TECHBRIEF Safety Evaluation of 

Centerline Plus Shoulder 

Rumble Strips

FHWA Publication No.: FHWA-HRT-15-064

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

roya.amjadi@dot.gov

This document is a technical summary of the Federal 

Highway Administration report Safety Evaluation 

of Centerline Plus Shoulder Rumble Strips (FHWAHRT-15-048).

Objective

The Federal Highway Administration (FHWA) organized 37 

States to participate in the FHWA Evaluation of Low-Cost 

Safety Improvements Pooled Fund Study as part of its 

strategic highway safety plan support effort. The purpose 

of the study was to evaluate the safety effectiveness of 

several low-cost safety improvement strategies through 

scientifically rigorous crash-based studies. One of the 

strategies evaluated for this study is the combination of 

centerline and shoulder rumble strips. This strategy is 

intended to reduce the frequency of crashes by alerting 

drivers that they are about to leave the travelled lane. 

While research into the performance of shoulder and 

centerline rumble strips applied separately has been 

conducted, the combination of shoulder and centerline 

rumble strips is still relatively rare and has not been 

studied to the same degree. This study sought to fill this 

knowledge gap.

Introduction

The dual application of centerline rumble strips 

in combination with shoulder rumble strips is a 

recommended strategy in volumes 4 and 6 of the National 

Cooperative Highway Research Program (NCHRP) 500 

Series Guidebooks.(1,2) These guidebooks describe 

shoulder rumble strips as 0.5-inch-deep, crosswise 

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.fhwa.dot.gov/research
2

grooves in the road shoulder that are 

spaced about 7 inches apart and cut in 

groups of four or five. States have developed various designs and methods of installation, including rolling the rumble strips 

into hot asphalt or concrete as they are laid 

or milled in later. The rumble strips produce 

a vibrotactile—or auditory—warning in the 

form of a sudden rumbling sound or vibration to inattentive, drowsy, or sleeping 

drivers that encroach on the shoulder. 

Shoulder rumble strips are used extensively 

in the United States on all types of roadways. 

Centerline rumble strips are similar to 

shoulder rumble strips but are placed on 

the center line, typically extending into the 

travel lane by 5 to 18 inches. They may be 

placed continuously or with periodic gaps. 

Shoulder and centerline rumble strips are 

compatible with other measures taken to 

reduce crashes (e.g., curve flattening) and 

may be included in existing construction 

plans with minimal extra cost.

A literature review revealed that while 

research into the safety performance of 

shoulder and centerline rumble strips that 

have been applied separately has been conducted, most notably for NCHRP Report 641, 

the combined application of shoulder and 

centerline rumble strips has been relatively 

rare, and evaluations to date have, as a 

result, been limited in scope.(3) The one U.S. 

study of relevance evaluated the safety 

impacts of applying centerline and shoulder 

rumble strips in combination using data 

for 80 mi of rural two-lane roads in Mississippi and applied the empirical Bayes 

(EB) before-after approach.(4) Target 

collisions were defined as the sum of 

head-on, sideswipe-opposite-direction, 

and single-vehicle run-off-road collisions. 

The results showed a 35-percent reduction 

in target collisions of all severities and a 

39.6-percent reduction in fatal+injury target 

collisions. A Canadian study evaluated the 

safety impacts of applying centerline 

and shoulder rumble strips alone and in 

combination on two-lane rural and four-lane 

divided rural highways in British Columbia, 

Canada.(5) The results of this EB before-after 

study indicated that the combined 

application on two-lane roads indicated a 

reduction of 21.4 percent in off-road right, offroad left, and head-on collisions combined. 

This study builds on these limited efforts 

using a multi-State database.

Methodology

This research examined the safety impacts 

of the combined application of centerline 

and shoulder rumble strips on two-lane 

rural roads in Kentucky, Missouri, and Pennsylvania. In Missouri and Pennsylvania, the 

rumble strips were installed where none 

existed before. In Kentucky, all roadways 

previously had shoulder rumble strips 

installed; therefore, the results for this State 

could be considered conservative in that 

even greater crash reductions would be 

expected for run-off-road crashes if shoulder 

rumble strips had not previously existed. It 

should be noted, however, that it is possible 

that the rumble strips had exceeded their 

service life, although this factor could not 

be determined. 

The objective was to estimate the safety 

effectiveness of this strategy as measured 

by changes in the frequency of crashes 
3

(excluding intersection-related and animalvehicle crashes). Target crash types included 

the following: 

• Total crashes (all types and severities 

combined).

• Injury crashes (K (fatal), A (incapacitating), B (non-incapacitating), and 

C (possible) injuries on KABCO scale).

• Run-off-road crashes (all severities 

combined).

• Head-on crashes (all severities 

combined).

• Sideswipe-opposite-direction crashes 

(all severities combined).

A further objective was to conduct a disaggregate analysis to investigate whether the 

safety effects varied by factors such the level 

of traffic volume, the frequency of crashes 

before treatment, vehicle speed, lane width, 

and shoulder width. It was also of interest 

to examine the differences between the 

combined effects of centerline and shoulder rumble strips and the effects of either 

in isolation.

The evaluation of overall effectiveness 

included the consideration of the installation costs and crash savings in terms of the 

benefit-cost (B/C) ratio. 

The EB methodology for observational 

before-after studies was used for the evaluation.(6) This methodology is considered 

rigorous in that it accounts for regressionto-the-mean using a reference group of 

similar but untreated sites. In the process, 

safety performance functions (SPFs) were 

not applied. SPFs are equations used to 

estimate the expected crash frequency 

of a site based on its characteristics that 

influence crashes (e.g., traffic volumes). 

The use of SPFs in the EB methodology 

addresses the following:

• It overcomes the difficulties of using 

crash rates in normalizing for volume 

differences between the before and 

after periods.

• It accounts for time trends.

• It reduces the level of uncertainty in the 

estimates of safety effect.

• It properly accounts for differences in 

crash experience and reporting practice 

in amalgamating data and results from 

diverse jurisdictions.

• The methodology provides a foundation for developing guidelines for 

estimating the likely safety consequences of a contemplated strategy.

The SPFs used in the EB methodology were 

estimated 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, an overdispersion parameter, 

which is used in the EB calculations, was 

estimated iteratively from the model and 

the data. For a given dataset, smaller values 

of this parameter indicate relatively better 

models. 

The full report includes a detailed explanation of the methodology, including a 

description of how the estimate of safety 

effects for target crashes was calculated.
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Results

Based on the data for all three States 

combined, results are presented in two 

parts. The first part contains aggregate 

results, and the second part is based on 

a disaggregate analysis that attempted to 

discern factors that may be most favorable 

to the installation of centerline plus shoulder rumble strips.

Aggregate Analysis

The aggregate results for all three States 

are shown in table 1, which provides the 

estimates of expected crashes in the after 

period without treatment, the observed 

crashes in the after period, and the estimated crash modification factor (CMF) and 

its standard error for all crash types considered. The percent change in crashes is 

100(1 − estimate of the CMF); thus, a CMF 

of 0.80 with a standard deviation of 0.025 

indicates a 20-percent reduction in crashes 

with a standard deviation of 2.5 percent.

The combined results in table 1 indicate 

reductions for all crash types analyzed that 

are statistically significant at the 95-percent 

confidence level. The crash type with the 

smallest CMF (which translates to the 

greatest reduction) is head-on, with a 

CMF of 0.632. Run-off-road and sideswipeopposite-direction crashes have estimated 

CMFs of 0.742 and 0.767, respectively. For 

all crash types combined, CMFs of 0.800 

for all severities and 0.771 for fatal+injury 

were estimated. It is important to remember that all crash types considered exclude 

intersection- and animal-related crashes.

The most comprehensive and reliable 

multi-State study to date of both shoulder 

and centerline rumble strips is published 

in NCHRP Report 641.(3) This report does 

not include findings for the combination 

of shoulder and centerline rumble strips 

but does recommend CMFs for these 

treatments separately. A comparison of 

the results for the combined treatment in 

table 1 with the recommended CMFs for 

the individual treatments is encouraging in 

that it appears that the effect of combining 

centerline and shoulder rumble strips further 

Total Injury ROR HO S-OD HO+

S-OD

ROR+HO+

S-OD

EB estimate of 

crashes expected 

in the after period 

without strategy

2409.00 986.63 712.11 102.64 101.41 204.05 916.15

Count of crashes 

observed in the after 

period

1,927 761 529 65 78 143 672

Estimate of CMF 0.800 0.771 0.742 0.632 0.767 0.700 0.733

Standard error of 

estimate of CMF 0.025 0.034 0.041 0.085 0.097 0.064 0.035

Table 1. Results for combined States.

HO = Head-on.

ROR = Run-off-road.

S-OD = Sideswipe-opposite-direction.
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reduces run-off-road crashes compared to 

shoulder rumble strips alone and total and 

fatal+injury crashes compared to centerline 

rumble strips alone. However, it appears 

that shoulder rumble strips did not reduce 

head-on+sideswipe-opposite-direction 

crashes further than applying centerline 

rumble strips in isolation, which is intuitive.

Disaggregate Analysis

The disaggregate analysis sought to identify 

those conditions under which the treatment 

is most effective. Run-off-road, head-on, 

and sideswipe-opposite-direction crashes 

were the focus of this analysis because they 

were the focus of this treatment. The analysis found no clear trend between the CMFs 

and values for posted speed, lane width, or 

shoulder width. Table 2 presents the results 

with respect to the other two variables.

For AADT, as shown in table 2, larger percentage crash reductions were found for 

run-off-road crashes for higher AADTs with 

some stability reached at approximately 

an AADT of 3,200. At AADTs above 3,200, 

the estimated CMF did not change significantly. At AADTs lower than 3,200, a runoff-road crash CMF of 0.851 was estimated 

versus 0.702 for AADTs at 3,200 or greater. 

For head-on+sideswipe-opposite-direction 

crashes, the stability in the CMF was 

reached at an AADT of approximately 9,200, 

and the trend was reversed with a CMF 

of 0.679 at AADTs lower than 9,200 and 

0.817 for AADTs greater than 9,200. A possible explanation for a larger CMF value for 

head-on+sideswipe-opposite-direction 

crashes is that at higher AADTs, there were 

fewer passing opportunities, and not all 

head-on or sideswipe-opposite-direction 

crashes were due to vehicles drifting out of 

their lane.

For the expected crash frequency per 

mi-year without treatment, as shown in 

table 2, larger percentage crash reductions 

were found for run-off-road crashes for 

higher crash frequencies with some stability 

reached at a crash rate of approximately 

0.500/mi-year. At rates lower than 0.500, a 

run-off-road crash CMF of 0.840 was estimated versus 0.621 for rates at 0.500 or 

greater. For head-on+sideswipe-oppositedirection crashes, the stability in the CMF 

was reached at a rate of approximately 

0.065, and the trend was reversed with a 

CMF of 0.608 at rates less than 0.065 and 

0.715 for rates greater than 0.065. Because 

expected crashes increased with volume 

Crash Type

AADT Expected Crashes/mi-year Without Treatment

Range CMF (Standard Error) Range CMF (Standard Error)

Run-off-road

< 3,200 0.851 (0.089) < 0.500 0.840 (0.058)

≥ 3,200 0.702 (0.045) ≥ 0.500 0.621 (0.055)

Head-on+sideswipeopposite-direction

< 9,200 0.679 (0.069) < 0.065 0.608 (0.147)

> 9,200 0.817 (0.172) ≥ 0.065 0.715 (0.071)

Table 2. Results disaggregated by ranges of average annual daily traffic (AADT) and expected crash frequency.
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as seen in the SPFs developed, the trend of 

a larger CMF at a higher crash rate for 

head-on+sideswipe-opposite-direction 

crashes would be expected, given the 

results for AADT.

Caution should be used in interpreting 

and applying these disaggregate results 

because they are not robust enough to 

develop CMFunctions. A CMFunction is an 

equation that would allow the estimation 

of CMFs for different levels of AADT and 

expected crash frequency. However, they 

may be used in prioritizing treatment sites. 

For example, sites with a high proportion 

of run-off-road crashes and high AADTs 

will have higher priority than sites with 

high AADTs and a high proportion of headon+sideswipe-opposite-direction crashes.

Economic Analysis

For the purposes of the economic analysis, 

the assumed treatment is, conservatively, 

the dual application of centerline and 

shoulder rumble strips for which the 

combined CMF of 0.800 for total crashes 

(table 1) is recommended. Treatment costs 

ranged from $3,000/mi for Missouri to 

$12,000/mi in Kentucky. Service lives are 

7 to 10 years and 12 to 15 years, respectively. Results are presented for these two 

extremities.

Based on information from the Office of 

Management and Budget’s Circular A-4, a 

real discount rate of 7 percent was applied 

to calculate the annual cost of the treatment 

for 7- and 12-year service lives, respectively.(7)

Applying the lower ends of the service life 

ranges conservatively gives annual costs 

of $557/mi and $1,551/mi for the two cost/

service life extremes.

The most recent FHWA mean comprehensive crash costs disaggregated by crash 

severity, location type, and speed limit are 

based on 2001 dollar values.(8) The 2001 

unit costs for property damage only (PDO) 

and fatal+injury crashes from the FHWA 

report ($7,428 and $158,177) were multiplied 

by the ratio of the 2014 value of a statistical life of $9.2 million to the 2001 value of 

$3.8 million.(9,8) By applying this ratio of 2.42 

to the unit costs for PDO and fatal+injury 

crashes and then weighting by the frequencies of these two crash types in the after 

period, an aggregate 2014 unit cost for total 

crashes of $162,045 was obtained. Fatal 

crashes were not considered on their own because of the very low numbers of such crashes in the data, which would skew the results. 

The total crash reduction was calculated by 

subtracting the actual crashes in the after 

period from the expected crashes in the 

after period had the treatment not been 

implemented. The number of crashes saved 

per mi-year was 0.1881, which was obtained 

by dividing the total crash reduction (482.0) 

by the number of after period mi-years per 

site (2,562). 

The annual benefit (i.e., crash savings) of 

$30,481 is the product of the crash reduction 

per mi-year (0.1881) and the aggregate 

cost of a crash (all severities combined) 

($162,045). The B/C ratio was calculated as 

the ratio of the annual benefit per mi to 

the annual cost per mi. The B/C ratios were 

estimated to be 20.2 for the higher cost/

higher service life assumption and 54.7 

for the lower cost/lower service life 

assumption. These results suggest that the 

treatment, even in its most expensive variation, can be highly cost effective.

Summary and Conclusions

The objective of this study was to perform a rigorous before-after evaluation of 
7

the safety effectiveness, as measured by 

crash frequency, of shoulder and centerline 

rumble strips applied in combination on twolane rural roads. The CMFs shown in table 3 

are recommended for various crash types.

To date, the most comprehensive and 

reliable study of both shoulder and 

centerline rumble strips individually 

applied is published in NCHRP Report 641—

Guidance for the Design and Application of 

Shoulder and Centerline Rumble Strips.

(3)

Compared to the recommended CMFs from 

that study, the results suggest that the 

effect of combining centerline and shoulder 

rumble strips further reduces run-off-road 

crashes compared to shoulder rumble

strips alone and both total and fatal+injury 

crashes compared to centerline rumble 

strips alone. However, it appears that 

shoulder rumble strips do not further reduce 

head-on+sideswipe-opposite-direction 

crashes than applying centerline rumble 

strips in isolation.

A disaggregate analysis of the results 

indicated that larger percentage crash 

reductions were found for run-off-road 

crashes for sites with higher AADTs. For 

head-on+sideswipe-opposite-direction 

crashes, smaller percentage crash reductions were found for higher AADTs. For 

the expected crash frequency per mi-year 

without treatment, larger percentage crash 

reductions were found for run-off-road 

crashes for higher crash frequencies. For 

head-on+sideswipe-opposite-direction 

crashes, smaller percentage crash reductions were seen at higher crash frequencies. 

Caution should be used in interpreting and 

applying these disaggregate results because 

they are not robust enough to develop 

CMFunctions that would allow the estimation of CMFs for different levels of AADT 

and expected crash frequency. However, 

they may be used in prioritizing treatment 

sites.

B/C ratios were estimated to range from 

20.2 for a higher cost/higher service life 

assumption to 54.7 for a lower cost/lower 

service life assumption. These results, which 

are based on conservative service life 

assumptions, suggest that the treatment, 

even in its most expensive variations, can 

be highly cost effective. 

Crash Type CMF Standard Error of CMF

Total 0.800 0.025

Injury 0.771 0.034

Run-Off-Road 0.742 0.041

Head-On 0.632 0.085

Sideswipe-Opposite-Direction 0.767 0.097

Head-On+Sideswipe-Opposite-Direction 0.700 0.064

Run-Off-Road+Head-On+SideswipeOpposite-Direction 0.733 0.035

Table 3. Recommended CMFs.
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SEPTEMBER 2015 FHWA-HRT-15-064

HRDS-20/09-15(200)E

Researchers—Craig Lyon, 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—Rumble strips, low-cost, safety improvements, safety evaluations, empirical Bayes.

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 

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References

1. Neuman, T.R., Pfefer, R., Slack, K.L., Hardy, 

K.K., Council, F., McGee, H., Prothe, L., and 

Eccles, K. Volume 6: A Guide for Addressing 

Run-Off-Road Collisions. Transportation 

Research Board, Washington, DC, 2003a.

2. Neuman, T.R., Pfefer, R., Slack, K.L., Hardy, 

K.K., Council, F., McGee, H., Prothe, L., and 

Eccles, K. Volume 4: A Guide for Addressing 

Head-On Collisions. Transportation Research 

Board, Washington, DC, 2003b.

3. Torbic, D., Hutton, J.M., Bokenkroger, C.D., 

Bauer, K.M., Harwood, D.W., Gilmore, D.K., 

Dunn, J.M., Ronchetto, J.J., Donnell, E.T., 

Sommer, H.J., Garvey, P., Persaud, B., and 

Lyon, C. NCHRP Report 641—Guidance for 

the Design and Application of Shoulder and 

Centerline Rumble Strips. Transportation 

Research Board, Washington, DC, 2009.

4. Torbic, D., Bauer, K., Hutton, J., and Campbell, 

J. Delta Region Transportation Development 

Program: Rural Safety Innovation Program 

Evaluation. Draft Final Report. Federal Highway Administration, 2013.

5. Sayed, T., deLeur, P., and Pump, J. “Impact of 

Rumble Strips on Collision Reduction 

on Highways in British Columbia, Canada: 

Comprehensive Before-and-After Safety 

Study.” Transportation Research Record 

2148. Transportation Research Board, 

Washington, DC, 2010, pp 9–15.

6. Hauer, E. Observational Before-After Studies 

in Road Safety—Estimating the Effect of 

Highway and Traffic Engineering Measures 

on Road Safety. Elsevier Science, Incorporated, 1997.

7. Office of Management and Budget. Circular 

A-4. September 17, 2003. https://www.

whitehouse.gov/omb/circulars_a004_a-4/.

8. U.S. Department of Transportation. Guidance 

on Treatment of the Value of a Statistical Life 

(VSL) in U.S. Department of Transportation 

Analyses—2014 Adjustment. Memo, June 

13, 2014. Accessed November 12, 2014. 

http://www.dot.gov/sites/dot.gov/files/docs/

VSL_Guidance_2014.pdf.

9. Zaloshnja, E., Miller, T. Council, F., and 

Persaud, B. “Crash Costs in the United States 

by Crash Geometry.” Accident Analysis and 

Prevention 38(4), 2006, pp 644–651.