small to 

obtain robust results.(9)

13

CHAPTER 2. OBJECTIVE

The research for this study examined the safety impacts of cable median barriers with inside 

shoulder rumble strips in Illinois, Kentucky, and Missouri. The objective of the study was to 

estimate the safety effectiveness of this strategy as measured by crash frequency. The primary 

target crash type was cross-median crashes, and the analysis excluded intersection-related and 

animal crashes.

In addition to cross-median crashes, the research team considered total injury and fatal, medianrelated, ROR left side, and winter-related crashes. Median-related crashes are those in which the 

vehicle does not cross the median but ends up in the median after a rollover and/or a collision 

with an object. Based on the available variables in the crash databases from the States, the 

research team was not able to determine whether a crash was median-related. Similarly, the 

research team could not determine run-off-left-side crashes from the crash reports. Consequently, 

the evaluation did not specifically examine these crash types. The evaluation examined winterrelated crashes, but because the sample of crashes was very small, the research team did not 

report the results here.

It is important to note that the treatment itself was not exactly the same in the three evaluated

States. In Illinois and Kentucky, the introduction of cable median barriers came many years after 

the introduction of rumble strips on inside shoulders. Conversely, Missouri installed cable 

median barriers and inside shoulder rumble strips about the same time. The before–after 

conditions in the three States can be summarized as follows:

• Illinois and Kentucky. The before-after conditions were as follows:

o Before condition: Inside shoulder rumble strips were present, but no cable median 

barriers were present.

o After condition: Both inside shoulder rumble strips and cable median barriers were 

present.

• Missouri. The before-after conditions were as follows:

o Before condition: No inside shoulder rumble strips or cable median barriers were 

present.

o After condition: Both inside shoulder rumble strips and cable median barriers were 

present.

In addition to determining the overall safety effect of the treatment(s), a further objective was to 

address the following questions:

• Do effects vary by level of traffic volume?

• Do effects vary by the frequency of crashes before treatment?

• Do effects vary by speed limit?

• Do effects vary by lane width and shoulder width?
14

The evaluation of overall effectiveness included consideration of the installation costs and crash 

savings in terms of the B/C ratio. 

Meeting these objectives placed some special requirements on the data collection and analysis 

tasks, including the need to do the following:

• Select a large enough sample size to detect—with statistical significance—what may be 

small changes in safety for some crash types.

• Identify appropriate untreated reference sites.

• Properly account for changes in safety due to changes in traffic volume and other 

nontreatment factors.

• Pool data from multiple jurisdictions to improve reliability of the results and facilitate 

broader applicability of the products of the research.
15

CHAPTER 3. STUDY DESIGN

When planning a before–after safety evaluation study, it is vital to ensure that enough data are 

included to detect the expected change in safety with statistical confidence. Even though those 

designing the study do not know the expected change in safety in the planning stage, it is still 

possible to make a rough determination of how many sites are required based on the best 

available information about the expected change in safety. Alternatively, one could estimate the 

statistically detectable change in safety for the number of available sites. For a detailed 

explanation of sample size considerations, as well as estimation methods, see chapter 9 of 

Hauer.

(10) The sample size analysis cases presented in this report address (1) how large a sample 

would be required to statistically detect an expected change in safety and (2) the change in safety 

that could be detected with available sample sizes.

CASE 1: SAMPLE SIZE REQUIRED TO DETECT AN EXPECTED CHANGE IN

SAFETY

For this analysis, the research team assumed that a conventional before–after study with 

comparison group design would be used because available sample size estimation methods were

based on this assumption. The sample size estimates from this method would be conservative in 

that the EB methodology proposed would likely require fewer sites. To facilitate the analysis, the 

research team also assumed that the number of comparison sites was equal to the number of 

treatment sites, which again, was a conservative assumption.

As discussed earlier in the literature review, the crash types that cable median barriers and rumble 

strips on the inside shoulder would most affect would be cross-median crashes. For the study 

design, the research team assumed that crashes that were coded as head-on and opposite-direction 

sideswipe were cross-median. The research team used crash rates from the reference groups for 

total crashes and injury and fatal crashes; they used crash rates from the treatment group after 

reducing by 25 percent for cross-median crashes. (The team chose this reduction percentage 

based on the work by Bahar, which indicated that possible bias due to regression-to-the-mean 

was not likely to be higher than 25 percent.)

(11) Table 2 provides the crash rate assumptions.

Table 2. Before-period crash rate assumptions.

Crash Type

Kentucky 

(A)

Missouri 

(B)

Illinois

(C)

Total 8.25 1.91 6.95

Injury and fatal 1.90 0.62 1.53

Cross-median 0.27 0.03 0.07

Note: All crash rates are mi/year.

Table 3 provides estimates of the required number of before- and after-period mile-years for

statistical significance at both a 90- and 95-percent confidence level for both crash rate 

assumptions. The minimum sample indicates the level at which a study would seem to be

worthwhile; that is, it would be feasible to detect with the desired level of confidence the largest 

effect that might reasonably be expected based on what was currently known about the strategy. 

The research team based these sample size calculations on the methodology in Hauer and on 
16

specific assumptions regarding the number of crashes per mile and years of available data.(10)

Mile-years are the number of miles of highway on which the strategy was implemented

multiplied by the number of years of data before or after implementation. For example, if a 

strategy was implemented at a 10-mi segment and data were available so far for 4 years since 

implementation, then there would be a total of 40 mi-year of after-period data available for the 

study.

Table 3. Minimum required before-period mile-years.

Expected 

Percent 

Reduction 

in 

Crashes A† B† C† A‡ B‡ C‡

All 10 153* 658* 181* 108* 465* 128*

All 20 32 136 37 22 97 27

All 30 11 49 13 8 34 10

All 40 5 21 6 4 15 4

Injury and 

Fatal 10

661 2033 825 467 1436 583

Injury and 

Fatal 20

136* 419* 170* 97* 298* 121*

Injury and 

Fatal 30

49 150 61 35 106 43

Injury and 

Fatal40

22 66 27 15 47 19

Crossmedian 104,666 45,737 18,516 3,296 32,306 13,079

Cross 

Median 20

963 9,438 3,821 685 6,715 2,719

Cross 

Median 30

344 3,376 1,367 244 2,396 970

Cross 

Median 40

152* 1,488* 603* 107* 1,053* 426*

MedianRelated 101,131 4,875 1,379 799 3,443 974

MedianRelated 20233 1,006 285 166 716 203

MedianRelated 3083 360 102 59 255 72

MedianRelated 4037 159 45 26 112 32

Note: Assumes equal number of mile-years for treatment and comparison sites and equal length of before and after 

periods.

†95-percent confidence level.

‡90-percent confidence level.

*Recommended sample sizes in this study.
17

The sample size values recommended in this study are indicated with an asterisk in table 3. These 

values are recommended based on the likeliness of obtaining the estimated sample size as well as 

the anticipated effects of the treatment. As noted, the sample size estimates provided were

conservative in that the state-of-the-art EB methodology proposed for the evaluations would 

require fewer sites than the less robust conventional before–after study with a comparison group

that had to be assumed for the calculations. Estimates can be predicted with greater confidence,

or a smaller reduction in crashes would be detectable, if there were more site-years of data 

available in the after period. The same holds true if the actual data used for the analysis had a 

higher crash rate for the before period than was assumed.

CASE 2: CHANGE IN SAFETY THAT CAN BE DETECTED WITH AVAILABLE 

SAMPLE SIZES

The standard deviations of the estimated percent change in safety describe the statistical accuracy 

attainable for a given sample size. From this, one can estimate p-values for various sample sizes 

and expected change in safety for a given crash history based on the method in Hauer.(10)

For the available data in the three States in this evaluation, the research team estimated the 

minimum percentage changes in crash frequency that could be statistically detectable at 5- and 

10-percent significance levels, as shown in table 4. For these calculations, the research team

assumed that data would be available until the end of 2012. The results indicated that the data 

should be able to detect the recommended crash reduction values from table 3 if such an effect 

were present. Using these results, the authors made a decision to proceed with the evaluation 

using the data available at that time.

Table 4. Sample analysis for crash effects.

Crash Type

Mile-Years

in Before 

Period

Mile-Years

in After Period

Minimum 

Percent

Reduction

Detectable for 

Crash Rate

Assumption*

p = 0.10

Minimum 

Percent

Reduction

Detectable for 

Crash Rate

Assumption*

p = 0.05

Total 2,512 4,249 3 4

Injury and fatal 2,512 4,249 6 7

Cross-median 2,512 4,249 16 19

Note: Results are to nearest 1 percent.

*Crash rate assumption based on crash rates in table 3.

19

CHAPTER 4. METHODOLOGY

This evaluation uses the EB methodology for observational before–after studies.

(10) This 

methodology is considered rigorous in that it accounts for regression-to-the-mean using a 

reference group of similar but untreated sites. In the process, the research team used safety 

performance functions (SPFs) for the following purposes:

• To overcome the difficulties of using crash rates in normalizing for volume differences 

between the before and after periods.

• To account for time trends.

• To reduce the level of uncertainty in the estimates of safety effects.

• To properly account for differences in crash experience and reporting practice in 

amalgamating data and results from diverse jurisdictions.

The methodology also provides a foundation for developing guidelines for estimating the likely 

safety consequences of a contemplated strategy.

In the EB approach, the change in safety for a given crash type at a site is given by figure 3.

Figure 3. Equation. Estimated change in safety.

Where:

= Expected number of crashes that would have occurred in the after period without the 

strategy.

= Number of reported crashes in the after period. 

In estimating , the authors used SPFs to explicitly account for the effects of regression-to-themean and changes in traffic volume, relating crashes of different types to traffic flow and other 

relevant factors for each jurisdiction based on untreated sites (reference sites). They calibrated 

annual SPF multipliers to account for temporal effects on safety (e.g., variation in weather, 

demography, and crash reporting).

In the EB procedure, the first step was to use the SPF to estimate the number of crashes that 

would be predicted in each year of the before period at locations with traffic volumes and other 

characteristics similar to the one being analyzed (i.e., reference sites). The sum of these annual 

SPF estimates (P) was then combined with the count of crashes (x) in the before period at a 

strategy site to obtain an estimate of the predicted number of crashes (m) before strategy. Figure 4

shows this estimate of m.

 Safety = λ - π

π
20

Figure 4. Equation. EB estimate of expected crashes.

Where w, the EB weight, is estimated from the mean and variance of the SPF estimate as figure 5

illustrates.

Figure 5. Equation. EB weight.

Where k is the overdispersion parameter of a negative binomial regression model, which was

estimated from the SPF calibration process with the use of a maximum likelihood procedure. k

could be assumed as a constant or as a function of site characteristics, including segment length. 

Based on the recommendation from Hauer, k was estimated based on segment length and 

assumed to be , where k1 is the overdispersion parameter for a 1-mi segment and l is the length 

of the segment.(12)

A factor was then applied to m to account for the length of the after period and differences in 

traffic volumes between the before and after periods. This factor was the sum of the annual SPF 

predictions for the after period divided by P, the sum of these predictions for the before period. 

The result, after applying this factor, was an estimate of . The procedure also produced an 

estimate of the variance of .

The estimate of was then summed over all sites in a strategy group of interest (to obtain sum) 

and compared with the count of crashes observed during the after period in that group ( sum). The 

variance of was also summed over all sites in the strategy group. 

The index of effectiveness ( ) is estimated as shown in figure 6.

Figure 6. Equation. Index of effectiveness.

Figure 7 shows how the standard deviation of is calculated.

Figure 7. Equation. Standard deviation of index of effectiveness. 

m = w(P)+(1-w)(x)

kP

w





1

1

l

k1

θ

















2

( )

1

sum

sum

sum

sum

Var











θ

2

2

2 2

2

( )

1

( ) ( )

( )







































sum

sum

sum

sum

sum

sum

Var

Var Var

StDev
















21

The percent change in crashes was calculated as 100(1  ); thus, a value of  0.7 with a 

standard deviation of 0.12 indicates a 30-percent reduction in crashes with a standard deviation of 

12 percent.

The analysis of the treatment sites in Missouri required a slightly different approach to the 

methodology. Missouri installed cable median barriers on a systemwide basis for certain road 

types. As a result, it was virtually impossible to identify comparable roadways without cable 

barriers for this road type presently or in the near future. For this reason, the research team did 

not identify a separate reference group of sites without rumble strips.

The research team applied an alternate approach to the standard method of estimating and 

applying SPFs for the EB before–after methodology. In short, this method used the before-period 

data at the treatment sites to develop SPFs to control for regression-to-the-mean and traffic 

volume changes. Because the State applied its policy of installation of cable barriers systemwide, 

regression-to-the-mean was not as big a concern as it otherwise might have been. The research 

team used SPFs calibrated from before-period data to account for time trends in the earlier part of 

the study period, before most of the sites had rumble strips installed. However, after a substantial 

number of sites had been treated, the number of sites was still too low to develop yearly factors to 

account for trends. Instead, the research team used time trends from the Missouri data that were 

used in the rural two-lane centerline plus shoulder rumble strips analysis to calculate the afterperiod trend when MoDOT had treated most or all of the sites.(13) The research team adjusted the 

before-period yearly factors based on the ratio of the after-period factors to common years in the 

rural two-lane data.

To illustrate, consider the fictitious information in table 5. Using the SPFs calibrated for both the 

before and after periods, annual multipliers were estimated for each year. In 2006, there were no 

data for the after period, so a multiplier did not exist for that year for the after-period SPF. 

Similarly, there was no multiplier for 2009–2011 using the before-period data. The average of the 

multipliers for the common years (2007–2008) was computed. The after-period multipliers post2007 were adjusted by dividing the values by the 2007–2008 average. Finally, the missing yearly 

multipliers for the before-period model were adjusted by multiplying the average from 2007–

2008 (1.03) by the value of the adjusted after-period multiplier for each year. These were the 

annual multipliers used in the evaluation.

θ θ
22

Table 5. Illustration of alternate approach.

Year

Using AfterPeriod Data

Adjusted

After-Period 

Multipliers

Using BeforePeriod Data

Adjusted

Before-Period 

Multipliers

2006 N/A — 0.98 —

2007 1.17 — 1.01 —

2008 0.99 — 1.05 —

Average 2007–

2008

1.08 — 1.03 —

2009 1.23 1.14 N/A 1.17

2010 0.84 0.78 N/A 0.80

2011 1.96 1.81 N/A 1.86

— Indicates no adjustment was required.

N/A = Not applicable.
23

CHAPTER 5. DATA COLLECTION

As mentioned earlier, Illinois, Kentucky, and Missouri provided data for this study. These States 

also provided data on roadway geometry, traffic volumes, and crashes for both installation and 

reference sites. They also provided crash injury severities according to the KABCO scale. This 

chapter summarizes the data assembled for the analysis.

ILLINOIS

This section describes the installation data, reference sites, roadway data, traffic data, crash data, 

and treatment cost data for Illinois sites used in this evaluation.

Installation Data

The Illinois Department of Transportation (IDOT) provided a list of interstate highway sections 

where it had installed cable median barriers, along with the date of installation, brand of barrier, 

and cost of contract. IDOT installed cable median barriers at these locations to reduce crossmedian crashes. The agency based its decisions on an examination of 5 years of crash data. 

Illinois used Fatality Analysis Reporting System (FARS) crashes on interstate highways and 

looked at head-on and opposite-direction sideswipe crashes. IDOT used this information to 

develop a warrant chart that could be used for cable median barrier installation. The final set of 

cable median barrier treatment sites covered approximately 100 mi.

IDOT did not install rumble strips at the same time as the cable median barriers. Thus, the 

research team wanted to know the exact time period when rumble strips were installed on the 

treatment sites. To obtain this information, IDOT ran a query of its contracts database to 

assemble a list of all contracts involving the installation of rumble strips. Owing to a lack of 

complete data from previous years, the research team was not able to determine a date at which 

IDOT first installed rumble strips on the treatment sites. However, the query did enable the 

research team to document any work involving rumble strips that IDOT performed on the 

treatment sites during the study period. If any record of work appeared for a site, the research 

team documented it along with the year it was done. 

Although IDOT did not know the exact date of rumble strip installation at these sites, the 

documentation provided an indication of the earliest known year of rumble strip presence based 

on a visual inspection of 2004 photolog images. This provided the starting point for the analysis 

period. 

Reference Sites

Reference sites for the cable median barrier treatment needed to be segments of road that 

matched the treatment sites in terms of the type of roadway, the potential for cable median 

barriers (appropriateness of median), and existence of shoulder rumble strips during the before 

period of study. The research team identified reference sites for Illinois from an IDOT list of all 

interstate highway segments in the State known to have shoulder rumble strips based on a visual

survey of 2004 photolog images. From this list, the research team selected interstate highway 

segments if they were on the same route as other segments that had been treated with cable 
24

median barriers but that had not received cable median barriers. This ensured that the reference 

sites would be similar to the treatment sites in terms of driver population, area type, and other 

factors. The research team selected only segments with a median width of less than 100 ft and no 

positive barriers. Once the team had assembled this list, they compared it with the query of 

rumble strip work that IDOT had developed based on a pay item query in its contracts database. 

The research team documented any work done during the study period involving rumble strips,

along with the year the work was done. 

This yielded a group of interstate highway segments with rumble strips, medians no wider than 

100 ft, and no positive barriers in the median and that were located along the same routes as the 

treatment sites. The final batch of reference sites comprised approximately 400 mi of road 

segments.

Roadway Data

The research team obtained roadway data for all treatment and reference sites from the FHWA 

Highway Safety Information System (HSIS) database. HSIS provided these data on an annual 

basis for each site from 2001 to 2010. The HSIS Guidebook for Illinois provided coding for these 

variables. (14)

The HSIS database yielded the following variables:

• Median Width.

• Total Surface Width.

• Total Number of Lanes.

• Average Lane Width.

• Outside Shoulder Type.

• Outside Shoulder Width.

• Inside Shoulder Type.

• Inside Shoulder Width.

• AADT.

• Functional Class.

• Speed Limit.

• Surface Type.

• Total Inside Shoulder Width.

• Total Outside Shoulder Width.

• Roadway Classification.

Traffic Data

The research team obtained traffic data for all treatment and reference sites as part of the roadway 

data file from the FHWA HSIS database. HSIS provided an AADT value for each site annually 

from 2001 to 2010.
25

Crash Data

The research team obtained crash data for all treatment and reference sites from the FHWA HSIS 

database. The crash data obtained covered the years 2001 to 2010. The HSIS crash data provided

location of crash, date of crash, severity, light condition at time of crash, crash type, and weather. 

The HSIS Guidebook for Illinois provides coding for the crash variables. The crash data can be 

linked to the treatment and reference sites by county and route (coded as CNTYRTE in HSIS 

data) and milepost.(14)

Treatment Cost Data

The research team obtained the cost of installing and maintaining cable median barriers and 

rumble strips from IDOT and summarized these data in table 6. 

Table 6. Illinois treatment cost and service life data.

Countermeasure

Initial Installation 

Cost Maintenance Cost

Service Life

(year)

Cable median barriers $180,000/mi $6,000–$12,000/mi 

per year

15 

Rumble strips on new 

surface

$2,000/mi No estimate provided 12 

Rumble strips on 

existing surface

$2,000/mi No estimate provided 8 

KENTUCKY

This section describes the installation data, reference sites, roadway data, traffic data, crash data, 

and treatment cost data for Kentucky sites used in this evaluation.

Installation Data

The Kentucky Transportation Cabinet (KYTC) provided a list of roadway sections where it had 

installed cable median barriers, along with the date of installation, brand of barrier, and cost of 

contract. KYTC generally selected sites for cable median barrier installations based on an annual 

evaluation process that considered crash experience, median width, median slope profile, traffic 

volume, traffic composition, and speed. From these variables, KYTC prioritized a list of 

recommended cable barrier projects. KYTC quantified crashes by examining cross-median fatal 

crashes per mile and total number of cross-median crashes per mile. The initial treatment group 

added up to 140 mi. 

KYTC revealed problems with crash reporting in Jefferson County during the earlier years of the 

study period. Because the change in the reporting practices could have negatively affected any 

before–after analysis using Jefferson County crash data, the research team removed these data. 

Because Jefferson County is one of Kentucky’s largest counties and had a significant number of 

cable barrier installations, omitting it from consideration reduced the sample size to 69 mi.
26

The State did not install rumble strips at the same time as the cable median barriers, but the prior 

condition of all treated sites included existing rumble strips on both the outside and inside 

shoulders. Thus, the research team chose these sites for an analysis of the effect of cable median 

barrier installation on facilities already equipped with rumble strips.

Reference Sites

KYTC provided a list of interstate highway segments from the entire State. Each segment had an 

indication of median type and median barrier presence. The research team selected reference sites 

from this list based on their having a depressed median, no median barriers, and a median width 

less than or equal to 200 ft. (Very wide medians are at low risk of cross-median crashes even 

without barriers.) The research team also cross-referenced this list against the cable barrier 

treatment list to remove treated sections. The final list of reference sites for the cable barrier 

treatment was approximately 323 mi. KYTC indicated that all reference sites could be assumed to 

have had median shoulder rumble strips by 2000.

Roadway Data

The authors obtained roadway data in geographic information system (GIS) shapefile format 

from Kentucky staff. Separate shapefiles, each segmented differently, contained various road 

characteristics (e.g., shoulder width and traffic volume). The research team obtained GIS files 

from the KYTC website.(15) The research team obtained characteristics of the treatment and 

reference sites by matching each study site to the appropriate inventory segment by county, route, 

and milepost. The team also used the KYTC photolog and Google® aerial and Google 

StreetviewTM imagery to confirm presence, type, and approximate installation dates of rumble 

strips at cable barrier treatment sites.

(16)

Because the sites in Kentucky included freeway sections, and crashes on freeways tend to 

concentrate near interchanges, the research team requested and obtained data on locations of 

interchanges from KYTC.

Traffic Data

KYTC maintained traffic volume data in the GIS inventory files, namely the traffic flow (TF)

shapefile. The authors obtained traffic data for the treatment and reference sites by matching each 

study site to the appropriate inventory segment by county, route, and milepost. Specifically, the 

research team used the TF file from 2010 because it provided two datapoints: current (i.e., 2010) 

AADT and the prior AADT (with an indication of the year taken). The research team used these 

volume points to extrapolate yearly AADT for the before period.

Crash Data

KYTC supplied crash data for the routes and counties indicated in the treatment and reference 

site lists. KYTC also provided a data dictionary that described the fields in the crash data. The 

crash data contained all crashes for the individual routes and could be linked to the sites with a 

match based on county, route, and beginning and ending milepost. The field labeled 

“RDWYIDTXT” was present in both the crash and road files to indicate the route. KYTC 

indicated that crash location quality improved significantly in 2008. This was because officers
27

used a mapping application, which allowed officers to select the crash location on a screen, which 

applied latitude/longitude coordinates to the crash record.

Treatment Cost Data

KYTC provided estimates of the costs and services lives of the treatments (see table 7). These 

data can be used to conduct a B/C analysis of the treatment.

Table 7. Kentucky treatment cost and service life data.

Countermeasure

Initial 

Installation Cost Maintenance Cost

Service Life

(years)

Cable median barriers $150,000/mi Difficult to determine 

because reporting system 

did not separate cable 

barriers from guardrail

20 

Edge-line or shoulder 

rumble strips (installed 

as part of resurfacing)

$2,500/mi No additional maintenance 

cost

12–15 for rumble 

strip, 2 for stripe

MISSOURI

This section describes the installation data, reference sites, roadway data, traffic data, crash data, 

and treatment cost data for Missouri sites used in this evaluation.

Installation Data

The State applied cable median barriers with inside shoulder rumble strip treatments statewide on

roadways identified as tier 1 or tier 2. (Tier 1 was the highest order of routes followed by tier 2.) 

MoDOT supplied the list of projects where center-line and shoulder rumble strips or cable 

median barriers were recently installed or planned to be installed. Among the data the reports 

provided were the location—including district, State route number, and milepost—and the 

construction dates. MoDOT reported no other construction activities on these road segments.

The research team identified treatment sites by looking for sites for which both before- and afterperiod data would be available. The dates of cable median barrier installation did not always 

match those for one or both sides of the rumble strip installation, but they were close to each 

other. The team considered before periods to be prior to the installation of both cable barriers and 

rumble strips and after periods to start after both treatments had been installed.

Reference Sites

Because Missouri had installed cable median barriers on a systemwide basis, it was almost 

impossible to identify comparable roadways without cable barriers for this road type at the time

or in the near future. For this reason, the research team did not identify a separate reference group 

of sites without rumble strips and applied an alternate approach to the standard method for 

estimating and applying SPFs in the EB before–after methodology. Chapter 4 of this report 

describes this approach in more detail.
28

Roadway Data

The research team obtained roadway data for the treatment sites from MoDOT, and the following 

variables were included in the data:

• Area type (urban/rural).

• Functional class.

• Divided versus undivided.

• Number of lanes.

• Lane width.

• Shoulder type.

• Shoulder width.

• Surface type.

• Speed limit.

The roadway, traffic, and crash data were stored in a bidirectional manner (i.e., there was a 

separate record for each direction of travel). MoDOT staff were able to match opposing 

directions of travel for each site. The research team limited the constructed database to one record 

per site and the geometric information taken from the primary direction of travel.

Traffic Data

The research team obtained traffic data in the form of AADT from 1999 to 2011 from MoDOT in 

electronic files for all treatment sites. 

Crash Data

The compiled crash data contain many variables related to the location, time, and characteristics 

of each crash. The crash types of interest included non-intersection- and non-animal-related 

crashes.

Treatment Cost Data

MoDOT provided estimates of the costs and service lives of the treatments (see table 8).

Table 8. Missouri cost and service life data.

Countermeasure

Initial Installation 

Cost Maintenance Cost

Service Life

(years)

Cable median barrier $100,000/mi Not available 20 

Edge-line or center-line 

rumble strip

$1,000/mi Not available 7 to 10 
29

DATA CHARACTERISTICS AND SUMMARY

Previous research indicated that cross-median crashes could be identified in many ways. In the 

crash reports from Kentucky and Missouri, there was a variable to indicate whether the crash was 

a cross-median crash. Illinois did not have such a variable. Staff from KYTC and the Kentucky 

Transportation Center indicated that the cross-median indicator in the Kentucky crash report was 

not reliable and suggested not using that variable. As a result, for Illinois and Kentucky, the 

research team used head-on plus opposite-direction sideswipe crashes as a proxy for crossmedian crashes. The research team removed crashes related to wrong-way driving based on the 

crash reports. For Missouri, the team used two proxies for cross-median crashes: one based only 

on the cross-median indicator and the other based on the total number of crashes where either the 

cross-median indicator was included or the crash was designated as head-on. (Missouri did not 

indicate whether a sideswipe crash was opposite-direction or same-direction.)

Table 9 and table 10 provide summary information for the data collected for the treatment sites. 

The information in these tables should not be used to make simple before–after comparisons of 

crashes per mile-year because they do not account for factors, other than the strategy, that might

cause a change in safety between the before and after periods. The research team made such 

comparisons properly with the EB analysis, as presented in chapter 7. Table 11 provides 

summary information for the reference site data. As discussed in chapter 4, the team used a 

different approach in Missouri where they could not find an appropriate reference group.
30

Table 9. Data summary for treatment sites in Illinois and Kentucky.

Variable Illinois Kentucky

Segment length (mi) 100.3 66.39

Mi-yr before 574.59 478.6

Mi-yr after 320.31 160.54

Outside shoulder width minimum (ft) 10 10

Outside shoulder width maximum (ft) 13 11

Outside shoulder width average (ft) 11.35 10.1

Inside shoulder width minimum (ft) 4 3

Inside shoulder width maximum (ft) 12 10

Inside shoulder width average (ft) 7.73 5.95

Median width minimum (ft) 32 40

Median width maximum (ft) 90 54

Median width average (ft) 53.85 47.95

AADT minimum before 14,960 28,148

AADT maximum before 68,283 85,501

AADT average before 35,498 41,684

AADT minimum after 15,300 29,399

AADT maximum after 75,908 73,055

AADT average after 38,213 42,289

Total crashes/mi-yr before 7.98 7.62

Total crashes/mi-yr after 9.61 9.04

Injury crashes (KABC)/mi-yr before 2.07 1.81

Injury crashes (KABC)/mi-yr after 2.27 2.27

Injury crashes (KAB)/mi-yr before 1.6 1.09

Injury crashes (KAB)/mi-yr after 1.37 1.2

Head-on + sideswipe opposite-direction/mi-yr before 0.09 0.21

Head-on + sideswipe opposite-direction/mi-yr after 0.02 0.11
31

Table 10. Data summary for treatment sites in Missouri.

Variable Minimum Maximum Sum Mean

Segment length (mi) 0 5.79 288.07 0.28

Mi-yr before 0.01 28.94 1,947.47 1.87

Mi-yr after 0 34.72 817.49 0.79

Outside shoulder width (ft) 4 12 10,072 9.69

Inside shoulder width (ft) 1 10 5,264 5.07

AADT before 6,005 52,059 N/A 24,164

AADT after 5,263 58,530 N/A 24,395

Total crashes/mi-yr before 0 125 3,019.68 2.91

Total crashes/mi-yr after 0 83.33 1,892.68 1.82

Injury crashes (KABC)/mi-yr before 0 66.67 997.51 0.96

Injury crashes (KABC)/mi-yr after 0 34.48 292.88 0.28

Injury crashes (KAB)/mi-yr before 0 20.41 235.78 0.23

Injury crashes (KAB)/mi-yr after 0 10 67.27 0.06

Cross-median crashes/mi-yr before 0 3.86 22.25 0.02

Cross-median crashes/mi-yr after 0 1.28 1.28 0

Cross-median + Head-on/mi-yr before 0 3.86 33.56 0.03

Cross-median + Head-on/mi-yr after 0 1.28 2.06 0

N/A = Not applicable.

Table 11. Data summary for reference sites.

Variable Illinois Kentucky

Segment length (mi) 401.02 323.47

Mi-yr 4,010.2 3,558.16

Outside_Shoulder_Width min (ft) 8 10

Outside_Shoulder_Width max (ft) 13 14

Outside_Shoulder_Width avg (ft) 11.09 10.39

Inside_Shoulder_Width (ft)_for_Divide (ft) min 4 2

Inside_Shoulder_Width (ft)_for_Divide (ft) max 10 14

Inside_Shoulder_Width (ft)_for_Divide (ft) avg 7.16 5.45

Median width min 40 30

Median width max 88 200

Median width avg 67.77 66.04

AADT min 9,200.6 12,158.64

AADT max 37,708.5 84,237.64

avg 21,204 35,383

Crashes/mi-yr 2.61 6.18

Injury crashes (KABC)/mi-yr 0.72 1.55

Injury crashes (KAB)/mi-yr 0.61 0.92

Head-On + sideswipe opposite-direction/mi/yr 0.03 0.18

33

CHAPTER 6. DEVELOPMENT OF SPFS

This chapter presents the SPFs developed for each State. The EB methodology uses SPFs to 

estimate the safety effectiveness of this strategy.(10) The research team used generalized linear 

modeling to estimate model coefficients assuming a negative binomial error distribution, which 

was consistent with the state of research in developing these models. Most previous studies had 

used the traditional power function as the default for AADT. In this effort, the team used the

hoerl function to provide more flexibility in the functional form for AADT.

(17) With the hoerl

function for AADT, the dependent variable (Y) is related to AADT as shown in figure 8.

Y = exp a1 + a2AADT + a3

ln AADT 

Y = e

a1 ea2AADTAADTa3

Figure 8. Equation. Functional form for AADT.

Where a1, a2, and a3 are parameters to be estimated. This allows the function for AADT to have a 

convex/concave shape with inflection points. The other variables were included in a loglinear/exponential form as shown in figure 9:

Y = exp[a1 + a2AADT + a3ln AADT + a4X4 + a5X5 + ⋯ anXn]

Figure 9. Equation. Functional form for SPFs.

Where X4 through Xn represent the other independent variables and a4 through an are parameters 

to be estimated. The equation included segment length as an offset. In specifying a negative 

binomial error structure, the dispersion parameter k was estimated iteratively from the model and 

the data. For a given dataset, smaller values of k indicate relatively better models. As discussed

earlier, k was estimated as a function of the segment length, and k1 (overdispersion for a 1-mi 

section) is shown in the tables in the following sections.

ILLINOIS AND KENTUCKY SPFs

The research team calibrated SPFs for each State separately using the reference sites from that

State. As discussed in chapter 5, the team developed the Missouri SPFs separately for the before 

and after periods at the treated sites. Table 12 and table 13 present the SPFs developed for Illinois 

and Kentucky, respectively.
34

Table 12. Illinois SPFs.

Parameter

Total Estimate

(SE)

KABC Estimate

(SE)

KAB Estimate

(SE)

Intercept −15.0452

(2.2366)

−7.9114

(0.7885)

−7.5992

(0.8107)

AADT/1,000 −0.0442

(0.0138)

—

(—)

—

(—)

In(AADT) 1.8019

(0.2564)

0.8541

(0.0768)

0.8065

(0.0789)

Median width (ft) −0.0115

(0.0015)

−0.0105

(0.0019)

−0.0106

(0.0019)

Rural −0.2385

(−0.2385)

−0.2501

(0.0835)

−0.2498

(0.0839)

Urban 0

(0)

0

(0)

0

(0)

k1 0.8332

(0.0861)

0.8832

(0.1641)

0.7739

(0.1747)

— Indicates that the specific variable was not significant and not included in the model.
35

Table 13. Kentucky SPFs.

Parameter

Total Estimate

(SE)

KABC 

Estimate

(SE)

KAB Estimate

(SE)

Cross-Median 

Estimate

(SE)

Intercept −8.0058

(0.9029)

−9.2966

(1.3347)

−10.9603

(1.6169)

−16.4789

(3.7323)

AADT/1,000 −0.0067

(0.0033)

−0.0096

(0.0046)

−0.0143

(0.0054)

−0.0378

(0.0119)

In(AADT) 1.0624

(0.1)

1.0777

(0.1467)

1.2226

(0.1755)

1.7697

(0.405)

No interchange 

influence area

−0.3332

(0.0291)

−0.2282

(0.042)

−0.2361

(0.0515)

−0.3142

(0.1161)

Interchange 

influence area

0

(0)

0

(0)

0

(0)

0

(0)

Rural −0.1381

(0.04)

−0.2666

(0.0554)

−0.1627

(0.0695)

−0.4636

(0.151)

Urban 0

(0)

0

(0)

0

(0)

0

(0)

Four lanes −0.6711

(0.0727)

−0.7012

(0.1077)

−0.8153

(0.1281)

−1.4294

(0.3108)

Six lanes —

(0)

0

(0)

0

(0)

0

(0)

Inside shoulder 

width (ft)

−0.0877

(0.0079)

−0.0953

(0.012)

−0.0926

(0.015)

−0.0757

(0.0367)

Median width (ft) 0.0026

(0.0004)

0.0028

(0.0006)

—

(—)

−0.0065

(0.002)

Speed limit lower

than 65 mi/h

0.9354

(0.0196)

0.6698

(0.1626)

0.5931

(0.1975)

—

(—)

Speed limit greater 

than 65 mi/h

0

(0)

0

(0)

0

(0)

—

(—)

k1 0.2317

(0.0137)

0.1858

(0.0277)

0.2038

(0.0432)

0.5144

(0.2568)

— Indicates that the specific variable was not significant and not included in the model.

MISSOURI SPFs

As discussed earlier, reference groups were not available in Missouri because the State 

implemented the treatment systemwide. Therefore, the research team used the before-period data 

for the treated sites to estimate the SPFs, which are shown in table 14. In Missouri, because the 

team could not reliably estimate SPFs for cross-median crashes, they based the predictions for 

cross-median crashes on the product of the SPFs for total crashes with the proportion of crossmedian crashes.
36

Table 14. Missouri before-period SPFs.

Parameter

Total Estimate

(SE)

KABC Estimate

(SE)

KAB Estimate

(SE)

Intercept −25.4506

(2.4438)

−22.0979

(3.6129)

−9.192

(1.9287)

AADT/1,000 −0.069

(0.0116)

−0.0485

(0.017)

—

(—)

In(AADT) 2.7106

(0.2687)

2.2077

(0.3966)

0.6729

(0.2004)

Speed limit lower than 65 mi/h 0.719

(0.1353)

0.6406

(0.2032)

—

(—)

Speed limit greater than 65 mi/h 0

(0)

0

(0)

—

(—)

Rural −0.4076

(0.0596)

−0.1754

(0.0898)

—

(—)

Urban 0

(0)

0

(0)

—

(—)

Full access control 1.0153

(0.1236)

0.9275

(0.1872)

1.0108

(0.2836)

Limited access control 0

(0)

0

(0)

0

(0)

k1 0.3084

(0.0219)

0.3346

(0.0541)

0.3813

(0.1656)

— Indicates that the specific variable was not significant and not included in the model.
37

CHAPTER 7. BEFORE–AFTER EVALUATION RESULTS

This chapter presents the results of the before–after evaluation, including aggregate and 

disaggregate analysis for Kentucky, Illinois, and Missouri. 

AGGREGATE ANALYSIS

Table 15 through table 26 present the resulting CMFs. These tables provide the estimates of 

predicted crashes in the after period without treatment, the observed crashes in the after period, 

and the estimated CMF and its SE for all crash types considered. CMFs statistically different 

from 1.0 at the 95-percent confidence level are indicated with an asterisk. The tables provide 

separate results for Illinois and Kentucky, the combined results for Illinois and Kentucky, and the 

results for Missouri. The research team did not combine the results from Missouri with the results 

from Illinois and Kentucky because the before conditions in Missouri did not include inside 

shoulder rumble strips, while the before conditions in Illinois and Kentucky included inside 

shoulder rumble strips.

Table 15. CMFs from 27 urban sites in Illinois.

Crash Type

EB Estimate of 

Crashes Predicted 

in the After Period 

Without Strategy

Count of Crashes 

Observed in the 

After Period CMF

SE of 

CMF

Total 1,844.73 2,335 1.265* 0.035

Injury and fatal (KABC) 671.87 455 0.676* 0.040

Injury and fatal (KAB) 493.52 370 0.749* 0.049

Cross-median 19.06 10 0.520* 0.170

Note: Cross-median was defined as head-on plus opposite-direction sideswipe.

*Statistically significant at the 95-percent confidence level.

Table 16. CMFs from 13 rural sites in Illinois.

Crash Type

EB Estimate of 

Crashes Predicted 

in the After Period 

Without Strategy

Count of Crashes 

Observed in the 

After Period CMF

SE of 

CMF

Total 323.38 430 1.327* 0.085

Injury and fatal (KABC) 100.87 64 0.631* 0.090

Injury and fatal (KAB) 80.97 59 0.725* 0.108

Cross-median 4.24 2 0.459 0.325

Note: Cross-median was defined as head-on plus opposite-direction sideswipe.

*Statistically significant at the 95-percent confidence level.
38

Table 17. CMFs from urban and rural sites in Illinois.

Crash Type

EB Estimate of 

Crashes Predicted 

in the After Period 

Without Strategy

Count of Crashes 

Observed in the 

After Period CMF

SE of 

CMF

Total 2,168.10 2,765 1.275* 0.033

Injury and fatal (KABC) 772.75 519 0.671* 0.036

Injury and fatal (KAB) 574.49 429 0.746* 0.045

Cross-median 23.30 12 0.512* 0.152

Note: Cross-median was defined as head-on plus opposite-direction sideswipe.

*Statistically significant at the 95-percent confidence level.

Table 18. CMFs from seven urban sites in Kentucky.

Crash Type

EB Estimate of 

Crashes Predicted

in the After Period 

Without Strategy

Count of Crashes 

Observed in the 

After Period CMF

SE of 

CMF

Total 168.79 162 0.957 0.090

Injury and fatal (KABC) 37.29 28 0.744 0.157

Injury and fatal (KAB) 22.57 18 0.787 0.204

Cross-median 4.85 1 0.193* 0.187

Note: Cross-median was defined as head-on plus opposite-direction sideswipe.

*Statistically significant at the 95-percent confidence level.

Table 19. CMFs from 32 rural sites in Kentucky.

Crash Type

EB Estimate of 

Crashes Predicted

in the After Period 

Without Strategy

Count of Crashes 

Observed in the 

After Period CMF

SE of 

CMF

Total 982.48 1,281 1.303* 0.045

Injury and fatal (KABC) 254.18 264 1.037 0.073

Injury and fatal (KAB) 149.37 137 0.916 0.086

Cross-median 31.47 18 0.569* 0.140

Note: Cross-median was defined as head-on plus opposite-direction sideswipe.

*Statistically significant at the 95-percent confidence level.
39

Table 20. CMFs from urban and rural sites in Kentucky.

Crash Type

EB Estimate of 

Crashes Predicted

in the After Period 

Without Strategy

Count of Crashes 

Observed in the 

After Period CMF

SE of 

CMF

Total 1,151.27 1,443 1.253* 0.041

Injury and fatal (KABC) 291.47 292 1.001 0.067

Injury and fatal (KAB) 171.95 155 0.900 0.080

Cross-median 36.32 19 0.520* 0.125

Note: Cross-median was defined as head-on plus opposite-direction sideswipe.

*Statistically significant at the 95-percent confidence level.

Table 21. CMFs from urban sites in Illinois and Kentucky combined.

Crash Type

EB Estimate of 

Crashes Predicted 

in the After Period 

Without Strategy

Count of Crashes 

Observed in the 

After Period CMF

SE of 

CMF

Total 2,013.51 2,497 1.240* 0.033

Injury and fatal (KABC) 709.16 483 0.680* 0.039

Injury and fatal (KAB) 516.09 388 0.751* 0.048

Cross-median 23.91 11 0.456* 0.142

Note: Cross-median was defined as head-on plus opposite-direction sideswipe.

*Statistically significant at the 95-percent confidence level.

Table 22. CMFs from rural sites in Illinois and Kentucky combined.

Crash Type

EB Estimate of 

Crashes Predicted 

in the After Period 

Without Strategy

Count of Crashes 

Observed in the 

After Period CMF

SE of 

CMF

Total 1,305.86 1,711 1.310* 0.040

Injury and fatal (KABC) 355.05 328 0.923 0.058

Injury and fatal (KAB) 230.34 196 0.850* 0.068

Cross-median 35.71 20 0.557* 0.130

Note: Cross-median was defined as head-on plus opposite-direction sideswipe.

*Statistically significant at the 95-percent confidence level.
40

Table 23. CMFs from urban and rural sites in Illinois and Kentucky combined.

Crash Type

EB Estimate of 

Crashes Predicted 

in the After Period 

Without Strategy

Count of Crashes 

Observed in the 

After Period CMF

SE of 

CMF

Total 3,319.37 4,208 1.267* 0.026

Injury and fatal (KABC) 1,064.22 811 0.762* 0.033

Injury and fatal (KAB) 746.43 584 0.782* 0.039

Cross-median 59.63 31 0.518* 0.097

Note: Cross-median was defined as head-on plus opposite-direction sideswipe.

*Statistically significant at the 95-percent confidence level.

Table 24. CMFs from 310 urban sites in Missouri.

Crash Type

EB Estimate of 

Crashes 

Predicted in the 

After Period 

Without Strategy

Count of Crashes 

Observed in the 

After Period CMF

SE of 

CMF

Total 345.37 264 0.763* 0.054

Injury and fatal (KABC) 102.33 54 0.526* 0.078

Injury and fatal (KAB) 24.73 9 0.361* 0.124

Cross-median indicator 4.86 0 0.000* —

Cross-median indicator + head-on 8.48 1 0.117* 0.116

— Indicates could not estimate SE when after-period crashes were 0.

*Statistically significant at the 95-percent confidence level.

Table 25. CMFs from 729 rural sites in Missouri.

Crash Type

EB Estimate of 

Crashes 

Predicted in the 

After Period 

Without Strategy

Count of Crashes

Observed in the 

After Period CMF

SE of 

CMF

Total 1,435.66 1,957 1.363* 0.041

Injury and fatal (KABC) 486.77 385 0.790* 0.046

Injury and fatal (KAB) 146.24 125 0.853* 0.083

Cross-median indicator 19.49 1 0.051* 0.051

Cross-median indicator + head-on 33.43 4 0.119* 0.060

*Statistically significant at the 95-percent confidence level.
41

Table 26. CMFs from urban and rural sites in Missouri.

Crash Type

EB Estimate of 

Crashes

Predicted in the 

After Period 

Without Strategy

Count of Crashes 

Observed in the 

After Period CMF

SE of 

CMF

Total 1,781.03 2,221 1.247* 0.034

Injury and fatal (KABC) 589.10 439 0.745* 0.040

Injury and fatal (KAB) 170.96 134 0.783* 0.073

Cross-median indicator 24.35 1 0.041* 0.041

Cross-median indicator + head-on 41.92 5 0.119* 0.053

*Statistically significant at the 95-percent confidence level.

Most of the Kentucky data were from rural roads, whereas most of the Illinois data were from 

urban roads. Because cross-median crashes are relatively rare, it would be difficult to draw any 

definitive conclusions unless a relatively large sample of sites were used. As a result, the focus 

should be on the combined rural and urban results. The combined urban and rural results from 

Kentucky indicated an increase in total crashes and a reduction in cross-median crashes. The 

results regarding total and cross-median crashes were similar in Illinois (for rural and urban 

combined), but the Illinois sites exhibited a reduction in injury and fatal crashes as well. The 

combined Illinois and Kentucky results (for rural and urban combined) indicate a 27-percent 

increase in total crashes, 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 crossmedian crashes. The increase in total crashes—along with a decrease in injury and fatal crashes—

reveals an increase in property-damage-only (PDO) crashes. Based on the difference between the 

EB predicted crashes in the after period for total and injury and fatal crashes, the predicted PDO 

crashes in the after period (without the cable barrier treatment) were about 2,255.2 (3,319.37 

minus 1,064.22), and the actual PDO crashes in the after period were 3,397 (4,208 minus 811). 

This implies that PDO crashes increased by approximately 51 percent following the 

implementation of the cable barriers.

The results from Missouri for total and injury and fatal crashes (for rural and urban combined) 

were very similar to the combined Illinois and Kentucky results. However, the reduction in crossmedian crashes in Missouri was much more dramatic, with CMFs of 0.041 (based on crossmedian indicator only) and 0.119 (based on cross-median indicator plus head-on).

DISAGGREGATE ANALYSIS

The disaggregate analysis sought to identify those conditions (i.e., before-period EB predicted 

crash frequency, median width, speed limit, and AADT) under which the treatment was most 

effective. The research team could discern no patterns, most likely because of the limited sample 

size for cross-median crashes. 

43

CHAPTER 8. ECONOMIC ANALYSIS

Because it was clear that cable median barriers resulted in an increase in total crashes and a 

reduction in injury and fatal crashes—thereby implying an increase in PDO crashes—the 

research team found it necessary to estimate the change in PDO crashes in order to conduct an 

economic analysis. The team undertook the following steps for the economic analysis:

1. The research team estimated the change in PDO crashes using the EB predicted 

crashes in the after period and the actual crashes in the after period for total and 

KABC crashes.

2. Using the number of mile-years in the after period, the research team determined

the change in PDO crashes per mile-year and the change in KABC crashes per 

mile-year. Based on combined data from Illinois and Kentucky, KABC crashes 

decreased by 0.53 per mi-year, and PDO crashes increased 2.38 per mi-yr. In 

Missouri, KABC crashes decreased by 0.18/year, and PDO crashes increased by 

0.72/mi-yr. The research team discussed the use of KAB crashes per-mile, but 

used KABC because it was based on a large sample of crashes.

3. The research team used the comprehensive cost estimate for PDO and KABC 

crashes shown in appendix D (which updated figures from an earlier report by 

Council et al.) to estimate the annual crash savings in economic terms.

(18) The 

team assumed the cost of a KABC crash was $498,579, and the cost of a PDO 

crash was $18,877. Using these numbers, the benefit per mile per year was 

$217,725 in Illinois and Kentucky and $77,917 in Missouri. Appendix D refers to 

a June 2013 U.S. Department of Transportation (USDOT) memo that prescribes 

sensitivity analysis based on low and high values of crash costs.

(19) Specifically, 

the USDOT memo suggests that sensitivity analysis should be done by estimating 

B/C ratios for 0.57 and 1.41 times the 2014 crash costs.

(19) Step 5 provides the 

results based on the sensitivity analysis.

4. The research team estimated the annualized cost of the treatment, as shown in 

Figure 10.

Figure 10. Equation. Determining annual cost.

Where:

C = Treatment cost.

R = Discount rate (as a decimal) and assumed to be 0.07. 

N = Expected service life (years).

The annualized treatment cost per mile was $26,286 in Illinois and Kentucky and $18,810

in Missouri.

Annual Cost =

C ∗ R

1 − 1 + R −N
44

• Step 5. The research team calculated the B/C ratio as the ratio of the annual crash savings 

to the annualized treatment cost. The resulting B/C ratio for Illinois and Kentucky was 

8.28, while the Missouri ratio was 4.14. Based on the sensitivity analysis, the B/C ratio 

for Illinois and Kentucky could range from 4.72 to 11.68, and the B/C ratio for Missouri 

could range from 2.36 to 5.84.
45

CHAPTER 9. SUMMARY AND CONCLUSIONS

The objective of this study was to undertake a rigorous before–after evaluation of the safety 

effectiveness—as measured by crash frequency—of cable median barriers in combination with 

inside shoulder rumble strips along divided roads. The study used data from three States—

Illinois, Kentucky, and Missouri—to examine the effects for specific crash categories, including 

total, fatal and injury (KAB and KABC), and cross-median crashes. The research team did not 

include crashes occurring at or related to an intersection and animal-related crashes. 

In Illinois and Kentucky, inside shoulder rumble strips were present prior to the implementation 

of cable barriers; as a result, the evaluation in Illinois and Kentucky determined the safety effect 

of adding cable barriers on divided roads where inside shoulder rumble strips were already 

present. On the other hand, Missouri installed inside shoulder rumble strips and cable median

barriers at about the same time (or within a few years of each other); therefore, the evaluation in 

Missouri determined the combined safety effect of cable median barriers and inside shoulder 

rumble strips. A disaggregate analysis of the results did not reveal any specific patterns, possibly 

because of the limited sample size for cross-median crashes.

Table 27 presents the recommended CMFs when the before condition included inside shoulder 

rumble strips. The B/C ratio for this treatment was 8.28. Table 28 provides the recommended 

CMFs when the before condition had neither inside shoulder rumble strips nor cable median

barriers. The associated B/C ratio for this treatment was 4.14.

Table 27. CMFs for the combination of cable median barriers and rumble strips when the 

before condition included inside shoulder rumble strips.

Crash Type CMF

SE of 

CMF

Total 1.267 0.026

Injury and fatal (KABC) 0.762 0.033

Injury and fatal (KAB) 0.782 0.039

Head-on plus opposite-direction sideswipe (proxy for cross-median crashes) 0.518 0.097

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. 

Crash Type CMF

SE of 

CMF

Total 1.247 0.034

Injury and fatal (KABC) 0.745 0.040

Injury and fatal (KAB) 0.783 0.073

Cross-median (cross-median indicator plus head-on) 0.119 0.053

The findings of this study indicate that the introduction of cable median barriers resulted in a 

reduction in head-on, opposite-direction sideswipe, and cross-median crashes. At the same time, 

the cable median barriers led to an increase in total crashes and a reduction in injury and fatal 

crashes.

47

APPENDIX A. ADDITIONAL INSTALLATION DETAILS FROM ILLINOIS

This appendix presents further details about the cable barrier installations from Illinois based on a 

questionnaire that was sent to the participating States.

Cable Median Barriers and Shoulder Rumble Strip Combination Questions—Illinois

1. What was the “before-period” condition for the treatment sites with respect to rumble strips 

and cable median barriers?

• No cable median barriers and no rumble strips.

• Cable median barriers present but no rumble strips.

• No cable median barriers but rumble strips present.

Answer: No cable median barriers but rumble strips present.

2. What type(s) of rumble strips were characteristic of the treatment sites evaluated by this 

study? (Check all that apply.)

• Milled.

• Rolled.

• Formed.

• Raised.

• Other.

Answer: Milled and rolled.

3. Can you provide specifications and/or standard drawings that address the following 

characteristics of the rumble strips evaluated by this study?

• Width.

• Length.

• Depth.

• Spacing.

• Lateral placement (i.e., in relation to pavement marking).

Answer:

Standard 642001-02  Shoulder rumble strip 16 inches, and

Standard 642006  Shoulder rumble strip 8 inches.

http://www.idot.illinois.gov/Assets/uploads/files/Doing-Business/Standards/HighwayStandards/HighwaysStandardsRevision215.pdf.

Specifications  Section 642 Shoulder Rumble Strip (p. 554–555).

http://www.idot.illinois.gov/Assets/uploads/files/Doing-Business/Manuals-Guides-&-

Handbooks/Highways/Construction/Standard-Specifications/12SpecBook.pdf.

Bureau of Design and Environment Manual CH 34 Section 34-2.02(e) Rumble strip 

(p. 15–16).
48

http://www.idot.illinois.gov/Assets/uploads/files/Doing-Business/Manuals-Split/DesignAnd-Environment/BDE-Manual/Chapter%2034%20Cross%20Section%20Elements.pdf

4. Can you provide specifications and/or standard drawings that address the following 

characteristics of the cable median barrier evaluated by this study?

• Number of cables.

• Post spacing.

• Other important design considerations (i.e., cable pre-stretch, tensioning, slope placement, 

footing design).

Answer:

Bureau of Design and Environment Manual  CH 38-7, Median barriers

http://www.idot.illinois.gov/Assets/uploads/files/Doing-Business/Manuals-Split/DesignAnd-Environment/BDE-Manual/Chapter%2038%20Roadside%20Safety.pdf

Supplemental Specification for Section 644 High Tension Cable Median Barriers (p. 58–59)

http://www.idot.illinois.gov/Assets/uploads/files/Doing-Business/Manuals-Guides-&-

Handbooks/Highways/Construction/Supplemental-StandardsSpecifications/2015Supp.pdf

Approved list of High Tension Cable Median Barriers

http://idot.illinois.gov/Assets/uploads/files/Doing-Business/SpecialtyLists/Highways/Materials/Materials-&-Physical-Research/Metals/htcmedianbarrier.pdf

5. What were the requirements (e.g., minimum paved shoulder width, minimum median width, 

number of lanes, etc.) for the installation of rumble strips and cable median barriers at the 

study sites?

Answer: Cable median barrier was installed at these locations to reduce cross-median 

crashes. This was based on an examination of 5 years of crash data. Illinois used FARS 

crashes on interstates and looked at head on and opposite-direction sideswipe. They used this 

information to develop a warrant chart [and] a subsequent list of sites for cable median barrier 

installation. For cable median barrier, the median width had to be no more than 100 ft.

6. What was the lateral offset from the road to the cable median barriers and how was that 

distance selected?

Answer: Variable.

7. Please describe any challenges related to the rumble strip and/or cable median barrier 

installation and how you overcame them.

Answer: Supervision required for both rumble strip and cable median barrier contractor was 

pretty extensive. Constant checking for uniformity was required to meet required 

specification.
49

8. Please describe any challenges related to the rumble strip and/or cable median barrier 

maintenance and how you overcame them.

Answer: Ongoing maintenance is maybe more than originally projected in some Districts due 

to the number of cable barrier hits. Since cable barrier is doing its job, monthly repairs are not 

uncommon and should be expected/budgeted.

9. What lessons learned or recommendations would you share with another state interested in 

the widespread application of cable median barrier and rumble strips? 

Answer: Both low cost safety improvements appear to be a good investment in saving lives 

on roads.

51

APPENDIX B. ADDITIONAL INSTALLATION DETAILS FROM KENTUCKY

This appendix presents further details about the cable barrier installations from Kentucky based 

on a questionnaire that was sent to the participating States.

Cable Median Barriers and Shoulder Rumble Strip Combination Questions—Kentucky

1. What was the “before-period” condition for the treatment sites with respect to rumble strips 

and cable median barriers?

• No cable median barriers and no rumble strips.

• Cable median barriers present but no rumble strips.

• No cable median barriers but rumble strips present.

Answer: No cable median barriers but rumble strips present.

2. What type(s) of rumble strips were characteristic of the treatment sites evaluated by this 

study? (Check all that apply.)

• Milled.

• Rolled.

• Formed.

• Raised.

• Other.

Answer: Milled and rolled.

3. Can you provide specifications and/or standard drawings that address the following 

characteristics of the rumble strips evaluated by this study?

• Width.

• Length.

• Depth.

• Spacing.

• Lateral placement (i.e., in relation to pavement marking).

Answer: 

http://transportation.ky.gov/Construction/Standard%20amd%20Supplemental%20Specificati

ons/400%20Asphalt%20Pavements%2012.pdf.

Section 403.03.08
52

4. Can you provide specifications and/or standard drawings that address the following 

characteristics of the cable median barriers evaluated by this study?

• Number of cables.

• Post spacing.

• Other important design considerations (i.e., cable pre-stretch, tensioning, slope placement, 

footing design).

Answer: 

http://transportation.ky.gov/Construction-Procurement/Proposals/201-CHRISTIANMARSHALL-MCCRACKEN-141230.pdf.

5. What were the requirements (e.g., minimum paved shoulder width, minimum median width, 

number of lanes, etc.) for the installation of rumble strips and cable median barrier at the 

study sites?

Answer: Cable installations are generally selected based on an annual evaluation process that 

considers crash experience, median width, median slope profile, traffic volume, traffic 

composition, and speed. From these variables, KYTC prioritizes a list of recommended cable 

barrier projects. Median width had to be narrower than 200 ft.

6. What was the lateral offset from the road to the cable median barriers and how was that 

distance selected?

Answer: Approximately 8 ft lateral offset from travel lanes based on deflection on 10 ft post 

spacing.

7. Please describe any challenges related to the rumble strip and/or cable median barrier 

installation and how you overcame them.

Answer: Monitor depth and alignment of rumbles. Ensure proper anchor placement for 

appropriate median coverage near bridges and coordinate minimum/maximum “gap” spacing 

for emergency vehicles.

8. Please describe any challenges related to the rumble strip and/or cable median barrier 

maintenance and how you overcame them.

Answer: Cable barrier maintenance is handled through contract. No substantial issues with 

maintenance of rumbles.

9. What lessons learned or recommendations would you share with another state interested in 

the widespread application of cable median barriers and rumble strips?

Answer: Have a defendable program to evaluate and select installations.
53

APPENDIX C. ADDITIONAL INSTALLATION DETAILS FROM MISSOURI

This appendix presents further details about the cable barrier installations from Missouri based on

a questionnaire that was sent to the participating States.

Cable Median Barrier and Shoulder Rumble Strip Combination Questions—Missouri

1. What was the “before-period” condition for the treatment sites with respect to rumble strips 

and cable median barriers?

• No cable median barriers and no rumble strips.

• Cable median barriers present but no rumble strips.

• No cable median barriers but rumble strips present.

Answer: The median guard cable was not installed in combination with the rumble strips.

Both were independent projects, and time of installation will vary. The “before-period” for 

the locations will not have median guard cable but would have potentially included a 30-inch

wide rolled rumble strip (very little value in noise and feel).

2. What type(s) of rumble strips were characteristic of the treatment sites evaluated by this 

study? (Check all that apply.)

• Milled.

• Rolled.

• Formed.

• Raised.

• Other.

Answer: Missouri uses a milled rumble strip (potential for stamped with concrete 

pavements). The majority of installed miles to date are milled rumble strips.

3. Can you provide specifications and/or standard drawings that address the following 

characteristics of the rumble strips evaluated by this study?

• Width.

• Length.

• Depth.

• Spacing.

• Lateral placement (i.e., in relation to pavement marking).

Answer: All of our specifications are in our Engineering Policy Guide (EPG), including 

standard drawings. MoDOT will install this same rumble strip specification for shoulder 

widths as low as 2 ft. The link below will provide the policy relating to our rumble strip 

program:

http://epg.modot.mo.gov/index.php?title=Category:626_Rumble_Strips
54

4. Can you provide specifications and/or standard drawings that address the following 

characteristics of the cable median barriers evaluated by this study?

• Number of cables.

• Post spacing.

• Other important design considerations (i.e., cable pre-stretch, tensioning, slope placement, 

footing design).

Answer: Our median guard cable program also has policy provided in our EPG. All current 

specifications and standard drawings are listed in the policy section. Also, please review the 

document called MoDOT’s Cable Median Barrier Program, as it provides a lot of good 

information on our program.

http://epg.modot.mo.gov/index.php?title=606.2_Guard_Cable

It is important to note our program began with the low-tension systems, but the majority of 

miles installed are high-tension.

5. What were the requirements (e.g., minimum paved shoulder width, minimum median width, 

number of lanes, etc.) for the installation of rumble strips and cable median barriers at the 

study sites?

Answer: The cable median barrier program was initially completed on our worst roadways 

first, and these roads featured very narrow medians (approx. 40 ft) and traffic volumes around 

30,000 daily, but we used a systemwide installation method to eliminate the cross-median 

crash type on the highest need routes. Please see question 4 for more detail, but the treatment 

has been applied statewide on roadways identified as tier 1 or tier 2.

6. What was the lateral offset from the road to the cable median barriers and how was that 

distance selected?

Answer: Historically, our program began with installing the median guard cable in the vertex 

of the ditch, which was basically in the middle of the median. In 2007, our program changed 

due to many factors (information on crash dynamics, maintenance issues, and other), and we 

began installing approximately 8 ft from the stripe. This means it will always be closer to one 

direction of travel. This information is available in the report identified in question 4 above.

7. Please describe any challenges related to the rumble strip and/or cable median barrier 

installation and how you overcame them.

Answer: When MoDOT began the median guard cable program, there was little information 

available on installation-related issues (location, type, etc.). This ultimately led to a team 

being formed to determine better policy. I believe the issues with rumble strips relate to both 

noise and the bicycle community. We initially did not see much pushback from the bicycle 

community, but we are beginning to see this now as other States have a different specification 

and standard drawing (more desired by bicycle community). The noise complaints have 

occurred but have not been a detriment to our program.
55

8. Please describe any challenges related to the rumble strip and/or cable median barrier 

maintenance and how you overcame them.

Answer: With our current specification of the median guard cable location and mow strip, we 

do not see many maintenance issues (we have an asphalt apron that extends from paved 

shoulder to approximately 1 ft past cable barrier—this was requested by maintenance). A 

larger issue related to maintenance of the cable system relates to vehicle crashes. This issue 

does create a financial issue as our agency is only able to receive about 1/3 back in claims 

compared to impacts with the system.

The rumble strips have not involved a great deal of maintenance issues due to failures.

However, where we have seen areas of failures near the joint, we have allowed sections (not 

longer than 200 ft) to not be re-milled after a pavement repair. Overall, our system has not 

seen large-scale failures.

9. What lessons learned or recommendations would you share with another state interested in 

the widespread application of cable median barriers and rumble strips?

Answer: The implementation needs to be based on a thorough crash type evaluation and 

involve widespread installations on a system of routes that would share similar characteristics 

(regardless of current crash information). In other words, when installing the median guard 

cable, evaluate characteristics of roads that share the cross-median crash type and install over 

a network of roadways that are similar (median width and roadway AADT could be your 

criteria). The same application can be applied on installation of rumble strips on improved 

shoulders. For instance, do roads with a minimum AADT have an over-representation of the 

roadway departure crash types? If so, all roads that have this minimum AADT threshold 

should have a rumble strip installed.

Also, it is critical to create policy on your safety initiatives to allow your programs to 

succeed. The policy you develop will drive each program.

57

APPENDIX D. METHODOLOGY FOR CALCULATING CRASH COSTS

This appendix presents the methodology for estimating cost per crash for current year by severity, 

crash type, and speed limit based on value of a statistical life.

(18)

The total cost of a fatal crash is larger than the value of a statistical life since there can be other 

injuries as well as property damage. The question is: how much larger? That is, how much is the 

cost of a fatal crash, given the value of a statistical life? 

The relationship between value of a statistical life and cost of a crash can be derived by using 

latest comparable information from two sources involving the same research team. For 2001, the 

value of a statistical life (VSL) recommended to the National Highway Traffic Safety 

Administration was $3.8 million, and the mean comprehensive fatal crash cost irrespective of 

location or speed limit was $4,008,085, giving a ratio of cost per fatal crash to value of a 

statistical life of 1.055.(19,20)

To update the cost of a fatal crash to 2014, for example, this ratio is applied to the VSL in that 

year ($9.2 million) as given annually in a USDOT memo to get an updated 2014 cost of a fatal 

crash of $9.7 million.(21)

For economic analyses in FHWA DCMF evaluations, researchers typically use total crashes (all 

severities combined), PDO crashes and fatal plus injury crashes combined, sometimes 

disaggregated for more precision by crash type, site type (e.g., signalized intersections, road 

segments), and environment (speed limit > 45 and > 45 mi/h). These costs are usually derived 

from the 2005 FHWA crash cost report using the 2001 costs and updating them using the 

procedure recommended in that report.(20) 

It is recommended that researchers continue to use the basic disaggregate cost from the 2005 

FHWA report (since that is the latest source of disaggregate crash cost, and disaggregation is 

desirable) and update it to current year considering the prescribed USDOT VSL number for that 

year. For example, for 2014:

2014 disaggregate crash cost = (2001 disaggregate crash cost/2001 fatal crash cost) x (updated 

2014 fatal crash cost derived from 2014 recommended VSL)

Substituting known values in millions, rounded to one decimal place per the USDOT memo, 

produces the following result:

2014 Disaggregate crash cost = (2001 disaggregate crash cost/4.0) x (9.7)

In other words, the 2001 crash costs are factored up by 9.7/4.0 = 2.425 to update them to 2014.

For example, the fatal and injury crash cost for all locations and speed limits in the FHWA report 

is $158,177 for 2001, which factors up to $383,579 for 2014.

Continuing the 2014 example, the USDOT memo also prescribes that sensitivity analysis be 

conducted based on low and high VSLs of $5.2 and $13.0 million for 2014. These translate to 
58

0.57 and 1.41 times the $9.2 million VSL recommended for use. By inference, sensitivity 

analysis should also be done for DCMF evaluations by estimating B/C ratios for 0.57 and 1.41 

times the 2014 crash costs derived using the method above. For example, for the fatal and injury 

crashes, B/C ratios would be estimated based on costs of $218,640 and $540,846. 
59

ACKNOWLEDGMENTS 

This report was prepared for the Federal Highway Administration, Office of Safety Research and 

Development under contract DTFH61-13-D-0001. The FHWA Program and Task Manager for 

this project was Roya Amjadi.

The research team gratefully acknowledges the participation of the following organizations for 

their assistance in this study:

• KYTC.

• Kentucky Transportation Center, University of Kentucky.

• MoDOT.

• ILDOT.

The authors would also like to thank the staff of the Wisconsin Department of Transportation for 

their insight on the study.

61

REFERENCES

1. Villwock, N., Blond, N., and Tarko, A. (2009). Safety Impact of Cable Barriers on Rural 

Interstates, presented at the 2009 Annual Meeting of the Transportation Research Board, 

Transportation Research Board, Washington, DC.

2. Cooner, S.A., Rathod, Y.K., Alberson, D.C., Bligh, R.P., Ranft, S.E., and Sun, D. (2009). 

Performance Evaluation of Cable Median Barrier Systems in Texas, Report No. 

FHWA/TX-09/0-5609-1, Texas Department of Transportation, Austin, TX.

3. Chandler, B. (2007). “Eliminating Cross Median Fatalities: Statewide Installation of 

Median Cable Barrier in Missouri,” TR News, 248, p. 29–30.

4. Hunter, W.W., Stewart, R., Eccles, K.A., Huang, H., Council, F., and Harkey, D. (2001).

“Three-Strand Cable Median Barrier in North Carolina,” Transportation Research Record

1743, p. 97–103.

5. Milton, J. and Albin, D. (2004). Improving Highway Safety: Cable Median Barrier,

presented at the 2004 Annual Meeting of the Transportation Research Board, 

Transportation Research Board, Washington, DC.

6. Griffith, M. (1999). “Safety Evaluation of Rolled-In Continuous Shoulder Rumble Strips 

Installed on Freeways,” Transportation Research Record, 1665, p. 28–34.

7. Torbic, D., Hutton, J., Bokenkroger, C., Bauer, K., Harwood, D., Gilmore, D., Dunn, J.,

Ronchetto, J., Donnell, E., Sommer III, H., Garvey, P., Persaud, B., and Lyon, C. (2009). 

Guidance for the Design and Application of Shoulder and Centerline Rumble Strips. 

NCHRP Report 641, National Cooperative Highway Research Program, Washington, DC.

8. American Association of State Highway and Transportation Officials. (2010). Highway 

Safety Manual, American Association of State Highway and Transportation Officials. 

Washington, DC.

9. Monsere, C.M., Sposito, B., and Johnston, S. (2003). Safety Effectiveness and Operating 

Performance of Three-Cable Median Barrier on Interstate 5 in Oregon, presented at the 

2003 Annual Meeting of the Institute of Transportation Engineers, Seattle, WA.

10. Hauer, E. (1997). Observational Before-After Studies in Road Safety—Estimating the 

Effect of Highway and Traffic Engineering Measures on Road Safety, Elsevier Science, 

Incorporated, Amsterdam, The Netherlands.

11. Bahar, G. (2010). Methodology for the Development and Inclusion of Crash Modification 

Factors in the Highway Safety Manual, Transportation Research Circular E-C142, 

Transportation Research Board, Washington, DC.
62

12. Hauer, E. (2001). “Overdispersion in Modelling Accidents on Road Sections in Empirical 

Bayes Estimation,” Accident Analysis and Prevention, 33(6), p. 799–808.

13. Lyon, C., Persaud, B., and Eccles, K. (2015). Safety Evaluation of Centerline Plus 

Shoulder Rumble Strips, Federal Highway Administration, Report No. FHWA-HRT-15-

048, Federal Highway Administration, Washington, DC.

14. Nujjetty, A., Sharma, S., and Council, F.M. (2015). Highway Safety Information System

Guidebook for Illinois Data Files, Prepared for Federal Highway Administration, 

https://www.hsisinfo.org/pdf/guidebook_IL.pdf. Last accessed on: December 1, 2014.

15. Kentucky Transportation Cabinet. (2014). Planning Highway Information (HIS) 

Database. Available online at: http://transportation.ky. gov/ Planning/Pages/RoadwayInformation-and-Data.aspx. Last accessed December 1, 2014.

16. Kentucky Transportation Cabinet. (2015). Photolog. KY.gov. Available online at: 

http://maps.kytc.ky.gov/photolog/. Last accessed on: December 1, 2014. 

17. Hauer, E. (2015). The Art of Regression Modeling in Road Safety, Springer International, 

New York, NY.

18. Council, F., Zaloshnja, E., Miller, T., and Persaud, B. (2005). Crash Cost Estimates by 

Maximum Police-Reported Injury Severity Within Selected Crash Geometries, Report No. 

FHWA-HRT-05-051, Federal Highway Administration, Washington, DC.

19. U.S. Department of Transportation. (2014). Guidance on Treatment of the Economic

Value of a Statistical Life (VSL) in U.S. Department of Transportation Analyses—2014 

Adjustment, Memo dated June 13, 2014. Available at http://www.dot.gov/sites/dot.gov/

files/docs/VSL_Guidance_2014.pdf. Last accessed December 1, 2015.

20. Persaud, B. (2014). How to Convert Value of a Statistical Life to Cost per Crash by 

Severity, Crash Type, and Speed Limit, Draft memo for DCMF Evaluations. Unpublished 

document.

21. Zaloshnja, E., Miller, T. Council, F. and Persaud, B. “Crash Costs in the United States by 

Crash Geometry,” Accident Analysis and Prevention 38(4), p. 644–651.

22. Council, F., Zaloshnja, E., Miller, T., and Persaud, B. (2005). Crash Cost Estimates by 

Maximum Police-Reported Injury Severity Within Selected Crash Geometries, FHWAHRT05-051, Federal Highway Administration, Washington, DC.

23. “Guidance on Treatment of the Value of a Statistical Life (VSL) in U.S. Department of 

Transportation Analyses–2014 adjustment”. USDOT memo, June 13, 2014. 

http://www.dot.gov/sites/dot.gov/files/docs/VSL_Guidance_2014.pdf.

HRDS-20/08-17(200)E