Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Safety Evaluation of Centerline Plus 

Shoulder Rumble Strips

PUBLICATION NO. FHWA-HRT-15-048 JUNE 2015
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). The 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 the implementation of the American Association 

of State Highway and Transportation Officials Strategic Highway Safety Plan. The ELCSI-PFS 

studies provide a crash modification factor (CMF) and benefit-cost (B/C) economic analysis for 

each of the targeted safety strategies identified as priorities by the pooled fund member states.

The combined application of centerline and shoulder rumble strips evaluated under this pooled 

fund study is intended to reduce the frequency of crashes by alerting drivers that they are about 

to leave the travelled lane. Geometric, traffic, and crash data were obtained at treated two-lane 

rural road locations in Kentucky, Missouri, and Pennsylvania. The results of this evaluation show 

that head-on, run-off-road, and sideswipe-opposite-direction crashes were significantly reduced, 

and application of centerline and shoulder rumble strips also has potential to reduce crash 

severity for all types of crashes.

Monique R. Evans, P.E.

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-15-048

2. Government

Accession No.

3. Recipient's Catalog No.

4. Title and Subtitle

Safety Evaluation of Centerline Plus Shoulder Rumble Strips

5. Report Date

June 2015

6. Performing Organization 

Code

7. Author(s) 

Lyon, Craig; Bhagwant Persaud; and Kimberly Eccles.

8. Performing Organization 

Report No.

9. Performing Organization Name and Address

Vanessa Hangen Brustlin, Inc (VHB) Persaud Lyon, Inc 

8300 Boone Blvd., Ste. 700 87 Elmcrest Road

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

10. Work Unit No.

11. Contract or Grant No.

DTFH61-13-D-00001

12. Sponsoring Agency Name and Address

Office of Safety Research and Development

Federal Highway Administration

6300 Georgetown Pike

McLean, VA 22101-2296

13. Type of Report and 

Period

Safety Evaluation

14. Sponsoring Agency 

Code: FHWA

15. Supplementary Notes. The Federal Highway Administration (Office of Safety Research and Development) 

managed this study. The project team members were Craig Lyon, Dr. Bhagwant Persaud, Kimberly Eccles, and 

Jonathan Soika. The FHWA Office of Safety Research and Development Contract Task Order Manager was Roya 

Amjadi.

16. Abstract

The Federal Highway Administration organized a pooled fund study of 38 States to evaluate low-cost safety 

strategies as part of its strategic highway safety effort. One of the strategies selected for evaluation was the 

combined application 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. Geometric, traffic, and crash data were 

obtained at treated two-lane rural road locations in Kentucky, Missouri, and Pennsylvania. To account for potential 

selection bias and regression-to-the-mean, an Empirical Bayes before-after analysis was conducted, using reference 

groups of untreated two-lane rural roads with similar characteristics to the treated sites. The analysis also controls 

for changes in traffic volumes over time and time trends in crash counts unrelated to the treatment. The combined 

results for all States indicate statistically significant crash reductions for all crash types analyzed. The crash type 

with the smallest crash modification factor (CMF) (i.e., the greatest crash reduction) is head-on, with a CMF of 

0.632. Run-off-road and sideswipe-opposite-direction crashes have estimated CMFs of 0.742 and 0.767, 

respectively. For run-off-road, head-on, and sideswipe-opposite-direction crashes combined (i.e., lane departure 

crashes), the estimated CMF is 0.733. For all crash types combined, CMFs of 0.800 for all severities and 0.771 for 

fatal+injury were estimated. Intersection-related and animal crashes were excluded from the evaluation. Benefit-cost 

ratios were estimated to range from 20.2 to 54.7, depending on the treatment cost and service life assumption, which 

varied by State. These results are based on conservative service life assumptions.

17. Key Words: Rumble strips, low-cost, safety 

improvements, safety evaluations, Empirical Bayesian.

18. Distribution Statement

No restrictions. This document is available to the 

public 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: 

59

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed pages authorized
ii
TABLE OF CONTENTS

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

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

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

BACKGROUND ON STUDY.................................................................................................. 3

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

SRS.......................................................................................................................................... 4

CLRS....................................................................................................................................... 8

Additional Research .............................................................................................................. 11

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

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

SAMPLE SIZE ESTIMATION OVERVIEW ..................................................................... 15

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

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

KENTUCKY............................................................................................................................ 23

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

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

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

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

Crash Data ............................................................................................................................. 24

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

MISSOURI............................................................................................................................... 25

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

Reference Sites...................................................................................................................... 25

Roadway Data ....................................................................................................................... 25

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

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

Treatment Cost Data.............................................................................................................. 26

PENNSYLVANIA................................................................................................................... 26

Installation Data .................................................................................................................... 26

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

Roadway Data ....................................................................................................................... 27

Traffic Data ........................................................................................................................... 27

Crash Data ............................................................................................................................. 27

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

Data Characteristics and Summary ....................................................................................... 28

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

KENTUCKY SPFS ................................................................................................................. 33

MISSOURI SPFS .................................................................................................................... 34

PENNSYLVANIA SPFS......................................................................................................... 35

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

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

DISAGGREGATE ANALYSIS............................................................................................. 40

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

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

APPENDIX: ADDITIONAL INSTALLATION DETAILS FROM STATES...................... 47

DETAILS OF THE KENTUCKY SITES............................................................................. 47

DETAILS OF THE MISSOURI INSTALLATIONS .......................................................... 48

DETAILS OF THE PENNSYLVANIA INSTALLATIONS............................................... 49

ACKNOWLEDGEMENTS........................................................................................................ 51

REFERENCES............................................................................................................................ 53

iv
LIST OF FIGURES

Figure 1. Equation. Estimated change in safety.............................................................................19

Figure 2. Equation. EB estimate of expected crashes....................................................................19

Figure 3. Equation. EB weight.......................................................................................................20

Figure 4. Equation. Index of effectiveness.....................................................................................20

Figure 5. Equation. Standard deviation of index of effectiveness .................................................20

Figure 6. Equation. SPF model form for Kentucky .......................................................................33

Figure 7. Equation. SPF model form for Missouri.........................................................................34

Figure 8. Equation. SPF model form for Pennsylvania..................................................................35

LIST OF TABLES

Table 1. Information on safety effects of SRS in table 4 of NCHRP Report 641............................5

Table 2. Information on safety effects of CLRS in table 5 of NCHRP Report 641.........................9

Table 3. Before period crash rate assumptions ..............................................................................15

Table 4. Minimum required before period mile-years for treated sites .........................................16

Table 5. Analysis for crash effects detectable with available sample size.....................................17

Table 6. Illustration of alternate approach .....................................................................................21

Table 7. Kentucky treatment cost and service life data..................................................................25

Table 8. Pennsylvania treatment cost data, 2009–2011 .................................................................28

Table 9. Definitions of crash types.................................................................................................29

Table 10. Data summary for treatment sites ..................................................................................30

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

Table 12. Kentucky SPFs...............................................................................................................33

Table 13. Missouri before period SPFs..........................................................................................34

Table 14. Missouri after period SPFs.............................................................................................34

Table 15. Pennsylvania SPFs.........................................................................................................35

Table 16. Results for Kentucky......................................................................................................37

Table 17. Results for Missouri .......................................................................................................38

Table 18. Results for Pennsylvania ................................................................................................38

Table 19. Results for combined States...........................................................................................40

Table 20. Comparison of Pennsylvania crash rates........................................................................41

Table 21. Results disaggregated by ranges of AADT and expected crash frequency ...................42

Table 22. Recommended CMFs.....................................................................................................45

Table 23. Installation details for Kentucky ....................................................................................47

Table 24. Installation details for Missouri .....................................................................................48

Table 25. Installation details for Pennsylvania ..............................................................................49

Table 26. Installation details for bicycle tolerable rumble strips for Pennsylvania .......................49

v
LIST OF ABBREVIATIONS

AADT Average annual daily traffic 

B/C Benefit-cost

C-G Comparison group

CLRS Centerline rumble strips

CMF Crash modification factor

EB Empirical Bayes 

FI Fatal+injury

FHWA Federal Highway Administration 

GIS Geographic information system

KYTC Kentucky Transportation Cabinet

MoDOT Missouri Department of Transportation

NCHRP National Cooperative Highway Research Program

PDO Property Damage Only

PennDOT Pennsylvania Department of Transportation

SPF Safety performance functions

SRS Shoulder rumble strip

SVROR Single-vehicle run-off-road

LIST OF SYMBOLS

Greek letter delta

Greek letter lamda

Greek letter pi

Greek letter theta

Δ

λ

π

θ

vi
EXECUTIVE SUMMARY

The Federal Highway Administration (FHWA) organized a pooled fund study of 38 States to 

evaluate low-cost safety strategies as part of its strategic highway safety effort. The purpose of 

the FHWA Low-Cost Safety Improvements Pooled Fund Study is to evaluate the safety 

effectiveness of several low-cost safety strategies through scientifically rigorous crash-based 

studies. One of the strategies selected for evaluation for this study was the application of shoulder 

rumble strips (SRS) and centerline rumble strips (CLRS) in combination. This strategy is 

intended to reduce the frequency of crashes by alerting drivers that they are about to leave the 

travelled lane. While research has been published on the safety effectiveness of SRS or CLRS

used in isolation, the effectiveness of the combined treatment has not been shown.

Geometric, traffic, and crash data were obtained at treated two-lane rural road locations in 

Kentucky, Missouri and Pennsylvania. To account for potential selection bias and regression-tothe-mean, an Empirical Bayes (EB) before-after analysis was conducted using reference groups 

of untreated two-lane rural roads with similar characteristics to the treated sites. A slightly 

different approach was required for the analysis of the treatment sites in Missouri, which is 

installing rumble strips on two-lane rural roads whenever a resurfacing project is undertaken. As 

a result, a suitable reference group with no rumble strips for this road type presently or in the near 

future did not exist. The analysis also controls for changes in traffic volumes over time and time 

trends in crash counts unrelated to the treatment.

The combined results for all States indicate reductions in crashes for all crash types analyzed that 

are statistically significant at the 95-percent confidence level (i.e., 5-percent significance level). 

The crash type with the smallest crash modification factor (CMF) (i.e., the great crash reduction) 

is head-on, with a CMF of 0.632. Run-off-road and sideswipe-opposite-direction crashes have 

estimated CMFs of 0.742 and 0.767, respectively. For run-off-road, head-on, and sideswipeopposite-direction crashes combined (i.e., lane departure crashes), the estimated CMF is 0.733. 

For all crash types combined, CMFs of 0.800 for all severities and 0.771 for fatal+injury (FI)

were estimated. It is important to remember that all crash types considered exclude intersectionrelated and animal crashes.

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

the CMF and values for posted speed, lane width, or shoulder width. Larger percentage crash 

reductions were found for run-off-road crashes at higher average annual daily traffic (AADT). 

For head-on+sideswipe-opposite-direction crashes, the trend is reversed with smaller percentage 

crash reductions at higher AADTs. 

For the expected crash frequency, larger percentage crash reductions were found for run-off-road 

crashes for higher crash frequencies. For head-on+sideswipe-opposite-direction crashes, the trend 

is reversed with smaller percentage crash reductions at higher crash rates. Because expected 

crashes increase with volume as seen in the Safety Performance Functions (SPF) developed, the 

trend of lower percentage crash reductions at higher crash rates for head-on+sideswipe-oppositedirection crashes would be expected given the results for AADT.

1
Benefit-cost (B/C) ratios are estimated to range from 20.2 for a higher cost/higher service life 

assumption (based on Kentucky information) to 54.7 for a lower cost/lower service life 

assumption (based on information from Missouri). 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.

2
CHAPTER 1. INTRODUCTION

BACKGROUND ON STRATEGY

This strategy involves the application of CLRS and SRS in combination. SRS may be placed on 

the edge line or offset some distance into a paved shoulder. 

As described in volume 6 and volume 4 of the National Cooperative Highway Research Program 

(NCHRP) 500 Series Reports, SRS are crosswise grooves in the road shoulder, generally 

0.5 inches deep, spaced about 7 inches apart, and cut in groups of 4 or 5.

(1,2) States have 

developed various designs and methods of installation, including rolling the rumble strips into hot 

asphalt or concrete as it is 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. SRS are used extensively in the United States on 

all types of roadways. 

CLRS are similar to SRS but are placed on the center line and typically extend into the travel lane 

by 5 inches to 1.5 ft. They may be placed continuously or with periodic gaps.

SRS and CLRS 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.

While research into the performance of SRS and CLRS has been conducted, the combination of 

SRS and CLRS is still relatively rare and has not been previously evaluated.

Additional details concerning current practice with rumble strips can be found on FHWA’s 

Office of Safety Rumble Strip Web site at the following URL: 

http://safety.fhwa.dot.gov/roadway_dept/pavement/rumble_strips/. This site provides technical 

advisories regarding SRS and CLRS along with other information of interest. 

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 emphasis areas that affect highway safety.

NCHRP published a series of guides to advance the implementation of countermeasures targeted 

to reduce crashes and injuries. Each guide addresses one of the emphasis areas and includes an 

introduction to the problem, a list of objectives for improving safety, 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; about 80 percent of the strategies 

are considered tried or experimental.

In 2005, to support the implementation of the guides, FHWA organized a pooled fund study to 

evaluate low-cost safety strategies as part of this strategic highway safety effort. Over the years 

3
the pooled fund has grown in size and now includes 38 States. The purpose of the pooled fund 

study is to evaluate the safety effectiveness of several tried and experimental, low-cost safety 

strategies through scientifically rigorous crash-based studies. The use of CLRS in combination 

with SRS was selected as a strategy to be evaluated as part of this effort. 

LITERATURE REVIEW

A recent and comprehensive study of SRS and CLRS is documented in NCHRP Report 641—

Guidance for the Design and Application of Shoulder and Centerline Rumble Strips.

(3) This 

report includes a thorough literature review, which is summarized herein. A list of the critical 

references from the NCHRP report is included at the end of this document, after the references 

for this report.

SRS

A summary of previous research is shown in table 1, which is a reproduction of table 4 from 

NCHRP Report 641. This table shows the location of the evaluation, facility type, collision types 

analyzed, estimated effects, and the methodology applied. Most of the evaluations at that time 

had focused on freeways, with a limited number looking at non-freeway facilities. Collision types 

included single-vehicle run-off-road (SVROR) and in some cases total collisions. Effects for runoff-road collisions ranged from a 10 to 80 percent reduction, with an average of 36 percent. 

Effects for total crashes ranged from a 13 to 33 percent reduction, with an average of 21 percent.

4
Table 1. Information on safety effects of SRS in table 4 of NCHRP Report 641.

(3)

State/Location

Type of 

Facility

Type of collisions 

targeted

Percent decrease (-) 

or percent increase 

(+) in target collision 

frequency from 

application of 

shoulder rumble 

strips (standard 

deviation)

Type of 

analysis

Arizona (16) Interstate SVROR -80 percent Cross-sectional 

comparison

California (17) Interstate

SVROR -49 percent Before-after 

with comparison 

sites Total -19 percent

Connecticut 

(18)

Limitedaccess 

roadways

SVROR -32 percent

Before-after 

with comparison 

sites

Florida (16) Fixed object -41 percent Naïve beforeRan-into-water -31 percent after

Illinois and 

California (1)

Freeways

SVROR (total) -18 percent

(±6.8 percent) Before-after 

with marked 

comparison sites 

and a 

comparison 

group

SVROR (injury) -13 percent

(±11.7 percent)

Rural 

freeways

SVROR (total) -21.1 percent

(±10.2 percent)

SVROR (injury) -7.3 percent

(±15.5 percent)

Kansas 

(unpublished; 

cited in Stutts 

(19))

Freeways SVROR -34 percent Unknown

Maine (20) Rural 

freeways

Total Inconclusive

Before-after 

with comparison 

sites

Massachusetts 

(unpublished; 

cited in Stutts 

(19))

SVROR -42 percent Unknown

Michigan (21) SVROR -39 percent Cross-sectional 

comparison

5
State/Location

Type of 

Facility

Type of collisions 

targeted

Percent decrease (-) 

or percent increase 

(+) in target collision 

frequency from 

application of 

shoulder rumble 

strips (standard 

deviation)

Type of 

analysis

Minnesota (3)

Rural 

multilane 

divided 

highways

Total -16 percent

Naïve beforeafter

Injury -17 percent

SVROR (total) -10 percent

SVROR (injury) -22 percent

Total -21 percent Before-after 

with comparison 

sites

Injury -26 percent

SVROR (total) -22 percent

SVROR (injury) -51 percent

Minnesota (2) Rural twolane roads

SVROR (total) -13 percent

(8 percent) Before-after EB 

analysis with a 

SVROR (injury) reference group -18 percent

(12 percent)

Montana (22)

Interstate 

and 

primary 

highways

SVROR -14 percent

Before-after 

with comparison 

sites

New Jersey 

(unpublished; 

cited in Stutts 

(19))

SVROR -34 percent Unknown

New York (23) Interstate 

Parkway SVROR -65 percent to 70 percent

Naïve beforeafter

Pennsylvania 

(24) Interstate SVROR -60 percent Naïve beforeafter

Tennessee (25) Interstate SVROR -31 percent Unknown

Utah (26) Interstate

SVROR -27 percent Before-after 

with comparison 

sites Total -33 percent

Virginia (27) Rural 

freeways

SVROR -52 percent

Before-after 

with comparison 

sites

Washington 

(15) Total -18 percent Naïve beforeafter

Multistate (16) Rural 

freeways

SVROR -20 percent

Before-after 

with comparison 

sites

Note: The follow reference callouts are numbered and presented in the same manner as in the original reference.

1. Griffith, M. S., Safety Evaluation of Rolled-in Continuous Shoulder Rumble Strips Installed on Freeways, In 

Transportation Research Record, No. 1665, TRB, National Research Council, Washington, D.C., 1999.

6
2. Patel, R. B., F. M. Council, and M. S. Griffith, Estimating the Safety Benefits of Shoulder Rumble Strips on Two 

Lane Rural Highways in Minnesota: An Empirical Bayes Observational Before-After Study, Presented at the 86th 

Annual Meeting of the Transportation Research Board, Washington, D.C., January 2007.

3. Carrasco, O., J. McFadden, P. Chandhok, and R. Patel, Evaluation of the Effectiveness of Shoulder Rumble Strips 

on Rural Multi- lane Divided Highways in Minnesota, Presented at the 83rd Annual Meeting of the 

Transportation Research Board, Washington, D.C., January 2004.

15. Harwood, D. W., NCHRP Synthesis of Highway Practice 191: Use of Rumble Strips to Enhance Safety, TRB, 

National Research Council, Washington, D.C., 1993.

16. Ligon, C. M., E. C. Carter, D. B. Joost, and W. F. Wolman, Effects of Shoulder Textured Treatments on Safety, 

Report No. FHWA/RD-85/027, FHWA, U.S. Department of Transportation, 1985.

17. Chaudoin, J. H., and G. Nelson, Interstate Routes 15 and 40 Shoulder Rumble Strips, Report Caltrans-08-85-1. 

California Department of Transportation, August 1985.

18. Annino, J.M., Rumble Strips In Connecticut: A Before/After Analysis of Safety Benefits, Connecticut Department 

of Transportation, 2003.

19. Stutts, J. C., NCHRP Synthesis of Highway Practice 287: Sleep Deprivation Countermeasures for Motorists 

Safety, TRB, National Research Council, Washington, D.C., 2000.

20. Gårder, P., and J. Alexander, Continued Research on Continuous Rumble Strips. Final Report, Technical Report 

94-4. Maine Department of Transportation, December 1995.

21. Morena, D.A., The Nature and Severity of Drift-Off Road Crashes On Michigan Freeways, and the Effectiveness 

of Various Shoulder Rumble Strip Designs. Presented at the 82nd Annual Meeting of the Transportation 

Research Board, Washington, D.C., 2003.

22. Marvin, R.R., and D.J. Clark, An Evaluation of Shoulder Rumble Strips In Montana, Montana Department of 

Transportation, 2003.

23. Perrillo, K., The Effectiveness and Use of Continuous Shoulder Rumble Strips, Federal Highway Administration, 

Albany, NY, 1998.

24. Hickey, J.J., Jr., “Shoulder Rumble Strip Effectiveness, Drift-Off-Road Accident Reductions on the 

Pennsylvania Turnpike.” In Transportation Research Record 1573. TRB, National Research Council, 

Washington, D.C., 1997, pp. 105–109.

25. Tennessee Department of Transportation, Statewide Installation of Shoulder Rumble Strips on Tennessee’s 

Interstates. http://www.tdot.state.tn.us/rumble/release.htm. Accessed September 2005.

26. Cheng, E. Y. C., E. Gonzalez, and M. O. Christensen, Application and Evaluation of Rumble Strips on 

Highways, Utah Department of Transportation, Compendium of Technical Papers, 64th ITE Annual Meeting, 

Dallas, Texas, 1994.

27. Chen, C., E. O. Darko, and T. N. Richardson, Optimal Continuous Shoulder Rumble Strips and the Effects on 

Highway Safety and the Economy, ITE Journal, Vol. 73, No. 5, May 2003.

The original research documented in NCHRP Report 641 focused on total, FI, SVROR, and 

SVROR FI collisions. Site types included urban freeways, rural freeways, rural multi-lane 

divided roads, and rural two-lane roads from Pennsylvania, Missouri, and Minnesota. The authors 

recommend that the following effects of SRS for rural freeways and rural two-lane roads be 

considered based on their research and previous credible studies: 

Rural Freeways:

• 11 percent reduction in SVROR crashes.

• 16 percent reduction in SVROR FI crashes.

Rural Two-Lane Roads:

• 15 percent reduction in SVROR crashes.

• 29 percent reduction in SVROR FI crashes.

7
For urban freeway and rural multi-lane divided roads, the results were deemed to be insignificant 

and unreliable, so there was no recommendation.

Subsequent disaggregate analyses indicated the following:

• On rural freeways, SRS placed closer to the edge line (i.e., edgeline rumble strips) are 

more effective in reducing SVROR FI crashes than those placed further from the edge 

line (i.e., non-edgeline rumble strips).

• On rural two-lane roads, there is no difference in the safety effect of SRS placed 

closer to the edge line (i.e., edgeline rumble strips) as compared with rumble strips 

placed further from the edge line (i.e., non-edgeline rumble strips).

• On rural freeways, SRS resulted in an estimated reduction of SVROR crashes 

involving heavy vehicles by approximately 40 percent.

• On rural two-lane roads, there is no evidence that suggests SRS may result in a 

reduction of SVROR crashes involving heavy vehicles.

• SRS appear to provide a positive safety benefit during low-lighting conditions.

CLRS

A summary of previous research is shown in table 2, a reproduced version of table 5 from 

NCHRP Report 641. This table shows the location of the evaluation, facility type, collision types 

analyzed, estimated effects, and the methodology applied. Although most of the previous studies 

used poor study methods, they are quite consistent in observing collision reductions for total and 

specific collisions related to a vehicle crossing the center line. All but one study looked at twolane rural roads. The one remaining study did consider rural multi-lane roads. Effects for head-on 

crashes ranged from 34 to 95 percent, with an average of 65 percent. It should be noted that the 

Highway Safety Manual, First Edition only recommends the results for one of the studies 

(reference 4 in table 2) concerning rural two-lane roads. The methodologies applied for the other 

studies are suspect, and therefore, those results are not recommended.

8
Table 2. Information on safety effects of CLRS in table 5 of NCHRP Report 641.

(3)

State/Location Type of facility

Type of collisions 

targeted

Percent decrease 

(-) or percent 

increase (+) in the 

target collision 

frequency from 

application of 

centerline rumble 

strips (95-percent

confidence 

interval)

Type of 

analysis

California (29) Rural two-lane Head-on (total) -42 percent Naïve 

Head-on (fatal) -90 percent before-after

Colorado (30) Rural two-lane road Head-on -34 percent Naïve 

Sideswipe -36.5 percent before-after

Delaware (31) Rural two-lane road

Head-on -95 percent

Naïve 

before-after

Drove left of center -60 percent

PDO +13 percent

Injury +4 percent

Fatal N/A

Total -8 percent

Massachusetts 

(32) Rural two-lane

Head-on

Inconclusive

Before-after 

with 

comparison 

group

Opposite-direction 

angle

Opposite-direction 

sideswipe

SVROR with 

centerline 

encounters

Minnesota (33) Rural two-lane 

roads

Total -42 percent

Crosssectional 

comparison

Total (fatal and 

severe injury) -73 percent

Head-on/oppositedirection/

sideswipe/

SVROR-to-the-left 

(all severities)

-43 percent

Head-on/oppositedirection 

sideswipe/SVRORto-the-left (fatal and 

severe injury

13 percent

9
State/Location Type of facility

Type of collisions 

targeted

Percent decrease 

(-) or percent 

increase (+) in the 

target collision 

frequency from 

application of 

centerline rumble 

strips (95-percent

confidence 

interval)

Type of 

analysis

Missouri (34) Rural two-lane 

roads Total -60 percent Naïve before-after

Nebraska (35) Rural two-lane 

roads Cross-over crashes -64 percent Naïve before-after

Oregon (36) Rural two- and 

four-lane highways Cross-over crashes

-69.5 percent

-79.6 percent

Naïve 

before-after

Before-after 

with 

comparison 

group

Multistate (4) Rural two-lane 

roads

Total -14 percent

(8–20 percent)

Empirical 

Bayes 

before-after

Injury -15 percent

(5–25 percent)

Frontal/oppositedirection sideswipe 

(total)

-21 percent

(5–37 percent)

Frontal/oppositedirection sideswipe 

(injury)

-25 percent

(5–45 percent)

Note: The follow reference callouts are numbered and presented in the same manner as in the original reference.

4. Persaud, B. N., R. A. Retting, and C. A. Lyon, Crash Reduction Following Installation of Centerline Rumble Strips 

on Rural Two-lane Roads. Insurance Institute for Highway Safety, Arlington, VA, September 2003. 

http://www.dot.state.mn.us/trafficeng/safety/rumble/IIHS_report.pdf. Accessed November 2005.

29. Fitzpatrick, K., K. Balke, D. W. Harwood, and I. B. Anderson, NCHRP Report 440: Accident Mitigation Guide for 

Congested Rural Two-Lane Highways, Transportation Research Board, National Research Council, Washington, 

D.C., 2000.

30. Outcalt, W., Centerline Rumble Strips, Report No. CDOT-DTD- R-2001-8. Colorado Department of 

Transportation, August 2001.

31. Delaware Department of Transportation (DelDOT). CRS: The Delaware Experience. 

http://www.deldot.net/static/projects/rumblestrip/handout.pdf. Accessed November 2005.

32. Noyce, D. A., and V. V. Elango, Safety Evaluation of Centerline Rumble Strips: A Crash and Driver Behavior 

Analysis, In Transportation Research Record, No. 1862, TRB, National Research Council, Washington, D.C., 

2004.

33. Briese, M., Safety Effects of Centerline Rumble Strips in Minnesota, Capstone Project for Infrastructure Systems 

Engineering Program, University of Minnesota, December 2006.

34. Missouri Department of Transportation, unpublished results provided to the research team.

35. Nebraska Department of Roads, unpublished results provided to the research team.

36. Russell, E. R., and M. J. Rys, NCHRP Synthesis 339: Centerline Rumble Strips. TRB, National Research Council, 

Washington, D.C., 2005.

10
The original research documented in NCHRP Report 641 focused on total, FI, target, and target 

FI collisions. Target collisions included head-on and sideswipe-opposite-direction. Site types 

included urban two-lane roads and rural two-lane roads from Pennsylvania, Minnesota, and 

Washington. The authors recommend that the following effects for CLRS be considered based on 

their research and previous credible studies:

Urban Two-Lane Roads:

• 40 percent reduction in total target crashes.

• 64 percent reduction in FI target crashes.

Rural Two-Lane Roads:

• 9 percent reduction in total crashes.

• 12 percent reduction in FI crashes.

• 30 percent reduction in total target crashes (head-on and sideswipe-oppositedirection).

• 44 percent reduction in FI target crashes (head-on and sideswipe-opposite-direction).

A disaggregate analysis indicated no difference in effectiveness between horizontal curves and 

tangent segments for total target collisions. There were some limited mileage installations of 

shoulder and CLRS in combination but not enough to allow a formal evaluation.

Additional Research

Sayed et al. evaluated the safety impacts of applying CLRS and SRS alone and in combination on 

two-lane rural and four-lane divided rural highways in British Columbia, Canada.(4) The EB 

before-after study approach was applied. Results for the combined application on two-lane roads 

indicated a reduction of 21.4 percent in off-road right, off-road left, and head-on collisions 

combined. SRS on their own indicated a reduction of off-road right collisions of 26.1 percent on 

two-lane roads and 18.4 percent on four-lane divided roads. CLRS on their own on two-lane 

roads indicated a reduction of 29.3 percent in off-road left and head-on collisions combined. It is 

of interest that the estimated reduction for the combined application on two-lane roads is smaller 

than the reduction of target crashes for single applications of either CLRS or SRS. It is possible 

that the locations subject to the dual application had lower target crash rates or were otherwise 

different from locations with single applications, prior to application. The paper does not provide 

enough details to assess if this is true.

Torbic et al. evaluated the safety impacts of applying CLRS and SRS in combination using data 

for 80 mi of rural two-lane roads in Mississippi by applying the EB before-after approach.(5)

Target collisions were defined as the sum of head-on, sideswipe-opposite-direction, and SVROR. 

The results showed a 35 percent reduction in target collisions of all severities and a 39.6 percent 

reduction in FI target collisions.

11

CHAPTER 2. OBJECTIVE

This research examined the safety impacts of the combined application of CLRS and SRS in 

Kentucky, Missouri, and Pennsylvania. The objective was to estimate the safety effectiveness of 

this strategy as measured by crash frequency. Intersection-related and animal crashes were 

excluded. Excluding these crash types, 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 address questions of interest such as the following:

• Do effects vary by level of traffic volumes?

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

• Do effects vary by vehicle speeds?

• Do effects vary by lane width and shoulder width?

• What is the difference between the combined effects of CLRS and SRS and effects of 

either in isolation?

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

crash savings in the form of 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.

13

CHAPTER 3. STUDY DESIGN

The study design involved a sample size analysis and prescription of needed data elements. The 

sample size analysis assessed the size of a sample required to statistically detect an expected 

change in safety and also determined what changes in safety can be detected with likely available 

sample sizes.

SAMPLE SIZE ESTIMATION OVERVIEW

Sample size estimations require assumptions of the expected treatment effect and the average 

crash rate at treatment sites prior to treatment. Minimum and desired sample sizes were 

calculated assuming a conventional before-after with comparison group (C-G) study design, as 

described in Hauer and a literature review of likely safety effects.

(6) The sample size analysis 

undertaken for this study addressed the size of sample required to statistically detect an expected 

change in safety. The sample size estimates are conservative because the more robust EB 

methodology is actually used in the before-after analysis rather than the C-G methodology. 

Sample sizes were estimated for various assumptions of the likely annual crash rate in the before 

period and likely safety effects of the strategy. Annual crash rates were assumed for five crash 

types (i.e., total, injury, run-off-road, head-on plus sideswipe-opposite-direction, and all target 

crashes (run-off-road plus head-on plus sideswipe-opposite-direction)), as shown in table 3. 

Intersection-related and animal crashes are not included in these crash rates. These crash rates, 

which were obtained from preliminary data for the untreated reference group data collected for 

the EB analysis, represent a range of mean crash rates. Only crash rates from Pennsylvania and 

Kentucky were used at the time of the sample size analysis. The study design assumed that the 

number of comparison sites would be equal to the number of treatment sites for a C-G study. 

Table 3. Before period crash rate assumptions.

Crash Type

Pennsylvania 

(Crashes/Mi/Year)

Kentucky 

(Crashes/Mi/Year)

All 0.96 1.21

Injury1 0.51 0.38

Run-Off-Road 0.15 0.19

Head-On+Sideswipe-OppositeDirection 0.06 0.08

All Target Crashes 0.21 0.27 1

Non-injury crash type proportions assumed to be same as Pennsylvania for run-off-road, head-on, 

and sideswipe-opposite-direction.

Table 4 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 for which a study seems worthwhile; that

is, it is feasible to detect with the level of confidence the largest effect that may reasonably be 

expected based on what is currently known about the strategy. These sample size calculations 

were based on specific assumptions regarding the number of crashes per mile and years of 

available data. Mile-years are the number of miles where the strategy was implemented

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

15
strategy was implemented at a 9-mi segment and data are available so far for 3 years since 

implementation, then there are a total of 27 mi-year of after period data available for the study.

Table 4. Minimum required before period mile-years for treated sites.

1,2

Expected Percent Reduction 

in Crashes

95-Percent 

Confidence Level

90-Percent 

Confidence Level

PA Rate KY Rate PA Rate KY Rate

Total

5 1,057 444 740 311

10 216 91 151 64

20 77 32 54 23

30 33 14 23 10

Injury

10 2,508 3,366 1,756 2,357

20 512 687 359 481

30 182 244 127 171

40 79 106 55 74

Run-Off-Road

10 8,528 6,733 5,971 4,714

20 1,742 1,375 1,219 963

30 618 488 432 341

40 269 212 188 149

HeadOn+SideswipeOpposite-Direction

10 21,321 15,991 14,927 11,195

20 4,354 3,265 3,048 2,286

30 1,544 1,158 1,081 811

40 672 504 471 353

All Target Crashes

10 6,092 4,738 4,265 3,317

20 1244 968 871 677

30 441 343 309 240

40 192 149 134 105 1

Assumes equal number of mile-years for treatment and comparison sites and equal length of 

before and after periods.

2

Bold indicates sample size values recommended in this study.

The sample size values recommended in this study are highlighted in bold in table 4. 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 are 

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 may be predicted with greater confidence 

or a smaller reduction in crashes will be detectable if there are 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.

Following the data collection for both the before and after periods, the total mile-years of data 

available was 6,392 for the before period and 2,623 for the after period. For the available data, 

the minimum percentage change in crash frequency that could be statistically detectable at 

95- and 90-percent confidence levels was estimated using the same crash rates found in table 3.

The calculations are based on methodology in Hauer.

(6) The results, which are shown in table 5,

indicate that the data should be able to detect the recommended crash reduction values from 

16
table 4, if such an effect were present. Using these results, a decision was made to proceed with 

the evaluation using the data available at the time.

Table 5. Analysis for crash effects detectable with available sample size.

Crash Type

Mile-Years in 

Before Period

Mile-Years in 

After Period

90-Percent 

Confidence 

Level1

95-Percent 

Confidence 

Level1

Total

6,392 2,623

5.0 7.5

Injury 7.5 10.0

Run-Off-Road 10.0 12.5

Head-On+SideswipeOpposite-Direction 17.5 20.0

All Target Crashes 7.5 10.0 1

Minimum percent reduction detectable for crash rate assumption. Minimum percent reduction is rounded to nearest 

2.5 percent.

17

CHAPTER 4. METHODOLOGY

The EB methodology for observational before-after studies was used for the evaluation.(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, SPFs are used for the following 

reasons:

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

differences between the before and after periods.

• They account for time trends.

• They reduce the level of uncertainty in the estimates of safety effect.

• They 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 in figure 1:

Figure 1. 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 effects of regression-to-the-mean and changes in traffic volume were 

explicitly accounted for using SPFs, relating crashes of different types to traffic flow and other 

relevant factors for each jurisdiction based on untreated sites (reference sites). Annual SPF 

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

demography, and crash reporting).

In the EB procedure, the SPF is used to first estimate the number of crashes that would be 

expected 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) is then combined with the count of crashes (x) in the before period at a strategy 

site to obtain an estimate of the expected number of crashes (m) before strategy. This estimate of 

m is seen in figure 2:

Figure 2. Equation. EB estimate of expected crashes.

W is estimated from the mean and variance of the SPF estimate as seen in figure 3:

∆ Safety = λ - π

λ

π

λ

,

19
Figure 3. Equation. EB weight.

Where:

k = constant for a given model and is estimated from the SPF calibration process with the use of a 

maximum likelihood procedure. In that process, a negative binomial distributed error structure is 

assumed, with k being the overdispersion parameter of this distribution.

A factor is 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 is 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, is an estimate of . The procedure also produces an estimate 

of the variance of .

The estimate of is 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 is also summed over all sites in the strategy group. 

The index of effectiveness ( ) is estimated as seen in figure 4:

Figure 4. Equation. Index of effectiveness.

The standard deviation of is in figure 5:

Figure 5. Equation. Standard deviation of index of effectiveness.

The percent change in crashes is 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.

,

λ

λ

λ λ

π

λ

θ





 



 +

=

2

( ) 1 sum

sum

sum

sum

Var

λ

λ

λ

π

θ ,

θ

2

2

2 2

2

( ) 1

( ) ( )

( )

















+

















+

=

sum

sum

sum

sum

sum

sum

Var

Var Var

StDev

λ

λ

λ

λ

π

π θ

θ ,

θ θ

20
A slightly different approach to the methodology was required for the analysis of the treatment 

sites in Missouri, which is installing rumble strips on two-lane rural roads whenever a resurfacing 

project is undertaken. As a result, it would be very difficult to identify comparable roadways with 

no rumble strips for this road type presently or in the near future. For this reason, a separate 

reference group of sites without rumble strips was not identified. 

An alternate approach to the standard EB before-after methodology was applied. In short, this 

method makes use of the before period data at the treatment sites to develop SPFs to control for 

regression-to-the-mean and traffic volume changes. Because the installation of rumble strips is a 

policy for all resurfacing projects, regression-to-the-mean is not as high a concern as it otherwise 

may be. The SPFs calibrated from before period data are also used to account for time trends in 

the earlier part of the study period, before most of the sites have had rumble strips installed. 

However, after a substantial number of sites have been treated, the number of sites is low for 

developing yearly factors and is not possible after all have been treated. For these later years, the 

after period data are used to develop SPFs for calculating yearly factors for the after period. The 

before period yearly factors are extrapolated based on the ratio of the after period factors to a 

common year.

To illustrate, consider the fictional information in table 6. Using the SPFs calibrated for both the 

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

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

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

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

multipliers for the before period model are adjusted by multiplying the average from 2007–2008 

(1.03) by the value of the adjusted after period multiplier for each year. These are the annual 

multipliers used in the evaluation.

Table 6. Illustration of alternate approach.

Year 

Using After 

Period Data 

Adjusted 

After Period 

Multipliers 

Using Before 

Period 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

N/A = Not applicable.

Blank cell = No adjustment is required.

21

CHAPTER 5. DATA COLLECTION 

Kentucky, Missouri, and Pennsylvania provided data containing locations and dates of the 

installation of CLRS and SRS. In Missouri, SRS are placed on the edge line; in Kentucky and 

Pennsylvania, they are both installed on the edge line and placed further into the shoulder. 

Throughout the report, the abbreviation SRS will be used to refer to both installation types. These 

States also provided roadway geometry, traffic volumes, and crash data for both installation and 

reference sites. This section provides a summary of the data assembled for the analysis.

KENTUCKY

Installation Data

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

had been installed, along with an indication of whether SRS or edgeline rumble stripes were 

installed concurrently. As stated previously, the report will refer to both as SRS in abbreviated 

form. In cases where the rumble strip is placed on the edge line, these are referred to as rumble 

stripes because the lane striping is applied on top. Both edgeline and shoulder applications have 

been applied in Kentucky. The final list of treated sites used for analysis comprises 12 sections 

(42 mi) where CLRS and SRS were installed at the same time as part of a resurfacing effort, and 

15 sections (122 mi) where CLRS had been installed as retrofits. All roadways previously had 

SRS installed, so the results will strictly pertain to the incremental effects of adding CLRS to 

roadways that have SRS. Thus, the estimated benefits could be considered as conservative in that 

even greater crash reductions would be expected for run-off-road crashes if SRS had not 

previously existed. It should be noted, however, that for the resurfaced sites, it is possible that the 

rumble strips had exceeded their service life, although this could not be determined. 

The two types of treatment groups are described as follows:

• Retrofit treatment sites are sections of road where CLRS were milled into the existing 

asphalt. These sites were selected as roadways that were wide enough to apply CLRS 

and had a history of crashes over a certain threshold (3 head-on, sideswipe, or 

opposite-direction crashes in the 2004–2008 period). The KYTC safety office funded 

and applied the installations. All sites had a prior condition of rolled SRS.

• Resurfacing treatment sites are sections of road where CLRS and new edgeline 

rumble stripes were installed simultaneously as part of a resurfacing effort. The 

KYTC safety office, working in conjunction with the roadway maintenance and 

pavement office, identified the locations. A contractor performed a field inspection to 

verify installation date and type. KYTC indicates that these sites were selected as part 

of the regular resurfacing schedule with no specific consideration of crash history in 

their selection. Almost all resurfacing sites were rural locations. All sites had a prior 

condition of SRS or texturing and no CLRS.

Reference Sites

As described above, one set of treated sites was selected for treatment under a retrofit program 

that installed CLRS on sites that were selected on the basis of high target crashes (i.e., head-on 

and sideswipe-opposite-direction). To match these retrofit treated sites, the KYTC identified sites 

23
that were selected to receive CLRS (with a projected installation date of summer 2012) but had 

not yet received the treatment. This retrofit reference group comprises 133 mi of road. 

The other set of treatment sites was selected as part of the resurfacing effort (and not on the basis 

of crashes). To match these resurfacing treatment sites, the research team desired to identify a 

reference group that would match the characteristics of the resurfaced sites and would be eligible 

to receive resurfacing but would not yet have received resurfacing. The reference group was 

identified initially by considering all statewide mileage that was undivided (as noted by a median 

width of 0 ft), had two lanes, had an 11-ft or greater lane width, and had a 50 mi/h or greater 

speed limit (and was thus eligible to receive CLRS in any resurfacing effort). These locations 

were cross-referenced against several lists to remove sites that were identified in the State’s 

Roadway Departure plan as having high target crashes (head-on and sideswipe-oppositedirection) and to remove sites that already received CLRS through past construction activities. 

This resurfacing reference group comprises 1,588 mi of road. 

Roadway Data

KYTC staff provided roadway data in geographic information system (GIS) shapefile formats. 

The various road characteristics (e.g., shoulder width) were contained in separate shapefiles for 

each segment. GIS files were obtained from the Kentucky Roadway Information and Data Web

site. Characteristics of the treatment and reference sites were obtained by matching each study 

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

Traffic Data

Traffic volume data are maintained by KYTC in the GIS inventory files. Traffic data were 

obtained for the treatment and reference sites by matching each study site to the appropriate 

inventory segment by county, route, and milepost. Specifically, the inventory file from year 2010 

was used because it provided two data points—a current (2010) AADT and the prior AADT 

(with an indication of the year taken). These volume points can be used as needed to extrapolate

yearly AADT for the before period. Subsequently, a similar file was obtained with 2012 traffic 

counts. The multiple traffic counts were used to develop annual trends, but these estimates were 

determined to be unreliable because extrapolating over a significant number of years often 

resulted in unreasonable values (e.g., negative AADT counts). As such, the average AADT using 

actual counts was used in both the before and after periods.

Crash Data

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

site lists for 2002–2012 and a data dictionary for interpreting the fields in the crash data. The 

crash data can be linked to the sites based on county, route, and begin and end mileposts. The 

field labeled “RDWYIDTXT” is present in both the crash and road files to indicate the route. Of 

note, KYTC indicated that crash location quality improved significantly in 2008. This

improvement is the result of law enforcement using the Map It application, which the officers 

could use to select the crash location on a screen, which would apply latitude/longitude 

coordinates to the crash record.

24
Treatment Cost Data

KYTC provided estimates of the costs and services lives of the treatments for use in conducting a 

benefit-cost analysis of the treatment (table 7).

Table 7. Kentucky treatment cost and service life data.

Countermeasure 

Initial Installation 

Cost Maintenance Cost Service Life1 

Edgeline strips or SRS

(installed as part of 

resurfacing)

$2,500/mi for 

rumble strip

$305/mi for stripe

No additional 

maintenance cost

12–15 years for 

rumble strip, 

2 years for stripe

CLRS (retrofit, milled 

into asphalt)

$4,000/mi for 

rumble strip

$350/mi for stripe

No additional 

maintenance cost

12–15 years for 

rumble strip, 

2 years for stripe 1

Stripes were used in cases when the rumble strip was placed directly on the edge line.

CLRS = Centerline rumble strips.

SRS = Shoulder rumble strips.

MISSOURI

Installation Data

The Missouri Department of Transportation (MoDOT) provided a list of projects where CLRS

and edgeline rumble strips were recently installed or planned to be installed. The total length of 

roadway with CLRS and SRS installations was 460 mi. Among the data provided by the reports 

were the location (including district, State route number, and mileposts) and the construction 

dates. MoDOT reported that some locations also had 6-inch striping, bigger and brighter signs, 

and delineation on guardrails.

Reference Sites

Missouri now installs rumble strips on two-lane rural roads whenever a resurfacing project is 

undertaken. As a result, it would be very difficult to identify roadways with no rumble strips for 

this road type presently or in the near future. For this reason, a separate reference group of sites 

without rumble strips has not been identified. An alternate approach to the standard EB beforeafter methodology was applied, which is further described in the section on study design. In 

short, this method used before period data at the treatment sites to develop SPFs to control for 

regression-to-the-mean and traffic volume changes. Because the installation of rumble strips is a 

policy for all resurfacing projects, regression-to-the-mean was not as high a concern as it 

otherwise may be. Time trends were accounted for using both early installations and later 

installations.

Roadway Data

MoDOT provided roadway data for the treatment sites and included the following variables:

• Area type (urban/rural).

• Functional class.

25
• Divided versus undivided.

• Number of lanes.

• Lane width.

• Shoulder type.

• Shoulder width.

• Surface type.

• Speed limit.

The roadway data are stored in a bidirectional manner, meaning there is a separate record for

each direction of travel. MoDOT staff matched opposing directions of travel for each site. The 

constructed database is limited to one record per site and the geometric information taken from 

the primary direction of travel.

Traffic Data

MoDOT provided traffic data in the form of AADT from 1999 to 2011 in electronic files for all 

treatment sites. 

Crash Data

MoDOT provided crash data from 1999 to 2011, including many variables related to the location, 

time, and characteristics of each crash.

Treatment Cost Data

MoDOT provided approximate installation costs of $1,000/mi for either edgeline or centerline. 

MoDOT estimates that the service lives of rumble strips is 7 to 10 year.

PENNSYLVANIA

Installation Data

The Pennsylvania Department of Transportation (PennDOT) provided a list of projects where 

both CLRS and SRS were recently installed or planned to be installed. The sites used for analysis 

totaled 218 mi. These data included information on the location (including PennDOT district, 

county, State route number, and Segment/Offset, which is PennDOT’s milepost system) and the 

project number of the installations. The project number is used for tracking project progress. The 

project team obtained construction start and end dates from a PennDOT Web site with this 

number. 

PennDOT reported that some locations may have had shoulders widened to accommodate the 

SRS.

Reference Sites

The project team derived a preliminary list of reference sites by matching PennDOT’s rumble 

strip inventory to the inventory of all rural two-lane roads. Roads having neither CLRS nor SRS

were retained for reference sites. PennDOT confirmed that rumble strips have not been applied to 

26
any of the reference sites. The list was further reduced by including only those sites whose 

characteristics matched the range of treatment sites as follows:

• ACC_CODE=3 (no access control).

• Divisor in 0,1,2,3 (none, painted divided, man-made barrier, earth divided).

• DIV_WDT = 0 (divided width equal to 0 ft.).

• Speed Limit 20–55 mi/h.

• Number of lanes = 2.

• AADT between 650 and 26,570.

The sum of reference site miles was 17,931 mi.

Roadway Data

The project team obtained roadway data for the treatment and reference sites from the PennDOT 

Roadway Management System and included the following variables:

• Surface type.

• Pavement width.

• Speed limit.

• Number of lanes.

• Year of resurfacing.

• Shoulder type.

• Shoulder width.

• Area type (urban/rural).

Traffic Data

The project team obtained traffic data in the form of AADT from PennDOT from 2003 to 2011 in 

electronic files for all treatment and reference sites. The percentage of trucks in the traffic stream 

was also provided.

Crash Data

The PennDOT Crash Database is maintained by the Bureau of Highway Safety and Traffic 

Engineering’s Crash Information Systems & Analysis Division. The compiled crash data contain 

many variables related to the location, time, and characteristics of each crash. Data from 2003 to 

2012 were obtained.

Treatment Cost Data

Table 8 provides a breakdown of installation costs/ft provided by PennDOT. This average has 

steadily decreased over the last decade. In the early 2000s, the average cost was $0.77/ft. Note 

that these costs are associated with a single “row” or “line” of rumble strips. That cost is for a 

two-lane roadway with CLRS and SRS; the costs in table 8 should be multiplied by three.

27
Table 8. Pennsylvania treatment cost data, 2009–2011.

District Average Cost/ft Average Quantity Installed (ft) 

1 $0.1136 111,135

2 $0.3568 14,655

3 $0.4532 12,795

4 $0.1866 533,779

5 $0.2071 65,161

6 $0.3017 37,553

8 $0.2309 23,353

9 $0.1391 93,192

10 $0.1778 45,805

11 $0.2070 63,525

12 $0.2504 129,780

State Average $0.2386 102,794

The costs are per ft. Taking into consideration consistent driveway, intersection, and other types 

of breaks, PennDOT provided an average cost of $1,267/mi for a single line of rumble strips. 

Therefore, the average cost for CLRS and SRS along a two-lane roadway would be 

approximately $3,800/mi. These costs assume there are no maintenance costs. PennDOT assumes 

a life cycle of 7 year for the rumble strips.

Data Characteristics and Summary

Table 9 defines the crash types used by each State. The project team attempted to make the crash 

type definitions consistent. In all States, intersection-related and animal-related crashes were 

excluded.

28
Table 9. Definitions of crash types.

State Total Injury Run-Off-Road Head-On 

SideswipeOppositeDirection 

Kentucky

Identified as 

nonintersection 

and non-ramp 

and excludes 

those where 

Event 

Collision With 

indicated an 

animal or deer 

involvement.

Resulted in 

an injury or 

possible 

injury.

Event Collision 

With indicates an 

object off 

roadway was 

struck, and 

Pre-Collision 

Action is 

“avoiding object 

in roadway,”

“going straight 

ahead,” or 

“slowing or 

stopped.”

Manner of 

Collision is 

“head-on,”

and 

Event 

Collision 

With is “other 

motor 

vehicle.”

Manner of 

Collision is 

“sideswipeoppositedirection,”

and

Event 

Collision 

With is 

“other motor 

vehicle.”

Missouri

Identified as 

nonintersection 

and nonanimal related.

Resulted in 

a fatal, 

disabling,

or minor 

injury.

Accident Type 

described as “ranoff-road,” “ranoff road-fixedobject,” “ran-offroadoverturning,”

“ran-off-roadparked-motorvehicle,” or “ranoff-road-other.”

Accident 

Type 

described as 

“head-on.”

Accident 

Type 

described as 

“sideswipe.”

Note that the 

data do not 

indicate 

whether a 

sideswipe 

crash was 

opposite or 

same 

direction.

Pennsylvania

Identified as a 

midblock 

crash and not 

“deer” or 

“other 

animal.”

If number 

of fatal or 

injured 

persons is 

greater than 

zero.

Relation To Road 

indicates the 

crash occurred 

outside the 

trafficway in an 

area not intended 

for vehicles.

Collision 

Type is 

“head-on.”

Collision 

Type is 

“sideswipeoppositedirection.”

Table 10 provides summary information for the data collected for the treatment sites. The 

information in table 10 should not be used to make simple before-after comparisons of crashes 

per mile-year because it does not account for factors, other than the strategy, that may cause a 

change in safety between the before and after periods. Such comparisons are properly done with 

the EB analysis as presented later. Table 11 provides summary information for the reference site 

data. As discussed previously, a different approach was used in Missouri where an appropriate 

reference group could not be found.

29
Table 10. Data summary for treatment sites.

Variable Kentucky Missouri Pennsylvania

Number of miles 164 460 218

Mile-years before 604 4,238 1,407

Mile-years after 764 1,286 512

Crashes/mile/year before 1.61 0.74 1.13

Crashes/mile/year after 0.94 0.49 1.13

Injury crashes/mile/year before 0.50 0.32 0.62

Injury crashes/mile/year after 0.27 0.18 0.62

Run-off-road crashes/mile/year 

before 0.62 0.30 0.16

Run-off-road crashes/mile/year 

after 0.20 0.21 0.18

Head-on crashes/mile/year before 0.06 0.04 0.05

Head-on crashes/mile/year after 0.02 0.02 0.05

Sideswipe-opposite-direction 

crashes/mile/year before 0.10 0.05 0.03

Sideswipe-opposite-direction 

crashes/mile/year after 0.04 0.02 0.03

AADT before

Average:

6,101

Minimum:

1,282

Maximum:

20,433

Average:

5,290

Minimum:

154

Maximum: 

15,848

Average:

4,990

Minimum:

782

Maximum:

25,796

AADT after

Average:

6,101

Minimum:

1,282

Maximum:

20,433

Average:

5,106

Minimum:

155

Maximum: 

13,522

Average:

4,657

Minimum:

562

Maximum:

26,118

Average paved shoulder width (ft)

Average:

8.19

Minimum:

2.00

Maximum:

12.00

Average:

7.21

Minimum:

0.00

Maximum: 

12.00

Average:

4.60

Minimum:

0.00

Maximum:

10.00

AADT = Annual average daily traffic.

30
Table 11. Data summary for reference sites.

Variable Kentucky Pennsylvania

Number of miles 1,532 17,931

Mile-years 16,852 161,377

Crashes/mile/year 1.07 1.23

Injury crashes/mile/year 0.34 0.64

Run-off-road 

crashes/mile/year 0.38 0.22

Head-on crashes/mile/year 0.03 0.05

Sideswipe-opposite-direction 

crashes/mile/year 0.05 0.03

AADT

Average:

2,702

Minimum:

10

Maximum:

17,701

Average:

4,350

Minimum:

473

Maximum:

25,067

Average paved shoulder 

width (ft)

Average:

6.16

Minimum:

0.00

Maximum:

14.00

Average:

2.18

Minimum:

0.00

Maximum:

16.00

AADT = Average annual daily traffic.

31

CHAPTER 6. DEVELOPMENT OF SPFS

This section presents the SPFs developed for each State. The SPFs are used in the EB 

methodology to estimate the safety effectiveness of this strategy.(6) Generalized linear modeling 

was used to estimate model coefficients 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 dispersion parameter, k, was estimated iteratively from the model 

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

SPFs were calibrated separately for Kentucky and Pennsylvania using the corresponding 

reference sites from each State. As discussed in the methodology section, the Missouri SPFs were 

developed separately for the before and after periods at the treated sites. The SPFs developed are 

presented by State in the following sections. The parameter estimates are presented by State with 

the standard error of the estimates. 

KENTUCKY SPFS

The form of the SPFs for Kentucky, which are presented in table 12, is seen in figure 6: 

Figure 6. Equation. SPF model form for Kentucky.

Where:

AADT = Average annual daily traffic volume.

reftype = 1 if a resurfacing reference site; 0 if a retrofit reference site.

a, b, c = Parameters estimated in the SPF calibration process.

k = The overdispersion parameter of the model.

Table 12. Kentucky SPFs.

Parameter Estimates (Standard Error)

Crash Type a b c k

Total -5.8124

(0.3410)

0.6304

(0.0355)

1.1548

(0.1859) 0.8803

Injury -6.3308

(0.3641)

0.5520

(0.0385)

1.0702

(0.1839) 0.6981

Run-Off-Road -5.0019

(0.3974)

0.3933

(0.0418)

1.1128

(0.2046) 0.9091

Head-On -9.3272

(0.6967)

0.6610

(0.0751)

0.7647

(0.2791) 0.8055

SideswipeOppositeDirection

-7.0892

(0.5705)

0.4372

(0.0612)

0.8975

(0.2597) 0.8536

Crashes/mile/year = exp(a)AADTbexp(reftype*c)

33
MISSOURI SPFS

As discussed in the methodology section, the analysis of the Missouri data required that SPFs be developed for both 

the before and after periods. The before period SPFs are shown in table 13. For the after period, the time trend is only 

based on total crashes because of the low numbers of other crash types. Thus, only total crashes were modeled, as 

indicated in n/a = not applicable.

table 14.

The form of the SPFs for Missouri is seen in figure 7: 

Figure 7. Equation. SPF model form for Missouri. 

Where:

AADT = Average annual daily traffic volume.

shldwid = Average shoulder width in ft.

urbrur = 1 if rural; 0 if urban.

a, b, c, d = Parameters estimated in the SPF calibration process.

k = The overdispersion parameter of the model.

Table 13. Missouri before period SPFs.

Parameter Estimates (Standard Error)

Crash Type a b c d k

Total -7.8094

(0.6409)

1.0091

(0.0747)

-0.0691

(0.0144)

-0.4479

(0.1205) 0.8958

Injury -8.7627

(0.7731)

0.9958

(0.0892)

-0.0581

(0.0166)

-0.3273

(0.1503) 0.8644

Run-Off-Road -5.3832

(0.7745)

0.6168

(0.0906)

-0.1116

(0.0179)

-0.2298

(0.1652) 0.9827

Head-On -12.5421

(1.5038)

1.1047

(0.1751) N/A N/A 0.8202

SideswipeOppositeDirection

-11.5757

(1.3814)

1.0508

(0.1662)

-0.0553

(0.0284) N/A 0.5565

N/A = Not applicable.

Table 14. Missouri after period SPFs.

Parameter Estimates (Standard Error)

Crash Type a b c d k

Total -6.7214

(1.0431)

0.8107

(0.1193)

-0.0440

(0.0202)

-0.6187

(0.1828) 0.8154

Crashes/mile/year = exp(a)AADTbexp(shldwid*c+urbrur*d)

34
PENNSYLVANIA SPFS

The form of the Pennsylvania SPFs, which are provided in table 15, is seen in figure 8:

Figure 8. Equation. SPF model form for Pennsylvania. 

Where,

AADT = Average annual daily traffic volume.

shldwid = Average shoulder width in ft.

width = Pavement width in ft.

a, b, c, d = Parameters estimated in the SPF calibration process.

k = The overdispersion parameter of the model.

Table 15. Pennsylvania SPFs.

Parameter Estimates (Standard Error)

Crash Type a b c d k

Total -5.9379

(0.0402)

0.7603

(0.0050)

-0.0471

(0.0019) N/A 0.4519

Injury -6.7027

(0.0461)

0.7703

(0.0057)

-0.0371

(0.0021) N/A 0.4493

Run-Off-Road -5.8811

(0.0728)

0.6254

(0.0106)

-0.0793

(0.0036)

-0.0233

(0.0016) 0.9507

Head-On -10.3415

(0.1232)

0.9024

(0.0146)

-0.0325

(0.0051) N/A 0.7623

SideswipeOppositeDirection

-10.0866

(0.1496)

0.8161

(0.0178)

-0.0452

(0.0064) N/A 0.7521

N/A = Not applicable.

Crashes/mile/year = exp(a)AADTbexp(shldwid*c+width*d)

35

CHAPTER 7. BEFORE-AFTER EVALUATION RESULTS

AGGREGATE ANALYSIS

Table 16 through table 19 provide the estimates of expected crashes in the after period without 

treatment, the observed crashes in the after period, and the estimated CMF and its standard error 

for all crash types considered. Results are provided separately for each State as well as all States 

combined.

The results for Kentucky in table 16 indicate reductions for all crash types that are statistically 

significant at the 95-percent confidence level. All treatment sites in Kentucky had SRS or 

edgeline rumble stripes prior to treatment, so the results indicate that CLRS further reduce runoff-road crashes.

Table 16. Results for Kentucky.

Total Injury

Run-OffRoad Head-On

SideswipeOppositeDirection

EB estimate of crashes expected 

in the after period without strategy 851.54 256.91 241.30 30.48 33.92

Count of crashes observed in the 

after period 719 210 149 15 31

Estimate of CMF 0.842 0.812 0.613 0.480 0.891

Standard error of estimate of CMF 0.054 0.088 0.073 0.142 0.210

Bold indicates CMF estimates that are statistically significant at the 95-percent confidence level.

CMF = Crash modification factor.

EB = Empirical Bayes.

The results for Missouri in table 17 also indicate reductions for all crash types that are 

statistically significant at the 95-percent confidence level. Prior to treatment, rumble strips were 

not present. It is logical that the CMF for total crashes is smaller than that for Kentucky, for 

which the CMF pertains to the addition of CLRS on roadways that previously had SRS or stripes. 

That the CMF for run-off-road crashes in Missouri is larger than in Kentucky, where SRS

previously existed, is not intuitive, but such comparisons for specific crash types can be 

influenced by how crash types are defined in different jurisdictions and the extent of the 

overrepresentation of specific crash types prior to treatment.

37
Table 17. Results for Missouri.

Total Injury

Run-OffRoad Head-On

SideswipeOppositeDirection

EB estimate of crashes expected 

in the after period without strategy 965.83 418.82 360.04 47.35 50.94

Count of crashes observed in the 

after period 631 234 273 24 32

Estimate of CMF 0.653 0.558 0.758 0.506 0.628

Standard error of estimate of CMF 0.029 0.039 0.050 0.105 0.113

Bold indicates CMF estimates that are statistically significant at the 95-percent confidence level.

CMF = Crash modification factor.

EB = Empirical Bayes.

The results for Pennsylvania, where rumble strips were not present prior to treatment, are shown 

in table 18. These results indicate reductions in total, run-off-road, and sideswipe-oppositedirection crashes, and increases in injury and head-on crashes. However, none of these results are 

statistically significant. Nevertheless, these results are still of interest because they could be used 

to increase the significance of a combined CMF based on the results of all three States. More 

discussion of the Pennsylvania results is provided in the next section on disaggregate analysis, 

while the combined results for the three States are presented next.

Table 18. Results for Pennsylvania.

Total Injury

Run-OffRoad Head-On

SideswipeOppositeDirection

EB estimate of crashes expected 

in the after period without strategy 591.63 310.91 99.82 24.43 15.41

Count of crashes observed in the 

after period 577 317 92 25 14

Estimate of CMF1 0.975 1.019 0.920 1.021 0.907

Standard error of estimate of CMF 0.046 0.063 0.103 0.210 0.246 1

None of the CMF estimates are statistically significant at the 95-percent confidence level.

CMF = Crash modification factor.

EB = Empirical Bayes.

The combined results in table 19 indicate reductions for all crash types analyzed that are 

statistically significant at the 95-percent confidence level. It should be noted that combining the 

results of the three States produces a more robust CMF because the standard error of the 

combined CMF estimate (relative to the CMF) is smaller than that from any State or from the 

combination of the two States with the most significant results (Kentucky and Missouri). 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 sideswipe-opposite-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 

38
0.771 for FI were estimated. For run-off-road, head-on, and sideswipe-opposite-direction crashes 

combined (i.e., lane departure crashes), the estimated CMF is 0.733. It is important to remember 

that all crash types considered exclude intersection-related and animal crashes.

As discussed in the literature review, the most comprehensive and reliable study to date of both 

SRS and CLRS is published in NCHRP Report 641. This report does not include recommended 

findings for the combination of SRS and CLRS but does recommend CMFs for these treatments 

separately. A comparison of the results for the combined treatment with the recommended CMFs 

is encouraging.

In NCHRP Report 641, for SRS, a CMF of 0.85 is recommended for SVROR crashes. The results 

for other crash types were not statistically significant and so were not recommended. These

results included a 6-percent reduction in total crashes and an 8-percent increase in FI crashes. In 

comparison with the new results, it appears that the effect of combining CLRS and SRS further 

reduces run-off-road crashes with a CMF of 0.742 for dual application versus 0.85 for SRS alone.

In NCHRP Report 641, CMFs of 0.91 for total crashes, 0.88 for FI crashes, and 0.70 for head-on 

plus sideswipe-opposite-direction crashes were recommended for CLRS. The new results, which 

estimated CMFs of 0.800 for all severities and 0.771 for FI for dual application, indicate that SRS

further reduce these crashes. However, the CMF of 0.70 for head-on plus sideswipe-oppositedirection crashes suggests that dual application does not further reduce crashes of this type, which 

is intuitive. 

39
Table 19. Results for combined States.

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 

CMF1 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

Bold indicates CMF estimates that are statistically significant at the 95-percent confidence level.

CMF = Crash modification factor.

EB = Empirical Bayes.

HO = Head-on.

ROR = Run-off-road.

S-OD = Sideswipe-opposite-direction.

DISAGGREGATE ANALYSIS

While the combined results for all States provide results that meet expectations and are 

statistically significant, the results are not consistent amongst the three States. The results for 

Kentucky and Missouri show statistically significant crash reductions for all crash types, with the 

exception of sideswipe-opposite-direction crashes in Kentucky, for which the CMF of 0.891 is 

not statistically significant. 

For Pennsylvania, the results are much different. The CMFs estimated for Pennsylvania are all 

very close to 1.0, ranging from 0.920 to 1.021—none of which are statistically significant. These 

results are initially surprising. They differ from the findings for Kentucky and Missouri, and

results in NCHRP Report 641 for two-lane roads in Pennsylvania indicated large crash reductions 

for some crash types. For SRS, the NCHRP Report 641 reports CMFs of 0.76 for total crashes 

and 0.56 for SVROR crashes, which were both statistically significant at the 95-percent

confidence level. For CLRS, a non-significant CMF of 0.74 was estimated for head-on plus 

sideswipe-opposite-direction crashes. Anecdotally, this may be explained by the fact that 

Pennsylvania has been installing rumble strips on two-lane roads for many years with a goal of 

blanket coverage of their two-lane rural road system. Given this fact, it is likely that most 

40
higher-crash locations have already been prioritized and treated and that the sites that were 

evaluated in the current study did not have a high target crash issue and so logically will not have 

exhibited a large safety benefit compared with those that did and were evaluated for NCHRP 

Report 641. PennDOT indicated that sites are selected to prioritize high-volume locations and 

those with a high run-off-road or head-on crash frequency. A comparison of the summary 

statistics for the Pennsylvania data in NCHRP Report 641 and the current study support this 

hypothesis. Table 20 shows the crash rates per mile-year before treatment and the proportion of 

total crashes for both the current and previous study. For run-off-road crashes, the crash rate of 

0.87 in NCHRP Report 641 is much higher in than the rate of 0.16 for data used in the current 

study. Similarly the NCHRP Report 641 crash rate of 0.31 for head-on plus sideswipe-oppositedirection is much higher than the rate of 0.08 (0.05 for head-on and 0.03 for sideswipe-oppositedirection) for data used in the current study.

Table 20. Comparison of Pennsylvania crash rates.

Study Crash Type 

Crash Rate 

Before (Per MileYear)Crash Proportion 

Before

Current Study

Run-Off-Road 0.16 0.38

Head-On 0.05 0.03

Sideswipe-Opposite-Direction 0.03 0.06

NCHRP 641

Run-Off-Road 0.87 0.69

Head-On+Sideswipe-OppositeDirection 0.31 0.14

The different results in the present study for Pennsylvania compared with Missouri and Kentucky 

illustrate how the extent of the target crash problem at a location will affect the crash reduction 

benefits that can be expected. The before period crash rates in table 10 show that run-off-road and 

sideswipe-opposite-direction crash rates were higher in Missouri and Kentucky than in 

Pennsylvania, and the head-on crash rate in Kentucky was higher than in Pennsylvania.

The disaggregate analysis sought to identify those conditions under which the treatment is most 

effective. Since run-off-road, head-on, and sideswipe-opposite-direction crashes are the focus of 

this treatment, these crash types are the focus of the disaggregate analysis. Several variables were 

identified as being of interest and available for all three States, including speed limit, shoulder 

width, lane width, AADT, and the expected crash frequency per mile prior to treatment. 

The analysis found no clear trend between the CMF and values for posted speed, lane width, or 

shoulder width.

For AADT, as shown in table 21, larger percentage crash reductions were found for run-off-road

crashes for higher AADTs with some stability reached at an AADT of approximately 3,200. At 

AADTs above 3,200, the estimated CMF does not change significantly. At AADTs lower than 

3,200, a run-off-road crash CMF of 0.851 is estimated versus 0.702 for AADTs at 3,200 or 

greater. For head-on+sideswipe-opposite-direction crashes, the stability in the CMF is reached at 

an AADT of approximately 9,200, and the trend is reversed with a CMF of 0.679 at AADTs 

under 9,200 and 0.817 for AADTs over 9,200. A possible explanation for a larger CMF value for 

41
head-on+sideswipe-opposite-direction crashes is that at higher AADTs, there are fewer passing 

opportunities, and not all head-on or sideswipe-opposite-direction crashes are due to vehicles 

drifting out of their lane.

For the expected crash frequency per mile-year without treatment as shown in table 21, 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 is estimated versus 0.621 for rates at 0.500 or greater. For 

head-on+sideswipe-opposite-direction crashes, the stability in the CMF is reached at 

approximately a rate of 0.065, and the trend is reversed with a CMF of 0.608 at rates under 0.065 

and 0.715 for rates over 0.065. Since expected crashes increase with volume as seen in the SPFs 

developed, the trend of a larger CMF at higher crash rate for head-on+sideswipe-oppositedirection 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-offroad crashes and high AADTs will have higher priority than sites with high AADTs and a high 

proportion of head-on+sideswipe-opposite-direction crashes.

Table 21. Results disaggregated by ranges of AADT and expected crash frequency.

Crash Type

AADT

Expected Crashes/MileYear Without Treatment

Range 

CMF 

(Standard 

Error) Range

CMF 

(Standard 

Error)

Run-off-road

< 3200 0.851 (0.089) < 0.500 0.840 (0.058)

> 3200 0.702 (0.045) > 0.500 0.621 (0.055)

Head-on+sideswipeopposite-direction

< 9200 0.679 (0.069) < 0.065 0.608 (0.147)

> 9200 0.817 (0.172) > 0.065 0.715 (0.071)

AADT = Average annual daily traffic.

CMF = Crash modification factor.

42
CHAPTER 8. ECONOMIC ANALYSIS

For the purposes of the economic analysis, the assumed treatment is, conservatively, the dual 

application of CLRS and SRS for which the combined CMF of 0.800 for total crashes (table 8) is 

recommended. Treatment costs used range from $3,000/mi for Missouri to $12,000/mi in 

Kentucky. Service lives are 7–10 and 12–15 years, respectively. Results are presented for these 

two extremities.

The FHWA Office of Safety R&D suggests that the Office of Management and Budget Circular 

A-4 be used to determine the conservative real discount rate of 7 percent that was applied to 

calculate the annual cost of the treatment for 7- and 12-year service lives, respectively. Applying 

the lower ends of the service life ranges conservatively gives annual costs of $557 and $1,511/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 FI 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.

(7,8) Applying this ratio of 2.42 to the unit costs for PDO and FI 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 mile-year was 0.1881, which was obtained by dividing the total crash reduction 

(482.0) by the number of after period mile-years per site (2,562). 

The annual benefit (i.e., crash savings) of $30,481 is the product of the crash reduction per mileyear (0.1881) and the aggregate cost of a crash (all severities combined) ($162,045). The B/C 

ratio is calculated as the ratio of the annual benefit per mile to the annual cost per mile. The B/C 

ratios are 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.

43

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 SRS and CLRS applied in combination on twolane rural roads. The study used data from three States (Kentucky, Missouri, and Pennsylvania)

to examine the effects for specific crash types, including total, FI, run-off-road, head-on, and 

sideswipe-opposite-direction crashes. Crashes occurring at or related to an intersection and 

animal-related crashes were not included. Based on the combined results, the CMFs shown in 

table 22 are recommended for the various crash types. The benefits indicated by these CMFs may 

be regarded as conservative for two reasons. First, the sites in Kentucky already had SRS,

although those at the retrofit sites had already exceeded their useful lives. Second, the results 

include Pennsylvania sites, which experienced fewer benefits (likely because they were lower 

priority sites for the strategy). 

Table 22. Recommended CMFs.

Total Injury ROR HO S-OD HO + S-OD ROR+HO+S-OD

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

CMF = Crash modification factor.

HO = Head-on.

ROR = Run-off-road.

S-OD = Sideswipe-opposite-direction.

To date, the most comprehensive and reliable study of both SRS and CLRS individually applied 

is published in NCHRP Report 641—Guidance for the Design and Application of Shoulder and 

Centerline Rumble Strips. When compared with the recommended CMFs from that study, the 

results suggest that the effect of combining CLRS and SRS further reduces run-off-road crashes 

versus applying SRS alone. It also appears that SRS do not further reduce head-on plus 

sideswipe-opposite-direction crashes further than applying CLRS 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-oppositedirection crashes, smaller percentage crash reductions were found for higher AADTs. For the 

expected crash frequency per mile-year without treatment, larger percentage crash reductions 

were found for run-off-road crashes for higher crash frequencies. For head-on+sideswipeopposite-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 are estimated to range from 20.2 for a higher cost/higher service life assumption based 

on Kentucky information to 54.7 for a lower cost/lower service life assumption based on 

45
information from Missouri. 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. 

46
APPENDIX: ADDITIONAL INSTALLATION DETAILS FROM STATES

The following appendix presents additional details provided by the three participating States 

regarding the installation of the subject strategies in their State. 

DETAILS OF THE KENTUCKY SITES

Kentucky provided the rumble strip specifications seen in table 23:

Table 23. Installation details for Kentucky.

Dimensions Centerline Rumble Strips Shoulder Rumble Strips

Width 7 inches minimum

71/2 inches maximum 7 inches ± 1

/2 inch

Length 12 inches 16 inches

Depth

1

/2 inch minimum

5

/8 inch maximum

1

/2 ± 1/8

Spacing 24 inches 12 inches ± 1 inch

Lateral Placement

Center of roadway, 

perpendicular to centerline 

pavement markings

Place 1 ft out from the 

mainline pavement

Kentucky was also asked to provide some additional insight on their experience with the strategy. 

Their responses to several topics are presented below. The following responses are provided from 

the perspective of the State and are phrased informally:

• Types of Rumble Strips Evaluated: All new rumble strips were milled.

• Before-Period Rumble Strip Condition: No CLRS were present, but all sites had a 

preexisting condition of rolled SRS.

• Retrofit or Resurfacing Projects?: Both; the retrofit sites had rolled SRS in place,

and CLRS were milled in as the treatment. In the resurfacing projects, milled rumble 

strips were installed.

• Installation Requirements: For CLRS through resurfacing, lane widths had to be 

11 ft or wider and the speed limit 50 mi/h or greater. For retrofit routes, we performed 

visual analysis on pavement condition.

• Installation Challenges: Finding a good contractor with appropriate equipment is 

difficult. One contractor had a pull-behind unit on a tractor that had difficulty with 

maintaining the offsets and alignments.

• Installation Mechanism: Rumble strips were installed by both retrofit- and 

resurfacing-type projects.

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• Additional Installed Countermeasures: No other safety countermeasures were 

installed with the rumble strips at the treatment sites.

• Lessons Learned: Do not wait for unanimous support; instead, plan your 

implementation and move forward.

• Maintenance Challenges: When the centerline joint starts to fail, rumbles make it 

look worse. However, it does not appear that it expedites the deterioration of the 

joint. We have not seen the CLRS joints deteriorating any quicker than the non-CLRS 

joints. Most challenges are overcome through information exchange and documented 

analysis and review of the benefits and pavement concerns.

DETAILS OF THE MISSOURI INSTALLATIONS

Missouri provided the rumble strip specifications seen in table 24:

Table 24. Installation details for Missouri. 

Dimensions

Centerline Rumble Strips 

(Typical Drawing Detail #1) Shoulder Rumble Strips

Width 7 inches ± 1/2 inch 7 inches ± 1/2 inch

Length 12 inches 12 inches

Depth 7/16 inch ± 1/16 inch 7/16 inch ± 1/16 inch

Spacing 12 inches and 24 inches

(alternating pattern) 12 inches

Lateral Placement Centered on the centerline of the 

roadway

Placed on the outside edge of 

the edge line

Missouri was also asked to provide some additional insight on their experience with the strategy. 

Their responses to several topics are presented below. As with Kentucky, the following responses 

are provided from the perspective of the State and are phrased informally:

• Types of Rumble Strips Evaluated: All new rumble strips were milled.

• Before-Period Rumble Strip Condition: No rumble strips. (Some locations 

potentially had the 30-inch wide rolled rumble strip, which is not at all aggressive.)

• Retrofit or Resurfacing Projects?: Resurfacing with paved shoulders.

• Installation Requirements: We do encourage a new pavement depth of at least 

13⁄4 inch.

• Installation Challenges: We are now beginning to hear more from the bicycle 

community, but we have over 10,000 line mi now milled. As we continue to do more, 

we do look at the road users.

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• Additional Installed Countermeasures: We did much of this effort with the 

combination of wider stripes (6 inches), bigger and brighter signs, and delineation 

(tabs at 50 ft on guardrails and cables).

• Lessons Learned: Determine a system of roadways that will benefit the most and 

then just do them. Yes, you need to work with your partners, but you need to develop 

a plan of action and go forth.

• Maintenance Challenges: Many locations have gone through several winters 

(freeze/thaw) and for the most part are in pretty good shape. When locations do fail, 

we have allowed gaps up to 200 ft to exist (in anticipation of repair).

DETAILS OF THE PENNSYLVANIA INSTALLATIONS

Pennsylvania provided the rumble strip specifications seen in table 25 and table 26:

Table 25. Installation details for Pennsylvania. 

Dimensions

Centerline Rumble 

Strips (Typical 

Drawing Detail #1)

Centerline Rumble 

Strips (Typical 

Drawing Detail #2)

Edge Line 

Rumble Strips

Width 7 inches ± 1/2 inch 7 inches ± 1/2 inch 5 inches ± 1/2 inch

Length 16 inches 14–18 inches 6 inches

Depth 1/2 inch ± 1/16 inch 1/2 inch ± 1/16 inch 1/2 inch ± 1/16 inch

Spacing 2 ft and 4 ft

(alternating pattern) 2 ft 7 inches

Lateral Placement

Placed on both sides of 

the centerline, from 

inner edge of double 

yellow line marking and 

extending 16 inches into 

lane

Center of roadway, 

perpendicular to 

centerline pavement 

markings

Centered over 

4-inch edge line 

and edge line 

placed 4–6 ft from 

outside edge of 

shoulder

Table 26. Installation details for bicycle tolerable rumble strips for Pennsylvania. 

Dimensions

Bicycle Tolerable Shoulder 

Rumble Strip (55 mi/h or more)

Bicycle Tolerable Shoulder 

Rumble Strip (less than 55 mi/h)

Width 5 inches ± 1/2 inch 5 inches ± 1/2 inch

Length 16 inches 16 inches

Depth 3/8 inch ± 1/16 inch 3/8 inch ± 1/16 inch

Spacing 7 inches 6 inches

Lateral Placement

6 inches from edge of travel lane 

and 4 ft minimum from outside 

edge of shoulder

6 inches from edge of travel lane 

and 4 ft minimum from outside 

edge of shoulder

49
Pennsylvania was also asked to provide some additional insight on their experience with the 

strategy. Their responses to several topics are presented below. As with Kentucky and Missouri, 

the following responses are provided from the perspective of the State and are phrased 

informally:

• Types of Rumble Strips Evaluated: All new rumble strips were milled.

• Installation Challenges: Again, minimum shoulder width needs to be installed for 

SRS to accommodate the bicycle community. Edgeline rumble strips should be

omitted on the inside of moderate to sharp curves, which encompass dwellings. 

Drivers tend to use the shoulder area more in these situations, increasing the noise 

level. We have seen cases of pavement cracking when installing rumble strips in older 

pavement. We recommend only installing rumble strips where pavement is less than 

3 years old. (Less than 1 year is ideal pavement age.)

• Additional Installed Countermeasures: Treatment sites must have a minimum 

paved shoulder width of 6 ft to install SRS and 4 ft to install edgeline rumble strips. 

Other than paved shoulders, no other safety countermeasures were installed.

• Lessons Learned: While the combination of CLRS and SRS does create tight travel 

lanes for the driver, our data does not show an increase in crashes at these locations. 

To the best of our knowledge, there has not been any issues or complaints with this 

application. As long as lane width is at least 11 ft and minimum shoulder width 

requirements are met, we will continue to deploy this application throughout 

Pennsylvania. Our Roadway Departure Implementation Plan has also identified sites 

recommended for both CLRS and SRS.

• Maintenance Challenges: Our biggest challenge with maintenance has been rumble 

strips being filled in by thin overlays and not being re-installed. We have collected 

locations from our Districts where this is happening and information on their 

experiences with preserving, filling in, and re-cutting rumbles on two-lane rural 

highways. We are currently developing a synthesis of best practices (estimated 

completion June 2014) to provide guidance related to rumble strips and thin overlays.

50
ACKNOWLEDGEMENTS

This report was prepared by Vanasse Hangen Brustlin, Inc (VHB) for the Federal Highway 

Administration Office of Safety under Contract DTFH61-13-D-00001. The FHWA Contracting 

Officer’s Technical Manager for this project was Roya Amjadi. Kimberly Eccles, P.E, of VHB 

was the study principal investigator. Dr. Bhagwant Persaud and Craig Lyon, subcontractors to 

VHB, developed the study design, conducted the analysis of the strategy, and are the primary 

authors of the report. 

The project team gratefully acknowledges the participation of the following organizations and 

individuals for their assistance in this study:

Mr. Tracy Allen Lovell, PE

Kentucky Transportation Cabinet

Division of Traffic Operations

Ms. Ashley Reinkemeyer, P.E.

Missouri Department of Transportation

Traffic and Highway Safety

Mr. John Miller, P.E.

Missouri Department of Transportation

Traffic and Highway Safety

Mr. Girish Modi, P.E.

Pennsylvania Department of Transportation

Bureau of Highway Safety & Traffic Engineering

Mr. Jeffrey Roecker

Pennsylvania Department of Transportation

Bureau of Highway Safety & Traffic Engineering

Mr. Steven Fink

Pennsylvania Department of Transportation

Bureau of Highway Safety and Traffic Engineering

51

REFERENCES

1. Neuman, T.R. et al., Volume 6: A Guide for Addressing Run-Off-Road Collisions,

Transportation Research Board, Washington, DC, 2003a.

2. Neuman, T.R. et al., Volume 4: A Guide for Addressing Head-On Collisions,

Transportation Research Board, Washington, DC, 2003b.

3. Torbic, D. et al., Guidance for the Design and Application of Shoulder and Centerline 

Rumble Strips, NCHRP Report 641, Transportation Research Board, Washington, DC, 

2009.

4. 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.

5. 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, McLean, VA, 2013.

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. 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, last accessed on November 12, 2014,

http://www.dot.gov/sites/dot.gov/files/docs/VSL_Guidance_2014.pdf.

8. 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. 

53


HRDS-20/06-15(200)E Recycled

Recyclable