TECHBRIEF Safety Evaluation of 

Cable Median Barriers in 

Combination with Rumble 

Strips on the Inside Shoulder 

of Divided Roads

FHWA Publication No.: FHWA-HRT-17-071

FHWA Contact: Roya Amjadi, HRDS-20, (202) 493-3383, 

roya.amjadi@fhwa.dot.gov

This document is a technical summary of the Federal 

Highway Administration (FHWA) report Safety Evaluation 

of Cable Median Barriers in Combination with Rumble 

Strips on the Inside Shoulder of Divided Roads (FHWAHRT-17-070).

Objective

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-to-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 provided technical feedback on safety 

improvements to the DCMF program and implemented 

new safety improvements to facilitate evaluations. These 

States are members of the Evaluation of Low-Cost Safety 

Improvements Pooled Fund Study, which functions under 

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.fhwa.dot.gov/research
2

the DCMF program. This study evaluated 

the application of cable median barriers in 

combination with rumble strips on the inside 

shoulder of divided roads. This strategy was 

intended to reduce the frequency of crashes 

by reducing the frequency and severity 

of cross-median crashes. Many studies 

have explored the safety effectiveness of 

cable median barrier or shoulder rumble 

strips; however, no study has conducted 

a rigorous evaluation of the effectiveness 

of the two strategies in combination. This 

study sought to fill this knowledge gap.

Introduction

Roadway or lane departure is an emphasis area in every State’s strategic highway 

safety plan. One of the most severe and 

high-profile types of roadway departure 

crashes is cross-median crashes on divided 

roads. Proven, cost-effective strategies to 

reduce the frequency and severity of these 

crashes are necessary to address this problem. Many agencies have installed widespread shoulder rumble strips to reduce the 

frequency of run-off-road crashes and rigid 

or cable median barriers to prevent errant 

vehicles from crossing over the median into 

oncoming traffic. 

States cite many reasons why cable barriers 

are becoming more favorable, including the 

following:

• Cable barriers contain vehicles in the 

median instead of redirecting them 

back into traffic. 

• Typical installation and maintenance 

costs are lower than rigid barriers, 

although they may require more 

frequent maintenance and can sustain 

damage from snow plows. 

• Snow can move through cable barriers 

more easily, preventing accumulation.

• Cable barriers are environmentally 

non-intrusive and more aesthetically 

pleasing. 

There are also concerns about cable barriers being susceptible to penetration and 

hits by heavier vehicles. In addition, several 

studies indicate an increase in total crash 

frequency after installation of cable median 

barriers.

A literature review focused on the safety 

effects of cable median barriers, shoulder 

rumble strips, and cable median barriers 

alongside shoulder rumble strips. Studies 

have shown cable median barriers to 

reduce cross-median fatalities crashes 

by 92 percent, head-on fatal crashes by 

93 percent, and multiple vehicle opposite 

direction crashes by 94 percent. However, 

studies also show them generally increasing 

run-off-road-left, single-vehicle, fixed-object, 

rear-end, and total crashes.(1–3) Studies have 

also widely shown that shoulder rumble 

strips reduce total, injury, single-vehicle 

run-off-road, and fixed-object crashes. 

Torbic et al. found an 11-percent reduction 

in single-vehicle run-off-road crashes and 

a 16-percent reduction in single-vehicle 

run-off-road crashes resulting in fatalities 

or injuries.(4) Monsere et al. examined the 

combined effects of cable median barriers 

and shoulder rumble strips using a simple 

before-after study. The researchers found 

a 100-percent reduction (i.e. from 3 to 0) 

in cross-median fatal and serious injury 
3

crashes across three-year before and after 

periods. The researchers compared this to 

an increase in overall barrier hits from 7 to 

60 during the same timeframe; however, 

maintenance logs did not specify which 

barrier (i.e. on the left or right side of the 

road) was hit.(5)

Methodology

This study examined the safety impacts of 

cable median barriers with inside shoulder rumble strips in Illinois, Kentucky, and 

Missouri on cross-median crash frequency. 

This study also evaluated the effects on 

total crashes and all fatal and injury crashes. 

Data collection and sample size limitations 

prevented separate evaluations of medianrelated, run-off-road-left, and winter-related 

crashes.

The treatment itself is not exactly the same 

in the three States used in this evaluation. 

In Illinois and Kentucky, the introduction 

of cable median barriers came many years 

after the introduction of rumble strips on 

inside shoulders. The before condition for 

these two States included shoulder rumble 

strips. Conversely, Missouri installed cable 

median barriers and inside shoulder rumble 

strips around the same time. This study 

combined and evaluated Illinois and 

Kentucky separately from Missouri. Across 

all three States, the study used a total of 455 

mi of treatment sites and more than 700 mi 

of reference sites in the evaluation.

The evaluation made use of the empirical 

Bayes (EB) methodology for observational 

before-after studies.(6) 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 use of safety performance functions (SPFs) addresses the 

following:

• Overcoming the difficulties of using 

crash rates in normalizing for volume 

differences between the before and 

after periods.

• Accounting for time trends.

• Reducing the level of uncertainty in the 

estimates of safety effect.

• Properly accounting for differences in 

crash experience and reporting practice 

in amalgamating data and results from 

diverse jurisdictions.

• Providing a foundation for developing 

guidelines for estimating the likely 

safety consequences of a contemplated 

strategy.

The project team estimated the SPFs 

used in the EB methodology through 

generalized linear modeling assuming a 

negative binomial error distribution, which 

is consistent with the state of research in 

developing these models. In specifying 

a negative binomial error structure, the 

project team estimated an overdispersion 

parameter based on segment length from 

the model and the data. For a given dataset, 

smaller values of this parameter indicate 

relatively better models.

When the data did not specify cross-median 

crash type, the project team used proxy 

crash types. In the Illinois and Kentucky 

analysis, the project team used head-on 
4

crashes plus opposite-direction-sideswipe 

crashes. The Missouri analysis used the 

cross-median crash indicator plus head-on 

crashes.

The full report includes a detailed explanation of the methodology and the development of SPFs, including a description of 

how the estimate of safety effects for target 

crashes were calculated.

Results

This brief presents the research results in 

two parts. The first part contains aggregate 

results. The second part is based on a 

disaggregate analysis that sought to identify 

the optimal conditions for installation of the 

treatment.

Aggregate Analysis

Table 1 and table 2 show the aggregate 

results for Illinois and Kentucky combined 

and Missouri, respectively. Both tables 

represent urban and rural sites together. 

For all crash types, the tables provide the 

estimates of expected crashes in the after 

period without treatment, the observed 

crashes in the after period, the estimated 

crash modification factor (CMF), and the 

standard error of the CMF. Fatal and injury 

crashes were defined as KAB or KABC.1

The results indicate statistically significant 

crash reductions at the 95-percent confidence level for all crash types analyzed in 

all States. The crash type with the smallest 

CMF, which marks the greatest reduction 

in crashes, is the cross-median CMF. This 

is consistent in Illinois and Kentucky combined and in Missouri. The evaluation 

yielded an estimated CMF of 1.267 for total 

crashes in Illinois and Kentucky and 1.247 

in Missouri. 

Crash Type

EB Estimate of 

Crashes Predicted 

in the After Period 

Without Strategy

Count of Crashes 

Observed in the 

After Period

CMF Standard Error

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

*Indicates CMF estimates that are statistically significant at the 95-percent confidence level.

Table 1. CMFs from urban and rural sites in Illinois and Kentucky combined.

___________________

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).
5

Disaggregate Analysis

The disaggregate analysis examined 

the before-period EB predicted crash 

frequency, median width, speed limit, and 

annual average daily traffic to identify the 

conditions under which the treatment was 

most effective. The project team did not 

identify any patterns, which was possibly 

because of the limited sample of crossmedian crashes.

Economic Analysis

Because cable median barriers resulted in 

an increase in total crashes and a reduction in injury and fatal crashes, implying 

an increase in property-damage-only (PDO) 

crashes, it was necessary to estimate the 

change in PDO crashes to determine accurate results. The project team calculated 

PDO crashes by subtracting KABC crashes 

from total crashes for the EB predicted 

crashes in the after period and the actual 

crashes in the after period.

Using the number of mile-years in the 

after period, the project team determined 

the change in PDO crashes per mile-year 

and the change in KABC crashes per 

mile-year. KABC crashes decreased by 

0.53/mi-year in Illinois and Kentucky and 

0.18/mi-year in Missouri. PDO crashes 

increased 2.38/mi-year in Illinois and 

Kentucky and 0.72/mi-year in Missouri. 

This study used comprehensive crash cost 

estimates for KABC and PDO crashes from 

the recent report by Persaud (updated 

from an earlier report by Council et al.) to 

estimate the annual economic benefits.(7,8) A 

KABC crash was assumed to cost $498,579, 

and the cost of a PDO crash was $18,877. 

Using these numbers, the safety benefit 

per mile-year was $217,725 in Illinois and 

Kentucky and $77,917 in Missouri. 

Crash Type

EB Estimate of 

Crashes Predicted 

in the After Period 

Without Strategy

Count of Crashes 

Observed in the 

After Period

CMF Standard Error

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

Table 2. CMFs from urban and rural sites in Missouri.

*Indicates CMF estimates that are statistically significant at the 95-percent confidence level.
6

Persaud refers to a June 2013 United States 

Department of Transportation (USDOT) 

memo that prescribes sensitivity analysis 

based on low and high values of crash 

costs.(7,9) 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.(9)

The research team estimated the annualized 

cost of the treatment, as shown in figure 1.

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 is 

$26,286 in Illinois and Kentucky and $18,810 

in Missouri.

The B/C ratio was calculated as the ratio 

of the annual crash savings to the annualized treatment cost. The resulting B/C ratio 

is 8.28 for Illinois and Kentucky and 4.14 

for Missouri. A sensitivity analysis indicated 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.

Summary and Conclusions

The objective of this study was to undertake a rigorous before-after evaluation of 

the safety effectiveness 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 types, including 

total, fatal and injury (KABC and KAB), and 

cross-median crashes. This study 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 barriers around the same 

time (or within a few years of each other). 

The evaluation in Missouri determined the 

combined safety effect of cable barriers and 

inside shoulder rumble strips. Therefore, 

for the purpose of this evaluation, the 

treatment in Missouri is different from the 

treatment in Illinois and Kentucky. A disaggregate analysis of the results did not 

reveal any specific patterns, possibly 

because of the limited sample of crossmedian crashes. 

Table 3 provides the recommended CMFs 

when the before condition includes inside 

shoulder rumble strips. The average B/C 

ratio for this treatment is 8.28.

Table 4 provides the recommended CMFs 

when the before condition has neither 

inside shoulder rumble strips nor cable 

median barriers. The average B/C ratio for 

the combined treatment is 4.14.

Figure 1. Determining annual cost.
7

The results indicate an overall increase 

in crashes, with nearly 50-percent reduction in cross-median crashes where inside 

shoulder rumble strips were already present a nearly 90-percent reduction in crossmedian crashes when a State installed 

cable median barrier and shoulder rumble 

strips together. This is generally consistent 

with prior research evaluating the installation of cable median barriers.

References

1. Chandler, B. (2007). “Eliminating Cross 

Median Fatalities: Statewide Installation 

of Median Cable Barrier in Missouri,” 

TR News 248, January–February 2007, 

p. 29–30. 

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. 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.

Crash Type CMF Standard Error

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 3. CMFs for combination of cable median barriers and rumble strips when before condition includes inside 

shoulder rumble strips.

Crash Type CMF Standard Error

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

Table 4. CMFs for combination of cable median barriers and rumble strips when before condition has neither inside 

shoulder rumble strips nor cable median barrier.
8

SEPTEMBER 2017 FHWA-HRT-17-071

HRDS-20/09-17(200)E

Researchers—Raghavan Srinivasan, Bo Lan, Daniel Carter, Bhagwant Persaud, and Kimberly Eccles.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct 

distribution is being made to the Divisions and Resource Center.

Availability—This TechBrief may be obtained from the FHWA Product Distribution Center by e-mail 

to report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at http://www.fhwa.

dot.gov/research.

Key Words—Cable median barrier, shoulder rumble strips, low-cost, safety improvements, safety 

evaluations, empirical Bayesian.

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 highquality information to serve the Government, industry, and 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.

4. 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 Project, Washington, DC. 

5. Monsere, C.M., Sposito, B., and 

Johnston, S. (2003). Safety Effectiveness 

and Operating Performance Three-Cable 

Median Barrier on Interstate 5 in Oregon, 

presented at the 2003 Annual Meeting of 

the Institute of Transportation Engineers, 

Seattle, WA. 

6. 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.

7. 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, 

November 2014. 

8. Council, F., Zaloshnja, E., Miller, 

T., and Persaud, B. (2005). Crash 

Cost Estimates by Maximum PoliceReported Injury Severity Within 

Selected Crash Geometries, Report No. 

FHWA-HRT-05-051, Federal Highway 

Administration, Washington, DC. 

9. U.S. Department of Transportation. 

(2014). “Guidance on Treatment of the 

Economic Value of a Statistical Life 

(VSL) in U.S. Department of 

Trans-portation Analyses—2014 

Adjustment,” Memo, U.S. Department 

of Transportation, Washington, DC. 

Available online at: http://www.dot.

gov/sites/dot.gov/files/docs/VSL_

Guidance_2014.pdf. Last accessed 

December 1, 2015.