Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Safety Evaluation of the 

Safety Edge Treatment

Publication No. FHWA-HRT-11-024 April 2011
FOREWORD

Advancing the safety of America’s highways is a top priority for the Federal Highway 

Administration (FHWA). Through roadway design, cost-effective countermeasures, and

advanced analytical practices, the FHWA Office of Safety Research and Development supports 

this objective by encouraging the development and implementation of improvements, such as

the safety edge, that exhibit real safety benefits for the driving public.

This study evaluated the safety effectiveness of the safety edge treatment in conjunction with 

resurfacing, a cost-effective safety improvement that can reduce crashes and fatalities. 

Development of the safety edge treatment was based on a need to reduce drop-off-related 

crashes and on engineering judgment. The evaluation utilized a before-after empirical Bayes 

analysis for determining a crash reduction factor for this roadway treatment. Furthermore, the 

study conducted a benefit-cost analysis to determine the advantages of applying this treatment

to rural highways. This analysis of the safety edge highlights the benefits of a low-cost 

improvement through improved roadway design and evaluation.

This report will interest safety and highway agency engineers who have a shared responsibility 

for public safety and an interest in implementing low-cost roadway safety treatments.

Monique R. Evans

Director, Office of Safety

Research and Development

Notice

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

in the interest of information exchange. The U.S. Government assumes no liability for the use 

of the information contained in this document. This report does not constitute a standard, 

specification, or regulation.

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-11-024

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

4. Title and Subtitle

Safety Evaluation of the Safety Edge Treatment

5. Report Date

March 2011

6. Performing Organization Code: 

440545878

7. Author(s)

J.L. Graham, K.R. Richard, M.K. O’Laughlin, and D.W. Harwood

8. Performing Organization Report No. 

110495.1.001

9. Performing Organization Name and Address

MRIGlobal

425 Volker Blvd.

Kansas City, MO 64110

10. Work Unit No.

11. Contract or Grant No.

DTFH61-06-C-00013

12. Sponsoring Agency Name and Address

Federal Highway Administration

Turner-Fairbank Highway Research Center

Office of Safety Research and Development

6300 Georgetown Pike

McLean, VA 22101-2296

13. Type of Report and Period Covered

Final Report, 9/1/2006–7/31/2010

14. Sponsoring Agency Code

HRDS-20

15. Supplementary Notes

The FHWA Contracting Officer’s Technical Representative (COTR) was Carol Tan, HRDS-20.

16. Abstract

Between periods of maintenance, pavement-edge drop-offs can form along the edge of highways. When a driver 

runs off the roadway, such drop-offs can hinder reentry and may lead to driver overcorrection, loss of control, or 

overturning on the roadway or roadside. The safety edge is a treatment that is implemented in conjunction with 

pavement resurfacing and is intended to help minimize drop-off-related crashes. 

This report examines the safety effects, costs, and benefits of this low-cost treatment for two-lane and multilane 

rural highways. The safety research was conducted as an observational before-after evaluation of treated sites 

using the empirical Bayes method. The economic appraisal consisted of a benefit-cost analysis. 

The safety evaluation found that the safety edge treatment appears to have a small positive crash reduction effect.

The best effectiveness measure for the safety edge treatment was a 5.7 percent reduction in total crashes on rural 

two-lane highways. However, this result was not statistically significant. The economic analysis showed that the 

treatment is very inexpensive and that its application is highly cost-effective for a broad range of conditions on 

two-lane highways. Inconsistent results were found for rural multilane highways due to a small data sample.

17. Key Words

Safety edge, Pavement-shoulder drop-off, Pavement resurfacing, 

Run-off-road crash

18. Distribution Statement

No restrictions. This document is available 

through the National Technical 

Information Service, Springfield, VA 

22161.

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

95

22. Price

N/A

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

SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha

mi2 square miles 2.59 square kilometers km2

VOLUME 

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3 

yd3 cubic yards 0.765 cubic meters m3 

NOTE: volumes greater than 1000 L shall be shown in m3

MASS oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius o

C 

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2 square millimeters 0.0016 square inches in2 

m2 square meters 10.764 square feet ft2 

m2 square meters 1.195 square yards yd2 

ha hectares 2.47 acres ac

km2 square kilometers 0.386 square miles mi2 

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3 

m3 cubic meters 1.307 cubic yards yd3 

MASS 

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit o

F 

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. 

(Revised March 2003)
iii

TABLE OF CONTENTS

CHAPTER 1. BACKGROUND AND RESEARCH OBJECTIVES ........................................1

1.1 PURPOSE OF THE SAFETY EDGE TREATMENT...................................................1

1.2 RESEARCH OBJECTIVES AND SCOPE.....................................................................2

1.3 SUMMARY OF EVALUATION PLAN..........................................................................2

1.4 ORGANIZATION OF REPORT.....................................................................................3

CHAPTER 2. PROJECT DATABASE........................................................................................5

2.1 PARTICIPATING STATES AND SITE SELECTION.................................................5

2.2 DATA COLLECTION......................................................................................................6

2.2.1 Project Locations and Roadway Characteristics.........................................................7

2.2.2 Crashes........................................................................................................................7

2.2.3 Traffic Volumes........................................................................................................11

2.2.4 Lane Width................................................................................................................12

2.2.5 Field Drop-Off Measurements..................................................................................13

CHAPTER 3. PRELIMINARY ANALYSIS RESULTS FOR FIELD MEASUREMENTS

OF PAVEMENT-EDGE DROP-OFFS......................................................................................15

3.1 COMPARISON OF DROP-OFF MEASUREMENTS FOR TREATMENT AND 

COMPARISON SITES BEFORE RESURFACING..........................................................15

3.2 COMPARISON OF DROP-OFF MEASUREMENTS BETWEEN THE BEFORE

AND AFTER RESURFACING PERIODS .........................................................................16

CHAPTER 4. ANALYSIS RESULTS FOR SAFETY EVALUATION .................................23

4.1 EVALUATION APPROACH.........................................................................................23

4.2 SAFETY PERFORMANCE FUNCTIONS...................................................................25

4.3 SAFETY EVALUATIONS .............................................................................................32

4.3.1 Safety Comparison of Treatment and Comparison Sites in the Period Before 

Resurfacing.........................................................................................................................32

4.3.2 Before-After Evaluation Using the EB Technique ...................................................38

4.3.3 Cross-Sectional Analysis..........................................................................................51

4.3.4 Analysis of Shifts in the Crash Severity Distribution...............................................55

CHAPTER 5. ESTIMATED COST OF THE SAFETY EDGE TREATMENT...................59

5.1 COMPARISON OF OVERALL COSTS OF RESURFACING PROJECTS............59

5.2 COST OF SAFETY EDGE TREATMENT BASED ON AMOUNT OF 

ASPHALT USED ...................................................................................................................61

CHAPTER 6. BENEFIT-COST ANALYSIS............................................................................63

6.1 BENEFIT-COST ANALYSIS APPROACH.................................................................63

6.2 COMPONENTS OF THE BENEFIT-COST ANALYSIS...........................................63

6.2.1 Crash Frequencies.....................................................................................................64

6.2.2 Treatment Effectiveness............................................................................................65

6.2.3 Crash Costs...............................................................................................................65

6.2.4 Service Life...............................................................................................................65
iv

6.2.5 Minimum Attractive Rate of Return.........................................................................65

6.2.6 Uniform Series Present Worth Factor.......................................................................66

6.2.7 Treatment Cost..........................................................................................................66

6.2.8 Benefit Cost Ratio.....................................................................................................66

6.3 BENEFIT-COST ANALYSIS RESULTS .....................................................................66

CHAPTER 7. CONCLUSIONS..................................................................................................71

CHAPTER 8. RECOMMENDATIONS....................................................................................73

APPENDIX A. IDENTIFICATION OF DROP-OFF-RELATED CRASHES......................75

APPENDIX B. PAVEMENT-EDGE DROP-OFF DATA COLLECTION 

METHODOLOGY.......................................................................................................................77

SELECTION OF DATA COLLECTION LOCATIONS ..................................................77

FIELD MEASUREMENTS ..................................................................................................77

SHOULDER TYPE AND WIDTH.......................................................................................79

DROP-OFF SHAPE...............................................................................................................79

LANE WIDTH AND PAVEMENT WIDTH ......................................................................79

DROP-OFF HEIGHT............................................................................................................79

APPENDIX C. SCATTER PLOTS OF ACCIDENTS AND AADT.......................................81

REFERENCES.............................................................................................................................87
v

LIST OF FIGURES

Figure 1. Diagram. Safety edge detail .............................................................................................1

Figure 2. Graph. Drop-off height measurement distributions for two-lane highways with 

paved shoulders in Georgia............................................................................................................18

Figure 3. Graph. Comparison of Georgia SPFs by crash severity and roadway and shoulder 

type.................................................................................................................................................29

Figure 4. Graph. Comparison of Indiana SPFs by crash severity and roadway and shoulder 

type.................................................................................................................................................29

Figure 5. Diagram. Typical cross section for the safety edge treatment on one side of the road .....61

Figure 6. Graph. Minimum benefit-cost ratios for the safety edge treatment as a function 

of AADT........................................................................................................................................69

Figure 7. Graph. Maximum benefit-cost ratios for the safety edge treatment as a function 

of AADT........................................................................................................................................69

Figure 8. Illustration. Sample data collection form .......................................................................78

Figure 9. Illustration. Data collection intervals .............................................................................79

Figure 10. Illustration. Measurement of drop-off perpendicular to pavement surface..................80

Figure 11. Photo. Measurement of pavement-edge drop-off height..............................................80

Figure 12. Graph. Georgia multilane roadway with paved shoulder.............................................82

Figure 13. Graph. Georgia two-lane roadway with paved shoulder..............................................83

Figure 14. Graph. Georgia two-lane roadway with unpaved shoulder..........................................84

Figure 15. Graph. Indiana two-lane roadway with paved shoulder...............................................85

Figure 16. Graph. Indiana two-lane roadway with unpaved shoulder...........................................86
vi

LIST OF TABLES

Table 1. Summary of number and total length of sites....................................................................6

Table 2. Summary of total non-intersection crash data for study sites..........................................10

Table 3. Summary of fatal and injury non-intersection crash data for study sites ........................11

Table 4. Summary of traffic volume data for study sites...............................................................12

Table 5. Summary of lane widths for study sites...........................................................................13

Table 6. Comparison of the proportions of drop-off heights exceeding 2 inches for the

period before resurfacing ...............................................................................................................16

Table 7. Summary of pavement-edge drop-off height measurements...........................................17

Table 8. Comparison of the proportions of drop-off heights exceeding 2 inches between 

the before and after resurfacing periods.........................................................................................19

Table 9. Comparison of the proportions of drop-off heights exceeding 2 inches between 

treatment and comparison sites for the final period after resurfacing ...........................................21

Table 10. SPFs for Georgia sites ...................................................................................................27

Table 11. SPFs for Indiana sites ....................................................................................................28

Table 12. Run-off-road and drop-off-related crash frequencies as a proportion of total 

crashes............................................................................................................................................30

Table 13. Georgia SPF calibration factors.....................................................................................31

Table 14. Indiana SPF calibration factors......................................................................................32

Table 15. Evaluation of treatment versus comparison site effect for the period before 

resurfacing using EB-adjusted crash frequencies..........................................................................34

Table 16. Evaluation of treatment versus comparison site effect for the period before 

resurfacing using observed crash frequencies................................................................................36

Table 17. Before-after EB evaluation results for total crashes......................................................40

Table 18. Before-after EB evaluation results for fatal and injury crashes.....................................41

Table 19. Before-after EB evaluation results for PDO crashes.....................................................42

Table 20. Before-after EB evaluation results for total run-off-road crashes .................................43

Table 21. Before-after EB evaluation results for fatal and injury run-off-road crashes................44

Table 22. Before-after EB evaluation results for PDO run-off-road crashes ................................45

Table 23. Before-after EB evaluation results for total drop-off-related crashes............................46

Table 24. Before-after EB evaluation results for fatal and injury drop-off-related crashes..........47

Table 25. Before-after EB evaluation results for PDO drop-off-related crashes...........................48

Table 26. Summary of safety effects from year 3, year 2, and year 1 results for before-after 

EB safety evaluations.....................................................................................................................50

Table 27. Cross-sectional analysis of safety edge treatment effect for the period after 

resurfacing......................................................................................................................................53

Table 28. Comparison of proportions of fatal and injury crashes before and after resurfacing ....55

Table 29. Summary of Georgia resurfacing project costs (2005)..................................................59

Table 30. Summary of Indiana resurfacing project costs (2005)...................................................59

Table 31. Summary of New York resurfacing project costs (2005)..............................................60

Table 32. Summary of combined Georgia, Indiana, and New York resurfacing project costs 

(2005).............................................................................................................................................60

Table 33. SPFs used in benefit-cost analysis.................................................................................64

Table 34. Benefit-cost analysis for application of safety edge treatment on Georgia two-lane 

roadways with paved shoulders.....................................................................................................67
vii

Table 35. Benefit-cost analysis for application of safety edge treatment on Indiana two-lane 

roadways with paved shoulders.....................................................................................................67

Table 36. Benefit-cost analysis for application of safety edge treatment on Georgia two-lane 

roadways with unpaved shoulders.................................................................................................68

Table 37. Benefit-cost analysis for application of safety edge treatment on Indiana two-lane 

roadways with unpaved shoulders.................................................................................................68

Table 38. Classification criteria for crashes...................................................................................75

1

CHAPTER 1. BACKGROUND AND RESEARCH OBJECTIVES

This chapter describes the background and objectives of this research and the organization of the 

remainder of the report. 

1.1 PURPOSE OF THE SAFETY EDGE TREATMENT

Many two-lane rural highways have unpaved shoulders immediately adjacent to the traveled way.

Other two-lane highways and many multilane rural highways have narrow paved shoulders with 

widths of 1–4 ft. If roadway maintenance crews do not keep material against the pavement edge, 

a pavement-edge drop-off may form. The drop-off height can vary from less than 1 inch to 6 inches

or more, even though maintenance performance standards usually require maintenance when the 

drop-off exceeds 1.5–2 inches.

(1)

When a vehicle leaves the traveled way and encounters a pavement-edge drop-off, it can be 

difficult for the driver to return safely to the roadway. As the driver attempts to steer back onto 

the pavement, the side of the tire may scrub along the drop-off, resisting the driver’s attempts. 

This resistance often leads the driver to overcorrect with a greater steering angle than desired to 

remount the drop-off. When the tire does remount the pavement, the increased tire angle may 

“slingshot” the vehicle across the road, resulting in a collision with other traffic or a loss of 

control and overturning on the roadway or roadside.

The safety edge is a treatment that is intended to minimize drop-off-related crashes. With this 

treatment, the pavement edge is sloped at an angle of 30 degrees to reduce the resistance of the 

tire remounting the drop-off (see figure 1). The reduced resistance is intended to allow a more 

controlled reentry onto the traveled way.

Figure 1. Diagram. Safety edge detail.

Research conducted by the Texas Transportation Institute (TTI) in the 1980s found that drivers 

rated a 45-degree wedge as a much safer pavement edge to remount than either the vertical or

rounded edges normally found with portland cement concrete and asphalt pavements.(2) Because 

drivers in the study were instructed to go off the pavement edge, the TTI research has been criticized

as not being representative of naïve drivers. Prior to this research, actual field evaluation of the 

safety edge had not been completed.
2

Selected highway agencies have begun to use the safety edge treatment as part of their pavement 

resurfacing projects. However, there has been no formal evaluation of the effectiveness of this 

treatment in reducing drop-off-related crashes on rural highways. Such an evaluation is needed to 

determine whether this treatment should receive more widespread use.

1.2 RESEARCH OBJECTIVES AND SCOPE

Eight State highway agencies joined the Federal Highway Administration (FHWA) in a pooledfund study to implement and evaluate the safety edge treatment in conjunction with pavement 

resurfacing projects. Four State agencies provided study sites for this evaluation: the Colorado 

Department of Transportation, the Georgia Department of Transportation, the Indiana Department

of Transportation, and the New York State Department of Transportation. The evaluation of the 

safety edge treatment extended over a 3-year period. Unpublished interim reports were prepared 

for the first and second years after implementation of the safety edge treatment. This final report 

presents the evaluation results for the entire 3-year study following implementation of the treatment. 

The primary objective of the evaluation was to quantify the safety effectiveness of the safety edge

treatment. An evaluation was performed to determine whether including the safety edge treatment

as part of a pavement resurfacing project reduces crashes in comparison to pavement resurfacing 

without the safety edge treatment. The evaluation results are presented in terms of the percentage 

reduction in specific crash types that can be expected from the provision of the safety edge treatment.

Other objectives of the study were to document the effectiveness of the safety edge treatment in 

reducing the presence of pavement-edge drop-offs and to perform an economic analysis of the 

safety edge treatment. The economic analysis used the safety effectiveness evaluation results and 

project cost data to define the types of roadways and traffic volume levels for which the safety 

edge treatment would be cost-effective. 

The project scope included two-lane rural roads with no paved shoulder and with a paved shoulder

no wider than 4 ft. Multilane roads with paved shoulders no wider than 4 ft were also studied. 

1.3 SUMMARY OF EVALUATION PLAN

The evaluation plan for the safety edge treatment was based on the following types of sites:

Sites that were resurfaced and treated with the safety edge (referred to as treatment sites).

Sites that were resurfaced but not treated with the safety edge (referred to as comparison

sites).

Sites that were similar to the treatment and comparison sites but were not resurfaced 

(referred to as reference sites). 

This final report is based on data for the characteristics and performance of treatment, comparison,

and reference sites during the period before the treatment and comparison sites were resurfaced and

for 3 years after resurfacing. Data collected and analyzed in this report include field measurements

of drop-offs present on the treated sites before and during the 3 years after resurfacing; crash records

for 2–5 years before the site was resurfaced and 3 years after resurfacing; traffic volumes and road

characteristics for each site; and the date and cost of resurfacing the treatment and comparison sites.
3

This report presents the results of a comparison of the presence of pavement-edge drop-offs 

between the treatment and comparison sites for the period before resurfacing and during the 

3 years after resurfacing.

The report also presents the safety evaluation results using traffic volume and crash data for the 

period before resurfacing of the treatment and comparison sites and the 3 years after resurfacing.

Two statistical approaches were used to analyze these data: (1) a before-after comparison using the

empirical Bayes (EB) technique and (2) a cross-sectional comparison of the safety performance of 

sites that were resurfaced with and without the safety edge treatment based on the after period only. 

To estimate the safety performance of the safety edge treatment in the before-after EB analysis, 

safety performance functions (SPFs) were developed from the reference site data using negative 

binomial regression analysis.

The frequencies of specific target crash types were used as the dependent variables for the safety 

evaluation. The target crashes for the safety evaluation exclude at-intersection and intersectionrelated crashes because the safety edge treatment is targeted primarily at non-intersection crashes.

Safety measures used as dependent variables for this report included the frequencies of total nonintersection crashes, run-off-road crashes, and drop-off-related crashes. Run-off-road crashes included

those crashes in which one or more involved vehicles left the road. Drop-off-related crashes were a 

subset of run-off-road crashes for which the crash data included specific evidence that a pavementedge drop-off may have been involved, such as the inclusion of “low shoulder” or “shoulder defect”

as a contributing factor. Separate analyses were conducted for each target crash type for fatal and

injury crashes, property-damage-only crashes, and all crash severity levels combined.

Cost data for the resurfacing projects at the treatment and comparison sites are included in the 

report, and findings are presented concerning the cost-effectiveness of the safety edge treatment.

1.4 ORGANIZATION OF REPORT

The remainder of this report is organized as follows:

Chapter 2 documents the project database, including a summary of the length of the sites 

studied, the crash data analyzed, traffic volumes and characteristics of the sites, and field 

measurements of the pavement-edge drop-offs. 

Chapter 3 presents the analysis results for the field measurements of pavement-edge drop-offs.

Chapter 4 presents the safety effectiveness evaluation.

Chapter 5 presents project cost comparisons for sites resurfaced with and without the 

safety edge. 

Chapter 6 presents the benefit-cost economic analysis.

Chapter 7 presents conclusions drawn from the analysis results.

Chapter 8 presents recommendations based on results of the 3-year evaluation.

5

CHAPTER 2. PROJECT DATABASE

Evaluation of the safety edge treatment required data on roadway geometrics, traffic volumes, 

crashes, construction costs, and implementation projects for sites where the safety edge treatment 

was implemented and for other similar sites. This chapter describes the selection of sites and 

assembly of the project database.

2.1 PARTICIPATING STATES AND SITE SELECTION

Three States agreed to implement the safety edge treatment and to participate in the study:

Georgia, Indiana, and New York. Colorado also agreed to participate in the study, but no sites 

were resurfaced with the safety edge treatment in time for inclusion in the analysis. Sites for the

study were selected with the assistance of the participating State highway agencies. However, the 

site selection approach varied for three types of study sites: sites that were resurfaced and treated 

with the safety edge (treatment sites); sites that were resurfaced but not treated with the safety 

edge (comparison sites); and sites that were similar to the treatment and comparison sites but 

were not resurfaced (reference sites).

Treatment sites were selected by the three participating States from among the sites considered 

for their normal resurfacing program for 2005. In Indiana and New York, the sites that received 

the safety edge treatment were selected by the State as representative resurfacing projects for

which the safety edge treatment would be appropriate. In Georgia, the transportation department 

made a policy decision to include the safety edge treatment in all resurfacing projects let to 

contract in April 2005 or thereafter. The treatment sites for this evaluation were drawn from

among the projects let to contract after that date. 

Most of the sites selected by the State highway agencies were used in this evaluation. A few sites 

that were distinctly different from the remainder of the study sites were dropped from the evaluation.

Based on a preliminary review of the available treated projects in Georgia, Indiana, and New York,

the decision was made to focus the analysis on the following three types of roadway segments:

Rural multilane roadways with paved shoulders with widths of 4 ft or less.

Rural two-lane roadways with paved shoulders with widths of 4 ft or less.

Rural two-lane roadways with no paved shoulders (i.e., unpaved shoulders only).

Comparison sites were selected from among projects that were resurfaced in 2005 but did not receive

the safety edge treatment. In Georgia, the comparison sites were resurfacing projects that were let

to contract prior to April 2005, the date on which the Georgia Department of Transportation began

implementing the safety edge treatment in all resurfacing projects. The comparison sites were 

selected to include the same roadway types as the treatment sites. The comparison sites were located

in the same highway districts as the treatment sites so they were in the same geographical area.

Reference sites in each participating State included sites that had not been resurfaced during the 

period before resurfacing of the treatment and comparison sites and were not expected to be 

resurfaced during the entire 3-year study period. The reference sites included the same roadway
6

types as the treatment and comparison sites. The total length of reference sites selected in each State

was at least the same length as the treated sites in the State and often larger. Reference sites were

chosen from the same highway districts as the treatment sites so they were in the same geographical

area. Input from district engineers was sought to ensure that the reference sites were similar to the

treatment sites. No reference sites were selected in New York because the reference sites were 

needed only for the before-after EB evaluation and it appeared unlikely that an EB evaluation 

could be conducted for the limited set of treatment sites available in New York. The New York 

data were included in other evaluations without the need for reference sites.

Each resurfacing project was divided into smaller roadway segments as needed based on a review

of site characteristics and traffic volumes to assure that each site was relatively homogenous with 

respect to lane width, shoulder type and width, and traffic volume. The project database included

415 sites: 261 in Georgia, 148 in Indiana, and 6 in New York. The individual sites ranged in length

from 0.1 to 25.8 mi. The total length of all segments considered in the study was 685 mi in Georgia,

514 mi in Indiana, and 25 mi in New York. Table 1 summarizes the number of sites by State, 

roadway type, shoulder type, and site type.

Table 1. Summary of number and total length of sites.

State Roadway type Shoulder type Site type Number of sites Length (mi)

GA

Multilane Paved

T 10 18.9

C 7 12.9

R 15 23.5

Two-lane

Paved

T 25 53.0

C 19 26.9

R 53 201.9

Unpaved

T 22 45.2

C 31 92.8

R 79 210.1

Combined 261 685.3

IN Two-lane

Paved

T 14 25.5

C 7 21.3

R 29 101.3

Unpaved

T 16 58.0

C 18 71.2

R 64 237.0

Combined 148 514.1

NY Two-lane Paved T 3 10.0C 3 15.2

Combined 6 25.2

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.

R = Reference sites not resurfaced.

Table 1 shows that the project database included 90 treatment sites with a total length of 211 mi, 

with 57 treatment sites in Georgia, 30 treatment sites in Indiana, and 3 treatment sites in New York.

The project database also includes 85 comparison sites with a total length of 240 mi and 240 reference

sites with a total length of 774 mi.

2.2 DATA COLLECTION

A substantial amount of data was collected and assembled into a database for consideration in the

analysis phase of the study. Data were collected for the period before resurfacing of the treatment 
7

and comparison sites and for 3 years after resurfacing. Information concerning data availability, 

data collection procedures, and contents is presented below for the following data types:

Project locations and roadway characteristics.

Crashes.

Traffic volumes.

Field measurements of pavement-edge drop-offs.

2.2.1 Project Locations and Roadway Characteristics

For each treatment, comparison, and reference site, the project database included the following 

data elements: location on the agency’s highway system, project construction dates, and basic 

roadway characteristics. The basic roadway characteristics obtained included road type, lane width, 

and shoulder type and width. These data were obtained from State highway databases or published

reports. All State data were verified and supplemented by field visits to the sites.

Analysis units for the study (i.e., study sites) were created by subdividing resurfacing projects 

into sections that were generally homogeneous with respect to roadway geometrics. The roadway 

characteristics used to define the site boundaries were monitored for changes other than resurfacing.

2.2.2 Crashes

The crash database for the study included all non-intersection crashes that occurred within the limits

of each site during the study period. Crash data, provided by the participating agencies from their 

electronic crash record databases, contained sufficient summary information to identify the target 

crash types most likely to be affected by provision of the safety edge.

Where possible, it was desirable to limit the evaluation to specific target crash types that were most

likely affected by the implementation of the safety edge. If the crash data for both the before and 

after periods included crash types that could not conceivably be affected by the safety edge treatment,

then this “noise” could introduce unnecessary variability into the crash counts and mask the safety

effect of the treatment. For example, the installation of the safety edge treatment is likely to have a 

greater effect on run-off-road crashes than on rear-end crashes. By limiting the analysis to include

only run-off-road crashes, the likelihood of finding statistically significant effects may be improved.

However, the more restrictive the crash type definition used, the smaller the crash counts available 

for analysis, making it more difficult to find statistically significant effects. Because of this 

tradeoff between the relevance of the target crash type to the treatment being evaluated and the 

number of crashes available for analysis, a range of target crash type definitions from more 

inclusive and less relevant to less inclusive and more relevant was considered.

The selection of the target crash types to be evaluated was guided by two recent studies of crashes

related to pavement-edge drop-offs by Council and Hallmark et al.(1) These studies identified five 

scenarios (crash sequences) in which over-steering may result in a crash related to a pavement-edge

drop-off. This report assumes that only these types of crashes and no others would be affected by 

provision of the safety edge.
8

The five types of crashes used to identify potential drop-off-related crashes are as follows:

Head-on collision with an oncoming vehicle.

Sideswipe collision with an oncoming vehicle.

Run-off-road crash on the opposite side of the road.

Overturning within the traveled way or on the opposite side of the road.

Same-direction sideswipe collision on multilane roads.

Head-on crashes may involve a vehicle that crossed the centerline without first running off the 

road. Such head-on crashes were not classified as drop-off-related nor treated as target crashes. 

The target crash types described represent potential drop-off-related crashes, defined as precisely 

as possible without obtaining and reviewing individual police crash forms. Past research by Council, 

which included a detailed analysis of hard-copy reports, indicated that a larger percentage of

potential crashes were judged as probable or possible drop-off crashes when the officer had noted a 

shoulder defect. Therefore, if the agency’s crash form had an item for “low shoulder” or “shoulder

defect,” then this item was used to identify potential drop-off-related crashes.

This methodology represents a narrow interpretation of drop-off-related crashes. Therefore, it was

also recommended that crashes that showed evidence of a vehicle leaving the road and run-offroad crashes be included, such as the following:

Run-off-road right, cross centerline/median, hit vehicle traveling in the opposite direction 

(head-on or sideswipe).

Run-off-road right, sideswipe with vehicle in same direction (multilane roads).

Run-off-road right, rollover (in road or on roadside).

Run-off-road right, then run-off-road left.

Single vehicle run-off-road right.

Selection of the crash types was based on descriptors in the crash database furnished by the 

participating States. The data fields used included sequence of events, location of first harmful 

event, type of collision, driver, and roadway contributing circumstances. The specific fields used

to identify drop-off-related crashes in this study for each participating State are described in

appendix A.

Crash severity levels considered in the evaluation are as follows:

Fatal, injury, and property-damage-only (PDO) crashes (i.e., all crash severity levels 

combined).
9

Fatal and injury crashes.

PDO crashes.

The highest priority in assessment of the safety edge treatment is the evaluation of its effect on 

fatal and injury crashes because these categories include the most severe crashes among the target

crash types of interest. Crashes of all severity levels (i.e., including PDO crashes) were considered

because the larger crash sample size made it easier to detect statistically significant effects. It would

have been more desirable to consider only PDO crashes that were severe enough for at least one 

vehicle to be towed from the crash scene since PDO tow-away crashes are more consistently 

reported than other PDO crashes. However, this exclusion was not applied because only one of 

the participating States (Indiana) identified tow-away crashes in its data.

Table 2 and table 3 summarize the crash data for total and fatal and injury crashes, respectively, 

including the breakdown of total, run-off-the-road, and drop-off-related crashes for each State, 

roadway type, shoulder type, and site type . 

Indiana was able to provide only reference-point (i.e., milepost) information and latitude and

longitude information for some of the crashes. Additionally, some of the reference-point 

information provided with the crashes indicated that the crashes occurred on side roads at 

intersections. Approximately 40 percent of the crashes had wrong or missing reference point or 

coordinate information but contained a verbal description of the crash. Extensive efforts to better 

locate these crashes were undertaken during the execution of the work plan. 
10

Table 2. Summary of total non-intersection crash data for study sites.

State

Roadway 

type

Shoulder 

type Site type

Number 

of sites

Dates for study periods

Site

length (mi)

Number of crashes during 

before and after study 

periods combined1

Before 

resurfacing

After 

resurfacing

Total 

crashes

Run-offroad 

crashes

Drop-offrelated 

crashes

GA

Multilane Paved

T 10 1999 to 2004 2006 to 2008 18.9 563 162 99

C 7 1999 to 2004 2006 to 2008 12.9 368 120 81

R 15 1999 to 2004 2006 to 2008 23.5 927 199 118

Two-lane

Paved

T 25 1999 to 2004 2006 to 2008 53.0 844 306 186

C 19 1999 to 2004 2006 to 2008 26.9 475 223 157

R 53 1999 to 2004 2006 to 2008 201.9 2,489 924 573

Unpaved

T 22 1999 to 2004 2006 to 2008 45.2 820 335 216

C 31 1999 to 2004 2006 to 2008 92.8 874 427 289

R 79 1999 to 2004 2006 to 2008 210.1 2,105 995 631

Combined 261 1999 to 2004 2006 to 2008 685.3 9,465 3,691 2,350

IN Two-lane

Paved

T 14 2003 to 2004 2006 to 2008 25.5 250 58 12

C 7 2003 to 2004 2006 to 2008 21.3 234 55 25

R 29 2003 to 2004 2006 to 2008 101.3 646 176 59

Unpaved

T 16 2003 to 2004 2006 to 2008 58.0 169 59 16

C 18 2003 to 2004 2006 to 2008 71.2 287 145 73

R 64 2003 to 2004 2006 to 2008 237.0 810 260 96

Combined 148 2003 to 2004 2006 to 2008 514.1 2,396 753 281

NY Two-lane Paved

T 3 1999 to 2004 2006 to 2008 10.0 130 66 3

C 3 1999 to 2004 2006 to 2008 15.2 218 79 4

Combined 6 1999 to 2004 2006 to 2008 25.2 348 145 7

Combined 415 1,224.6 12,209 4,589 2,638

1 Does not include at-intersection or intersection-related crashes.

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.

R = Reference sites not resurfaced.
11

Table 3. Summary of fatal and injury non-intersection crash data for study sites.

State

Roadway 

type

Shoulder 

type Site type

Number 

of sites

Dates for study periods

Site 

length (mi)

Number of fatal and injury 

crashes during before and 

after study periods combined1

Before 

resurfacing

After 

resurfacing

Total 

crashes

Run-offroad 

crashes

Drop-offrelated 

crashes

GA

Multilane Paved

T 10 1999 to 2004 2006 to 2008 18.9 154 64 47

C 7 1999 to 2004 2006 to 2008 12.9 121 49 37

R 15 1999 to 2004 2006 to 2008 23.5 366 108 71

Two-lane

Paved

T 25 1999 to 2004 2006 to 2008 53.0 313 137 99

C 19 1999 to 2004 2006 to 2008 26.9 229 125 96

R 53 1999 to 2004 2006 to 2008 201.9 856 437 315

Unpaved

T 22 1999 to 2004 2006 to 2008 45.2 279 162 120

C 31 1999 to 2004 2006 to 2008 92.8 374 225 166

R 79 1999 to 2004 2006 to 2008 210.1 892 512 366

Combined 261 1999 to 2004 2006 to 2008 685.3 3,584 1,819 1,317

IN Two-lane

Paved

T 14 2003 to 2004 2006 to 2008 25.5 37 14 3

C 7 2003 to 2004 2006 to 2008 21.3 57 20 7

R 29 2003 to 2004 2006 to 2008 101.3 129 73 29

Unpaved

T 16 2003 to 2004 2006 to 2008 58.0 31 18 5

C 18 2003 to 2004 2006 to 2008 71.2 83 58 32

R 64 2003 to 2004 2006 to 2008 237.0 141 91 35

Combined 148 2003 to 2004 2006 to 2008 514.1 478 274 111

NY Two-lane Paved

T 3 1999 to 2004 2006 to 2008 10.0 59 42 3

C 3 1999 to 2004 2006 to 2008 15.2 75 42 3

Combined 6 1999 to 2004 2006 to 2008 25.2 134 84 6

Combined 415 1,224.6 4,196 2,177 1,434

1 Does not include at-intersection or intersection-related crashes.

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.

R = Reference sites not resurfaced.

2.2.3 Traffic Volumes

Annual average daily traffic (AADT) volume data for all study locations were obtained through 

agency databases or published sources from each of the participating agencies, so no field traffic 

counts were required as part of the database development. When possible, separate AADT values 

for each year of the study period were obtained. When AADT values were not available for all 

years of the study period, values were interpolated or extrapolated for the missing years.

Table 4 summarizes the traffic volume data assembled for the project database. Ideally, the AADT

ranges should be as similar as possible for the various site types within each State/road type/shoulder

type combination. In particular, it was desirable for reference sites to cover the entire range of 

values of the treatment and comparison sites, as SPF performance outside the range of the reference

sites is not optimum. It was also desirable that the comparison and reference sites have nearly 

identical ranges. The AADT ranges were found to be similar for most cases except for multilane 

highway sites with paved shoulders in Georgia. For these sites, the AADT ranges were higher for 
12

treatment sites than for comparison or reference sites. To a lesser extent, the same is true for twolane highway sites with paved shoulders in Indiana.

Table 4. Summary of traffic volume data for study sites.

State

Roadway 

type

Shoulder 

type Site type

Number 

of sites

Site

length (mi)

AADT (vehicles/day)

Minimum

Mean 

before 

resurfacing

Mean

after 

resurfacing Maximum

GA

Multilane Paved

T 10 18.9 7,639 15,417 14,966 23,825

C 7 12.9 4,467 9,988 11,148 22,160

R 15 23.5 6,087 10,060 10,373 22,302

Combined 32 55.3 4,467 11,874 12,124 23,825

Two-lane

Paved

T 25 53.0 410 4,046 3,983 13,237

C 19 26.9 1,453 4,929 6,104 11,247

R 53 201.9 397 4,118 4,122 18,697

Combined 97 281.9 397 4,182 4,285 18,697

Unpaved

T 22 45.2 1,285 3,418 3,601 9,650

C 31 92.8 413 3,134 2,976 15,000

R 79 210.1 310 2,996 3,001 9,660

Combined 132 348.1 310 3,087 3,073 15,000

IN Two-lane

Paved

T 14 25.5 2,198 6,584 6,561 14,662

C 7 21.3 3,406 5,067 5,047 7,457

R 29 101.3 1,170 4,046 4,056 8,958

Combined 50 148.0 1,170 4,629 4,629 14,662

Unpaved

T 16 58.0 376 1,444 1,436 3,158

C 18 71.2 996 1,858 1,845 6,423

R 64 237.0 478 2,554 2,548 13,615

Combined 98 366.1 376 2,243 2,235 13,615

NY Two-lane Paved

T 3 10.0 1,058 3,601 3,776 5,797

C 3 15.2 1,110 3,687 3,693 7,047

Combined 6 25.2 1,058 3,653 3,726 7,047

Combined 415 1,224.6 310 3,682 3,712 23,825

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.

R = Reference sites not resurfaced.

2.2.4 Lane Width

Lane widths ranged from 9 to 13 ft across all sites and States, with the majority of lanes being 

12-ft wide. The distribution of lane width is summarized in table 5 by State and site type. The 

variability in lane width was most evident for the unpaved shoulder type, so it was decided to 

include this variable in modeling efforts for these sites.
13

Table 5. Summary of lane widths for study sites.

State

Road 

type

Shoulder 

type Site type

Number 

of sites

Site 

length (mi)

Lane width (ft)

Minimum Mean Maximum

GA

Multilane Paved

T 10 18.9 12 12.3 13

C 7 12.9 12 12.7 13

R 15 23.5 12 12.3 13

Combined 32 55.3 12 12.4 13

Two-lane

Paved

T 25 53.0 11 12.0 13

C 19 26.9 12 12.6 13

R 53 201.9 11 12.3 13

Combined 97 281.9 11 12.3 13

Unpaved

T 22 45.2 11 11.9 13

C 31 92.8 10 12.0 13

R 79 210.1 10 12.2 13

Combined 132 348.1 10 12.1 13

IN Two-lane

Paved

T 14 25.5 12 12.0 13

C 7 21.3 12 12.2 13

R 29 101.3 9 11.5 13

Combined 50 148.0 9 11.8 13

Unpaved

T 16 58.0 10 11.4 13

C 18 71.2 9 10.2 11

R 64 237.0 9 11.3 13

Combined 98 366.1 9 11.1 13

NY Two-lane Paved

T 3 10.0 10 10.6 11

C 3 15.2 9 11.0 12

Combined 6 25.2 9 10.8 12

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.

R = Reference sites not resurfaced.

2.2.5 Field Drop-Off Measurements

Field visits were made to each treatment and comparison site to collect pavement-edge drop-off

measurements and additional geometric design variables. Field measurements of pavement-edge 

drop-offs were made before resurfacing and during each of the 3 years after resurfacing. However,

some of the project sites were resurfaced before field visits could be made, which prevented 

supplemental data collection before resurfacing at some sites. Drop-off height was measured 4 inches

from the pavement edge for all sites. The types of data collected and the methodology for collecting

these data are documented in appendix B. 

15

CHAPTER 3. PRELIMINARY ANALYSIS RESULTS FOR FIELD MEASUREMENTS 

OF PAVEMENT-EDGE DROP-OFFS

This chapter presents preliminary analysis results for field measurements of pavement-edge 

drop-offs. Field measurements of drop-off heights were made to evaluate the comparability of 

existing pavement-edge drop-offs for the treatment and comparison sites in the period before 

resurfacing and to verify that the safety edge treatment does not encourage the development of 

pavement-edge drop-offs in the period after resurfacing.

Field data for pavement-edge drop-off heights were collected for each participating agency for 

both treatment and comparison sites in the period before resurfacing and during each year after 

resurfacing. The field data collection methodology is presented in appendix B. A few sites were 

resurfaced before field visits could be made. Consequently, these sites were excluded from the 

analysis of before-period drop-off height data presented in this chapter.

3.1 COMPARISON OF DROP-OFF MEASUREMENTS FOR TREATMENT AND 

COMPARISON SITES BEFORE RESURFACING

A formal assessment of the comparability of the treatment and comparison sites with respect to 

the presence of pavement-edge drop-offs in the period before resurfacing was undertaken. The 

measure used for this comparison was the proportion of drop-off heights exceeding 2 inches. 

This criterion was used based on research indicating that pavement-edge drop-off heights 

exceeding 2 inches may affect safety.(1) It should be noted that this previous research was

conducted on sites without the safety edge treatment.

It would be desirable if the proportion of sites with pavement-edge drop-off heights exceeding

2 inches were similar for the treatment and comparison sites in the period before resurfacing. An 

analysis to make this comparison was conducted by performing a logistic regression analysis using

the LOGISTIC procedure in SAS®.

(3) This procedure uses the Fisher scoring method to estimate 

the statistical significance of differences in proportions between the treatment and comparison sites.

Ideal results for this analysis would have been obtained if the difference between the proportions 

of drop-off heights exceeding 2 inches for the treatment and comparison sites were not statistically

significant at some predetermined significance level. A statistically significant result would be

indicated by an odds ratio point estimate that was significantly greater than or less than 1.0 (i.e., the

confidence interval for the odds ratio does not contain 1.0). Conversely, for a difference that is not

statistically significant, the odds ratio for the difference would contain 1.0. If the odds ratio could 

not be determined by maximum likelihood due to small sample size or poor variation of responses

(i.e., identical responses for each site type or non-overlapping responses between site types), then 

an exact test was performed and a median unbiased estimate of the odds ratio was provided. 

The results of this analysis for each State, roadway type, shoulder type, and treatment type 

combination, including the frequency and proportion of measurements exceeding 2 inches, the 

odds ratio point estimate, the odds ratio confidence interval, and the statistical significance of the 

odds ratio point estimate, are given in table 6. Odds ratio values above 1.0 in this table indicate 

that comparison sites had a greater probability of experiencing drop-offs exceeding 2 inches than 

treatment sites.
16

Table 6. Comparison of the proportions of drop-off heights exceeding 2 inches 

for the period before resurfacing.

State

Roadway 

type

Shoulder 

type

Site 

type

Drop-off heights that 

exceed 2 inches Odds ratio 

point 

estimate

Lower 

confidence 

limit

Upper 

confidence 

limit

Statistically 

significant at 

Number Proportion 0.05 level?

GA

Multilane Paved T 2 0.07

C 5 0.06 0.909 0.184 6.596 No

Two-lane

Paved T 10 0.03

C 25 0.14 4.591 2.211 10.259 Yes

Unpaved T 23 0.09

C 29 0.13 1.557 0.876 2.799 No

IN Two-lane

Paved T 6 0.04

C 10 0.10 2.519 0.902 7.642 No

Unpaved T 150 0.39

C 53 0.22 0.423 0.291 0.608 Yes

NY Two-lane Paved T 36 0.38

0.028 0.000 1.620 No1 C 0 0.00

1 Indicates that median unbiased estimate was used.

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.

The results in table 6 indicate that in the period before resurfacing, there were relatively equal 

proportions of extreme drop-off heights between treatment and comparison sites for Georgia sites

on multilane highways with paved shoulders and two-lane highways with unpaved shoulders. 

This finding indicates that these two types of sites were relatively well matched in terms of 

shoulder conditions in the period before resurfacing. By contrast, the findings for Georgia sites 

on two-lane highways with paved shoulders suggest that there was a statistically significant 

chance that comparison sites had greater proportions of drop-offs exceeding 2 inches.

For Indiana sites on two-lane highways with paved shoulders, there was a greater proportion of 

extreme drop-off heights for the comparison sites than for the treatment sites in the period before 

resurfacing, but the difference was not statistically significant. The opposite was the case for Indiana 

sites on two-lane highways with unpaved shoulders and for New York sites on two-lane highways

with paved shoulders. In these cases, the treatment and comparison sites were not perfectly matched 

in terms of shoulder conditions in the period before resurfacing. For Indiana, this difference was 

statistically significant. Some differences of this sort may have been inevitable because resurfacing 

projects that received the safety edge treatment were not selected based on consideration of the 

existing shoulder condition. This is a potential confounding factor that should be considered in 

interpreting the research results. 

3.2 COMPARISON OF DROP-OFF MEASUREMENTS BETWEEN THE BEFORE AND 

AFTER RESURFACING PERIODS

The field measurement data for pavement-edge drop-offs were initially reviewed by State, roadway

type, shoulder type, and treatment type. Table 7 presents summary descriptive statistics for these 

measures for each study period. Figure 2 presents histograms for a sample of the distributions and

shows the impact of resurfacing for both treatment and comparison sites.
17

Table 7. Summary of pavement-edge drop-off height measurements.

State

Road 

type

Shoulder 

type

Site 

type

Before resurfacing After resurfacing (Year 1)

Number of 

measurements

Drop-off height (inches) Coefficient 

of variation 

%

Number of 

measurements

Drop-off height (inches) Coefficient 

of variation 

Minimum Mean Median Maximum %

Standard 

deviation Minimum Mean Median Maximum

Standard 

deviation

GA

Multilane Paved

T 30 0 0.783 0.750 2.000 0.618 79 59 0.375 1.047 0.875 2.875 0.504 48

C 82 0 0.811 0.750 3.000 0.710 88 86 0.250 1.038 1.000 2.375 0.467 45

Two-lane

Paved

T 291 0 0.546 0.500 3.750 0.611 112 289 0.000 0.960 1.000 2.375 0.495 52

C 178 0 0.912 0.750 4.000 0.912 100 150 0.000 0.887 0.875 1.875 0.471 53

Unpaved

T 270 0 0.881 0.750 3.750 0.695 79 273 0.000 0.941 0.875 2.500 0.495 53

C 229 0 1.076 1.000 4.750 0.804 75 466 0.000 0.945 0.875 2.875 0.556 59

IN Two-lane

Paved

T 136 0 0.630 0.500 3.500 0.598 95 158 0.000 0.703 0.625 1.875 0.356 51

C 96 0 0.960 0.750 3.250 0.708 74 137 0.250 1.340 1.125 4.250 0.707 53

Unpaved

T 380 0 1.758 1.625 5.125 0.778 44 367 0.250 1.653 1.500 4.500 0.737 45

C 245 0 1.353 1.250 6.875 0.930 69 279 0.125 1.168 1.000 5.250 0.673 58

NY Two-lane Paved

T 94 0 1.681 1.500 5.125 1.270 76 77 0.000 1.110 0.875 4.000 0.886 80

C 42 0 0.777 0.750 1.750 0.487 63 83 0.000 1.065 1.000 2.750 0.480 45

State

Road 

type

Shoulder 

type

Site 

type

After resurfacing (Year 2) After resurfacing (Year 3)

Number of 

measurements

Drop-off height (inches) Coefficient 

of variation 

%

Number of 

measurements

Drop-off height (inches) Coefficient 

of variation 

Minimum Mean Median Maximum %

Standard 

deviation Minimum Mean Median Maximum

Standard 

deviation

GA

Multilane Paved

T 65 0.500 1.175 1.000 3.000 0.448 38 65 0.500 1.175 1.000 3.000 0.448 38

C 86 0.250 0.906 0.813 2.500 0.455 50 86 0.250 0.907 0.875 2.500 0.442 49

Two-lane

Paved

T 212 0.000 0.956 0.875 2.250 0.455 48 254 0.000 1.087 1.000 3.375 0.432 40

C 152 0.375 1.166 1.125 2.250 0.356 31 164 0.250 1.104 1.125 2.250 0.372 34

Unpaved

T 238 0.125 1.179 1.000 3.563 0.571 48 259 0.250 1.107 1.000 3.563 0.566 51

C 426 0.000 1.163 1.125 3.250 0.548 47 448 0.000 1.119 1.063 3.250 0.526 47

IN Two-lane

Paved

T 187 0.000 0.788 0.750 2.250 0.379 48 189 0.125 0.780 0.750 2.250 0.398 51

C 102 0.250 1.456 1.250 4.375 0.857 59 147 0.000 1.344 1.250 3.875 0.609 45

Unpaved

T 370 0.250 1.916 1.750 6.875 0.993 52 373 0.250 1.584 1.375 4.500 0.774 49

C 280 0.000 1.353 1.250 5.500 0.764 56 290 0.125 1.236 1.125 4.500 0.676 55

NY Two-lane Paved

T 78 0.375 1.786 1.344 5.125 1.191 67 78 0.375 1.786 1.344 5.125 1.191 67

C 81 0.625 1.446 1.375 3.250 0.497 34 81 0.625 1.446 1.375 3.250 0.497 34

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.
18

Before

Treatment Site

0

10

20

30

40

50

60

Percent

# Measurements 291

Min 0.0

Mean 0.5

Median 0.5

Max 3.8

STD 0.6

CV 111.8

After1

# Measurements 289

Min 0.0

Mean 1.0

Median 1.0

Max 2.4

STD 0.5

CV 51.5

After2

# Measurements 212

Min 0.0

Mean 1.0

Median 0.9

Max 2.3

STD 0.5

CV 47.6

After3

# Measurements 300

Min 0.0

Mean 0.7

Median 0.8

Max 2.0

STD 0.4

CV 52.1

Comparison Site

0 0.8 1.6 2.4 3.2 4.0

0

10

20

30

40

50

60

Percent

# Measurements 178

Min 0.0

Mean 0.9

Median 0.8

Max 4.0

STD 0.9

CV 100.0

0 0.8 1.6 2.4 3.2 4.0

# Measurements 150

Min 0.0

Mean 0.9

Median 0.9

Max 1.9

STD 0.5

CV 53.1

0 0.8 1.6 2.4 3.2 4.0

# Measurements 152

Min 0.4

Mean 1.2

Median 1.1

Max 2.3

STD 0.4

CV 30.6

0 0.8 1.6 2.4 3.2 4.0

# Measurements 155

Min 0.0

Mean 1.1

Median 1.0

Max 2.3

STD 0.3

CV 30.3

Drop-off Height (in)

Figure 2. Graph. Drop-off height measurement distributions for two-lane highways with 

paved shoulders in Georgia.

In each graph shown in figure 2, the vertical blue line marks a 2-inch drop-off height. The mean 

drop-off height did not vary between the before and after periods. For almost all roadway type/

shoulder type/treatment type combinations, the coefficient of variation (i.e., relative standard

deviation) of drop-off height decreased substantially between before resurfacing and each of the 

first 2 years after resurfacing but increased again following the second year after resurfacing.

To formally assess whether the safety edge treatment has any effect on pavement-edge drop-offs, a 

trend analysis evaluating the change in drop-offs from before to after resurfacing was conducted.

Specifically, the proportion of drop-off height measurements exceeding 2 inches was evaluated to

determine if there were differences between the before and after study periods. This analysis was 

carried out using the same logistic regression approach presented in section 3.1. However, in this 

case, the proportions of drop-off heights exceeding 2 inches were compared between the periods

before and after resurfacing for each type of site rather than between treatment and comparison sites.

The ideal trend for this analysis would be indicated by a substantial decrease in drop-off height 

for the first year after resurfacing, possibly followed by a slow increasing trend in the later years 

back to the drop-off height that existed before resurfacing. To evaluate this trend, all pairwise 

comparisons between years were evaluated for statistical significance. Four of the comparisons: 

before versus after year 1, after year 1 versus after year 2, after year 2 versus after year 3, and 

before versus after year 3 have been summarized.

For before versus after year 1, an odds ratio point estimate less than 1.0 indicates that after year 1 

had more drop-off heights exceeding 2 inches than the period before resurfacing. A confidence 

interval for the odds ratio that does not contain the value 1.0 indicates statistical significance. 
19

Since the odds ratios were less than 1.0 in 3 of the 12 cases shown in table 8, the sites in after 

year 1 generally had fewer drop-off heights exceeding 2 inches than the sites in the period before

resurfacing. Also, the three cases when after year 1 had more drop-off heights exceeding 2 inches

than the period before resurfacing were not significant. Thus, it appears that resurfacing tends to 

reduce the proportion of extreme drop-off heights.

Table 8. Comparison of the proportions of drop-off heights exceeding 2 inches between 

the before and after resurfacing periods.

State

Roadway 

type

Shoulder 

type

Site 

Type Test

Proportion 

Period 1

Proportion 

Period 2

Odds ratio 

point 

estimate

Lower 95% 

confidence 

limit

Upper 95% 

confidence 

limit

Statistically 

significant at 

the 0.05 level?

GA

Multilane Paved

C Period Before vs AfterY1 0.06 0.06 1.05 0.28 3.92 No

C Period Before vs AfterY2 0.06 0.03 1.80 0.43 8.99 No

C Period Before vs AfterY3 0.06 0.08 0.73 0.21 2.39 No

C Period AfterY1 vs AfterY2 0.06 0.03 0.59 0.12 2.46 No

C Period AfterY1 vs AfterY3 0.06 0.08 1.44 0.44 5.03 No

C Period AfterY2 vs AfterY3 0.03 0.08 2.45 0.66 11.68 No

T Period Before vs AfterY1 0.07 0.07 0.98 0.13 5.35 No

T Period Before vs AfterY2 0.07 0.08 0.86 0.12 4.25 No

T Period Before vs AfterY3 0.07 0.09 0.70 0.10 3.27 No

T Period AfterY1 vs AfterY2 0.07 0.08 1.15 0.29 4.83 No

T Period AfterY1 vs AfterY3 0.07 0.09 1.40 0.38 5.72 No

T Period AfterY2 vs AfterY3 0.08 0.09 1.22 0.35 4.44 No

Two-lane

Paved

C Period Before vs AfterY1 0.14 0 infinity 12.13 infinity Yes

C Period Before vs AfterY2 0.14 0.05 3.38 1.49 8.70 Yes

C Period Before vs AfterY3 0.14 0.03 6.17 2.33 21.32 Yes

C Period AfterY1 vs AfterY2 0 0.05 infinity 3.24 infinity Yes

C Period AfterY1 vs AfterY3 0 0.03 infinity 1.60 infinity Yes

C Period AfterY2 vs AfterY3 0.05 0.03 0.55 0.14 1.86 No

T Period Before vs AfterY1 0.03 0.03 1.11 0.44 2.83 No

T Period Before vs AfterY2 0.03 0.02 1.85 0.61 6.82 No

T Period Before vs AfterY3 0.03 0 10.64 2.02 195.83 Yes

T Period AfterY1 vs AfterY2 0.03 0.02 0.60 0.16 1.86 No

T Period AfterY1 vs AfterY3 0.03 0 0.10 0.01 0.56 Yes

T Period AfterY2 vs AfterY3 0.02 0 0.17 0.01 1.19 No

Unpaved

C Period Before vs AfterY1 0.13 0.06 2.36 1.36 4.10 Yes

C Period Before vs AfterY2 0.13 0.1 1.29 0.78 2.12 No

C Period Before vs AfterY3 0.13 0.08 1.68 0.99 2.84 No

C Period AfterY1 vs AfterY2 0.06 0.1 1.83 1.11 3.04 Yes

C Period AfterY1 vs AfterY3 0.06 0.08 1.40 0.83 2.38 No

C Period AfterY2 vs AfterY3 0.1 0.08 0.77 0.48 1.23 No

T Period Before vs AfterY1 0.09 0.03 2.73 1.28 6.34 Yes

T Period Before vs AfterY2 0.09 0.13 0.62 0.35 1.10 No

T Period Before vs AfterY3 0.09 0.09 0.99 0.53 1.85 No

T Period AfterY1 vs AfterY2 0.03 0.13 4.39 2.13 9.99 Yes

T Period AfterY1 vs AfterY3 0.03 0.09 2.76 1.28 6.46 Yes

T Period AfterY2 vs AfterY3 0.13 0.09 0.63 0.35 1.12 No

See notes at end of table.
20

Table 8. Comparison of the proportions of drop-off heights exceeding 2 inches between 

the before and after resurfacing periods—Continued.

State

Roadway 

type

Shoulder 

type

Site 

Type Test

Proportion 

Period 1

Proportion 

Period 2

Odds ratio 

point 

estimate

Lower 95% 

confidence 

limit

Upper 95% 

confidence 

limit

Statistically 

significant at 

the 0.05 level?

IN Two-lane

Paved

C Period Before vs AfterY1 0.10 0.17 0.58 0.25 1.24 No

C Period Before vs AfterY2 0.10 0.27 0.31 0.13 0.66 Yes

C Period Before vs AfterY3 0.10 0.1 0.70 0.30 1.52 No

C Period AfterY1 vs AfterY2 0.17 0.27 1.88 1.01 3.53 Yes

C Period AfterY1 vs AfterY3 0.17 0.14 0.83 0.43 1.57 No

C Period AfterY2 vs AfterY3 0.27 0.14 0.44 0.23 0.83 Yes

T Period Before vs AfterY1 0.04 0 infinity 3.18 infinity Yes

T Period Before vs AfterY2 0.04 0.01 8.58 1.44 163.10 Yes

T Period Before vs AfterY3 0.04 0.02 2.86 0.74 13.75 No

T Period AfterY1 vs AfterY2 0.00 0.01 infinity 0.15 infinity No

T Period AfterY1 vs AfterY3 0.00 0.02 infinity 0.94 infinity No

T Period AfterY2 vs AfterY3 0.01 0.02 3.00 0.38 60.92 No

Unpaved

C Period Before vs AfterY1 0.22 0.11 2.21 1.37 3.61 Yes

C Period Before vs AfterY2 0.22 0.16 1.48 0.95 2.31 No

C Period Before vs AfterY3 0.22 0.14 1.68 1.07 2.64 Yes

C Period AfterY1 vs AfterY2 0.11 0.16 1.49 0.91 2.46 No

C Period AfterY1 vs AfterY3 0.11 0.14 1.32 0.80 2.18 No

C Period AfterY2 vs AfterY3 0.16 0.14 0.88 0.56 1.40 No

T Period Before vs AfterY1 0.39 0.28 1.65 1.22 2.24 Yes

T Period Before vs AfterY2 0.39 0.42 0.88 0.66 1.18 No

T Period Before vs AfterY3 0.39 0.30 1.52 1.12 2.06 Yes

T Period AfterY1 vs AfterY2 0.28 0.42 1.86 1.37 2.54 Yes

T Period AfterY1 vs AfterY3 0.28 0.30 1.09 0.79 1.49 No

T Period AfterY2 vs AfterY3 0.42 0.30 0.58 0.43 0.79 Yes

NY Two-lane Paved

C Period Before vs AfterY1 0 0.02 -infinity -infinity 3.18 No

C Period Before vs AfterY2 0 0.12 -infinity -infinity 0.37 Yes

C Period Before vs AfterY3 0 0.18 -infinity -infinity 0.23 Yes

C Period AfterY1 vs AfterY2 0.02 0.12 5.70 1.44 37.92 Yes

C Period AfterY1 vs AfterY3 0.02 0.18 9.07 2.44 58.83 Yes

C Period AfterY2 vs AfterY3 0.12 0.18 1.59 0.67 3.89 No

T Period Before vs AfterY1 0.38 0.31 2.79 1.39 5.84 Yes

T Period Before vs AfterY2 0.38 0.27 1.68 0.88 3.26 No

T Period Before vs AfterY3 0.38 0.27 1.72 0.91 3.30 No

T Period AfterY1 vs AfterY2 0.18 0.27 1.66 0.78 3.63 No

T Period AfterY1 vs AfterY3 0.18 0.27 1.62 0.77 3.52 No

T Period AfterY2 vs AfterY3 0.27 0.27 0.98 0.49 1.98 No

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.

The odds ratio for the treatment sites was less than 1.0 for one out of six cases, indicating that 

resurfacing with the safety edge treatment is effective in reducing the proportion of extreme drop-off

heights. Resurfacing without the safety edge treatment was effective in reducing the proportion 

of extreme drop-off heights in four of six cases. Additionally, none of the observed odds ratios less

than 1.0 and almost all of the observed odds ratios greater than 1.0 were statistically significant.
21

For after year 1 versus after year 2, an odds ratio point estimate greater than 1.0 indicates that the 

second year after resurfacing had more drop-off heights exceeding 2 inches than the first year 

after resurfacing. Since there were more drop-off heights exceeding 2 inches in after year 2 as

compared to after year 1 (10 of the 12 cases shown in table 8), there appears to be deterioration 

of the shoulder condition in the second year after resurfacing. However, only about half of these 

observed differences in the proportion of drop-off heights exceeding 2 inches were statistically 

significant at the 5 percent significance level. 

For after year 2 versus after year 3, an odds ratio point estimate greater than 1.0 indicates that the 

third year after resurfacing had more drop-off heights exceeding 2 inches than the second year 

after resurfacing. Since 7 of the 12 cases shown in table 8 have an odds ratio point estimate of 

1.0 (or nearly 1.0), which indicates no change in the proportion of drop-off heights exceeding 

2 inches, there appears to be minimal deterioration of the shoulder condition in the third year after 

resurfacing.

The before period drop-off height data were compared to the after year 3 drop-off height data to 

determine whether drop-off heights had increased to the levels that existed before resurfacing. 

For this comparison, an odds ratio point estimate less than 1.0 indicates that after year 3 had more

drop-off heights exceeding 2 inches than the period before resurfacing. Since the odds ratios were

greater than 1.0 in 7 of the 12 cases shown in table 8, there does not seem to be much evidence to 

suggest the proportion of high drop-offs after year 3 differs from the before period. 

A final comparison of drop-off height data was made between sites resurfaced with and without the

safety edge treatment in the third year after resurfacing to determine if the safety edge treatment 

has any role in development of drop-offs. The results of this analysis are given in table 9. Odds 

ratio values above 1.0 indicate that comparison sites had more drop-off heights exceeding 2 inches

than treatment sites.

Table 9. Comparison of the proportions of drop-off heights exceeding 2 inches between 

treatment and comparison sites for the final period after resurfacing.

State

Road 

type

Shoulder 

type

Site 

type

Drop-off heights that 

exceed 2 inches Odds ratio 

point 

estimate

Lower 95% 

confidence 

limit

Upper 95% 

confidence 

limit

Statistically 

Number Proportion significant

GA

Multilane Paved

C 2 0.02

0.286 0.040 1.374 No

T 5 0.08

Two-lane

Paved

C 6 0.04

1.034 0.341 2.922 No

T 9 0.04

Unpaved

C 38 0.08

0.796 0.476 1.349 No

T 27 0.1

IN Two-lane

Paved

C 21 0.14

10.332 3.470 44.394 Yes

T 3 0.02

Unpaved

C 41 0.14

0.384 0.256 0.567 Yes

T 112 0.3

NY Two-lane Paved C 10 0.12 0.382 0.161 0.858 Yes

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.
22

The results in table 9 indicate that there were no differences in extreme drop-offs between sites 

resurfaced with and without the safety edge in Georgia. In Indiana, sites with paved shoulders

resurfaced with the safety edge had fewer drop-offs. However, sites with unpaved shoulders 

showed the reverse trend. In New York, sites resurfaced without the safety edge had fewer 

proportions of extreme drop-off heights. Taken together, these results are inconclusive.

The analysis of the field measurements of drop-off-heights suggests that resurfacing is effective 

in reducing the proportion of extreme drop-off heights. It also suggests that resurfacing with the 

safety edge treatment does not increase the number of extreme drop-off heights and is similar to 

resurfacing without the safety edge treatment in reducing the proportion of extreme drop-off 

heights over time.
23

CHAPTER 4. ANALYSIS RESULTS FOR SAFETY EVALUATION

This chapter presents the safety evaluation approach, the development of SPFs, and the safety 

evaluation results. The safety evaluation results include the findings of a before-period compatibility 

study, a before-after evaluation using the EB technique, a cross-sectional analysis, and an analysis

of shifts in crash severity.

4.1 EVALUATION APPROACH

Two statistical approaches were used to evaluate the safety effectiveness of the safety edge 

treatment: (1) a before-after comparison of the effect of pavement resurfacing with and without

the safety edge treatment using the EB technique and (2) a cross-sectional comparison of the 

effect of pavement resurfacing with and without the safety edge treatment based on after-period 

data only. These two evaluation approaches were applied concurrently to provide alternative 

statistical approaches to the key issues being addressed. The following discussion describes these

evaluations, including issues related to the specific nature of the safety edge treatment.

A key objective of the evaluation was to determine the safety effectiveness of the safety edge 

treatment while avoiding the potential confounding effects of regression to the mean and the safety

effect of pavement resurfacing. Regression to the mean is a characteristic of repeated measures data 

in which observations move toward the mean value over time. That is, if an observation in a year 

is unusually high, then the observation in the following year will nearly always be lower (and vice 

versa), returning to the mean. This phenomenon often leads to an overestimation or underestimation

of safety for some sites. Thus, the effect of the treatment is likely to be partially confounded with 

the expected decrease or increase in crash experience from regression to the mean. Regression to 

the mean can only be accounted for with knowledge of the “normal” or expected value of beforeperiod crash experience at the treated sites. The EB technique has the advantage of compensating 

for regression to the mean. The cross-sectional approach does not explicitly compensate for 

regression to the mean. This concern is lessened by the availability of 3 years of crash data for 

the period after resurfacing.

The second potential confounding effect is the safety effect of pavement resurfacing since it is 

always used in conjunction with the safety edge treatment. Previous research has indicated that 

pavement resurfacing by itself may have an effect on safety, increasing crashes because of increased

speeds. This effect was found in one study to be statistically significant but was found to persist 

for only 12–30 months after resurfacing.(4) However, a more recent, larger study in National 

Cooperative Highway Research Program Project 17-9(2) found inconsistent results; increases 

in crash frequency with resurfacing were found in some States, but decreases in crash frequency 

with resurfacing were found in others.(5) Therefore, the safety effects of the pavement resurfacing

and installation of the safety edge treatment will be confounded, at least for some time, following

resurfacing.

The study design was developed to address the safety effect of resurfacing and the safety edge 

treatment as well as the confounding effect of resurfacing. First, the study period after resurfacing

was selected to be 3 years. This is sufficiently long as to extend beyond the duration of any shortterm resurfacing effect. Annual interim evaluations to monitor time trends were conducted to 
24

address this issue. Thus, the results for safety effectiveness of the safety edge treatment in the 

first- and second-year interim reports may be confounded by the safety effect of pavement 

resurfacing, but it is expected that this confounding effect is lessened in the final results. Second, 

resurfaced sites both with and without the safety edge treatment were considered. The ratio of

safety between resurfaced sites with and without the safety edge treatment (i.e., the treatment and 

comparison sites) may represent an effect of the safety edge treatment as long as the sites can be 

assumed comparable in other respects. 

The first evaluation approach is an observational before-after comparison using the EB technique,

as formulated by Hauer.(6,7) The specific version of the EB technique used in this evaluation was 

developed for the FHWA SafetyAnalyst software tools.(8)The primary objective of the before-after 

evaluation is to compare the observed number of crashes after the treatment is implemented to 

the expected number of crashes in the after period had the countermeasure not been implemented.

This provides an estimate of the overall safety effectiveness of the countermeasure expressed as 

a percent change in the crash frequency. 

When performing before-after evaluations using the EB technique, it is typical to collect data at 

sites where countermeasures were implemented (i.e., treatment sites) and at sites similar to the 

treatment sites with respect to area type (rural/urban), geometric design, and traffic volumes, but 

where no countermeasures were installed. Data from this comparison group of sites are used to 

create SPFs, which are then used with the observed crash counts at the treated sites in the before 

period to estimate the number of crashes that would have occurred at the treated sites in the after 

period if no improvement had been made. These SPFs are discussed in section 4.2.

The comparability before resurfacing of the two types of sites (treatment and comparison sites) is 

critical to interpreting the difference of the two estimated before-after effects as an effect of the safety

edge treatment. For example, if one of the site types had a higher mean in the before period and both

site types had the same mean in the after period, then the effectiveness of one treatment may be 

presumed greater than the other treatment. The comparability of sites was established through 

analysis of the before-period crash data. These analyses are discussed in section 4.3.1.

The EB before-after evaluation produced separate estimates of the effectiveness of resurfacing with

the safety edge (treatment sites) and resurfacing only (comparison sites) for each target crash type 

in each State. From each pair of estimated percent changes in safety (treatment and comparison), 

the effect of the safety edge alone was estimated as the ratio between the two measures of

effectiveness. For every combination of site characteristics under consideration, the mean and 

standard error of the percent change in target crash frequency and its statistical significance are 

presented in section 4.3.2.

It was anticipated that the effectiveness measure for the safety edge treatment would be relatively 

small since it was expected that the safety edge treatment would affect only certain crash types and

would have the greatest impact on two-lane highways with no paved shoulders. Most such sites 

have relatively low traffic volume and therefore are not expected to have a high frequency of 

run-off-the-road and drop-off-related crashes.

The EB-based before-after comparison technique is theoretically the strongest approach to

evaluations of this type. However, because of the confounding of the pavement resurfacing effect 
25

and the safety edge treatment effect, it cannot be assured that this approach correctly identifies the 

treatment effectiveness. Therefore, an alternative cross-sectional comparison was also conducted.

A cross-sectional evaluation of the after data at the treated sites was conducted to directly compare

the crash data between the two types of treatment—resurfacing with the safety edge treatment and

resurfacing without the safety edge treatment. Assuming that all roadway factors except resurfacing

are held constant, one could hypothesize that the differences in either after-period crash frequencies 

or crash severity distributions between treatment and comparison sites are due to the provision of 

the safety edge treatment. This comparison was made with a cross-sectional approach using data 

for the period after resurfacing while accounting for the effects of AADT. 

The cross-sectional comparison of crash data for the period after resurfacing was conducted using

negative binomial regression models to compare the crash frequencies for the period after resurfacing

for the sites with the safety edge treatment to those of the sites resurfaced without the safety edge

treatment. Site type (i.e., treatment versus comparison) was the main factor of interest in the analysis.

The effect of AADT was accounted for in this approach by quantifying the relationship between 

AADT and specific target crash types. When significant, the effect of lane width was also accounted

for in the model. The safety edge treatment effect and its standard error were then calculated for each

target crash type. The treatment effect was converted to a percent change in crash frequency for ease

in interpreting the results. The results of the cross-sectional analysis are presented in section 4.4.3.

In addition to evaluating mean crash frequencies, a comparison of the before-after data by crash 

severity level was performed to determine shifts in the crash severity distribution. These comparisons

were accomplished by calculating a confidence interval for the average difference in proportions 

across all sites at a preselected significance level of 10 percent. However, a non-parametric 

statistical test, the Wilcoxon signed-rank test, was also applied as the differences in proportions 

may not follow a normal distribution. Results from this analysis are presented in section 4.4.4.

(9)

4.2 SAFETY PERFORMANCE FUNCTIONS

This section documents the SPFs and calibration factors developed for use in the before-after EB 

evaluation of the safety effectiveness of the safety edge treatment. SPFs are regression relationships

between target crash frequencies and traffic volumes that can be used to predict the long-term crash

frequency for a site. SPFs are used in the before-after EB evaluation to estimate what the safety 

performance of a treated site would be in the after period if the treatment had not been implemented.

Negative binomial regression models were developed using data from the reference group of 

untreated sites for use in three categories of target crashes (all crash types combined, run-offroad crashes, and drop-off-related crashes) and two severity levels (total crashes and fatal and

injury crashes). Thus, a total of six dependent variables were considered. Traffic volume and 

lane width were the only independent variables considered in the SPFs. Separate models were 

developed for Georgia and Indiana for each of the three classifications, as follows: 

Rural multilane highways with paved shoulders with widths of 4 ft or less.

Rural two-lane highways with paved shoulders with widths of 4 ft or less.

Rural two-lane highways with no paved shoulders (i.e., unpaved shoulders only).
26

Regression models were not developed for New York due to the limited number of treated sites.

All regression models were developed to predict target crash frequencies per mile per year as a 

function of traffic volume and, in some cases, lane width in the functional forms shown in 

equation 1 and equation 2.

N = exp (a + b lnAADT) (1)

N = exp (a + b lnAADT + c LW) (2)

Where:

N = predicted number of target crashes per mile per year

AADT = average daily traffic volume (vehicles per day) for the roadway segment

LW = lane width for the roadway segment (ft)

a, b, c = regression coefficients

The AADT in the regression models was statistically significant in all cases. The lane width term 

was included in the regression model only when it was statistically significant.

Two generalized linear modeling techniques were used to fit the data. The first method used a 

repeated measures correlation structure to model yearly crash counts for a site. In this method, the

covariance structure, assuming compound symmetry, is estimated before final regression parameter

estimates are determined by general estimating equations. Consequently, model convergence for 

this method is dependent on the covariance estimates as well as parameter estimates. When the 

model failed to converge for the covariance estimates, an alternative method was considered. In 

this method, yearly crash counts for a site were totaled and annual daily traffic (ADT) values were

averaged to create one summary record for a site. Regression parameter estimates were then directly 

estimated by maximum likelihood without an additional covariance structure being estimated.

Both methods produced an estimate of the overdispersion parameter, the estimate for which the 

variance exceeds the mean. Overdispersion occurs in traffic data when a number of sites being 

modeled have zero accident counts, which creates variation in the data. When the estimate for 

dispersion was very small or even slightly negative, the model was refit assuming a constant 

value. Both methods were accomplished with the GENMOD procedure of SAS®.

(3)

Statistically significant models were not found for all dependent variables for some road type/

shoulder type combinations. In these three cases, the intercept coefficient of the total crashes or 

fatal and injury crashes model was adjusted by the proportion of the applicable dependent variable

to produce the final model. The model coefficients with their standard errors are presented in

table 10 for Georgia and in table 11 for Indiana. All AADT coefficients shown are significant at 

the 10 percent significance level or better. Lane width coefficients shown are significant at the 

20 percent significance level or better. Total crash and fatal and injury crash SPFs are illustrated 

in figure 3 for Georgia and in figure 4 for Indiana.
27

Table 10. SPFs for Georgia sites.

Roadway 

type

Shoulder 

type

Number 

of siteyearsIntercept 

(standard error)

AADT coefficient 

(standard error)

Lane width 

coefficient 

(standard error)

Overdispersion 

parameter

R2

LR 

(%)

Total crashes

Multilane Paved 192 -4.801 (1.608) 0.642 (0.172) 0.487 9.2

Two-lane Paved 582 -8.921 (1.189) 1.108 (0.141) 0.724 36.4

Two-lane Unpaved 792 -7.730 (0.783) 0.978 (0.095) 0.425 25.1

Fatal and injury crashes

Multilane Paved 192 -2.204 (1.752) 0.252 (0.184) 0.588 0.2

Two-lane Paved 582 -7.818 (1.116) 0.853 (0.132) 0.401 21.3

Two-lane Unpaved 792 -8.556 (0.796) 0.958 (0.098) 0.346 16.0

PDO crashes 

Multilane Paved 192 -6.611 (1.747) 0.787 (0.189) 0.540 14.0

Two-lane Paved 582 -11.414 (1.397) 1.349 (0.164) 0.982 34.6

Two-lane Unpaved 792 -8.470 (0.981) 1.011 (0.119) 0.623 19.3

Total run-off-road crashes

Multilane Paved 192 -3.475 (2.145) 0.360 (0.228) 0.213 1.9

Two-lane Paved 582 -2.625 (1.710) 0.783 (0.134) -0.376 (0.109) 0.464 19.9

Two-lane Unpaved 132 -4.405 (1.443) 0.757 (0.141) -0.199 (0.106) 0.472 14.8

Fatal and injury run-off-road crashes

Multilane Paved 192 -3.425(1.752) 0.252 (0.184) 0.588 0.2

Two-lane Paved 582 -1.848(1.618) 0.544 (0.128) -0.339 (0.110) 0.374 8.1

Two-lane Unpaved 132 -5.556(1.543) 0.743 (0.139) -0.151 (0.115) 0.341 15.8

PDO run-off-road crashes 

Multilane Paved 192 -7.742(3.004) 0.750 (0.320) 0.117 5.6

Two-lane Paved 582 -5.029(2.236) 1.033 (0.154) -0.406 (0.144) 0.598 19.2

Two-lane Unpaved 132 -4.544(1.709) 0.752 (0.173) -0.238 (0.126) 0.636 9.7

Total drop-off-related crashes

Multilane Paved 192 -3.583(2.126) 0.318 (0.226) 0.131 1.6

Two-lane Paved 582 -4.586(2.069) 0.884 (0.169) -0.327 (0.125) 0.585 16.3

Two-lane Unpaved 132 -4.140(1.495) 0.770 (0.141) -0.270 (0.114) 0.427 14.0

Fatal and injury drop-off-related crashes

Multilane Paved 192 -2.344(1.974) 0.113 (0.141) 0.294 0.1

Two-lane Paved 582 -3.297(1.894) 0.604 (0.154) -0.290 (0.121) 0.558 6.2

Two-lane Unpaved 132 -4.869(1.654) 0.699 (0.148) -0.209 (0.127) 0.357 11.9

PDO drop-off-related crashes

Multilane Paved 192 -6.690(3.194) 0.574 (0.340) 0.101 2.7

Two-lane Paved 582 -8.291(3.272) 1.269 (0.217) -0.359 (0.195) 0.754 16.3

Two-lane Unpaved 792 -4.345(3.899) 0.872 (0.157) -0.388 (0.290) 0.565 6.6

Note: Blank cells indicate lane width coefficient was not significant.
28

Table 11. SPFs for Indiana sites.

Road type

Shoulder 

Type

Number 

of siteyearsIntercept 

(standard error)

AADT coefficient 

(standard error)

Lane width 

coefficient 

(standard error)

Overdispersion

parameter

R2

LR

(%)

Total crashes

Two-lane Paved 100 -5.500(1.317) 0.737(0.154) 0.444 15.3

Two-lane Unpaved 98 -3.865(1.118) 0.701(0.146) -0.156(0.086) 0.654 15.5

Fatal and injury crashes

Two-lane Paved 100 -6.279(1.977) 0.642(0.233) 0.563 5.1

Two-lane Unpaved 196 -2.707(1.305) 0.427(0.139) -0.198(0.098) 0.211 7.2

PDO crashes

Two-lane Paved 100 -5.572(1.373) 0.718(0.161) 0.398 14.8

Two-lane Unpaved 98 -4.348(1.153) 0.694(0.148) -0.128(0.089) 0.661 15.9

Total run-off-road crashes

Two-lane Paved 100 -3.250(1.962) 0.303(0.231) 0.413 1.5

Two-lane Unpaved 196 -1.700(1.221) 0.490(0.119) -0.278(0.103) 0.438 10.9

Fatal and injury run-off-road crashes

Two-lane Paved 296 -3.127(1.034) 0.346(0.105) -0.132(0.078) 0.154 2.5

Two-lane Unpaved 196 -1.467(1.432) 0.331(0.129) -0.284(0.102) 0.027 6.4

PDO run-off-road crashes

Two-lane Paved 100 -4.764(2.398) 0.426(0.286) 0.212 2.5

Two-lane Unpaved 196 -2.752(1.260) 0.573(0.133) -0.279(0.112) 0.540 8.6

Total drop-off-related crashes

Two-lane Paved 100 -4.477(3.598) 0.313(0.421) 0.738 0.6

Two-lane Unpaved 98 -2.352(1.489) 0.356(0.192) -0.232(0.111) 0.310 1.5

Fatal and injury drop-off-related crashes

Two-lane Paved 100 -7.772(1.977) 0.642(0.233) 0.563 5.1

Two-lane Unpaved 98 -2.943(1.989) 0.227(0.258) -0.167(0.147) 0.276 0.3

PDO drop-off-related crashes

Two-lane Paved 100 -7.464(5.554) 0.597(0.653) 0.623 1.4

Two-lane Unpaved 98 -3.006(1.593) 0.419(0.209) -0.266(0.122) 0.069 1.7

Note: Blank cells indicate lane width coefficient was not significant.
29

Georgia Crashes/Mile/Year

0

1

2

3

4

5

6

7

8

AADT (veh/day)

0 10,000 20,000 30,000

F&I Accident SPF for Multilane Roadway with Paved Shoulder

F&I Accident SPF for Two-lane Roadway with Paved Shoulder

F&I Accident SPF for Two-lane Roadway with Unpaved Shoulder

Total Accident SPF for Multilane Roadway with Paved Shoulder

Total Accident SPF for Two-lane Roadway with Paved Shoulder

Total Accident SPF for Two-lane Roadway with Unpaved Shoulder

Figure 3. Graph. Comparison of Georgia SPFs by crash severity and roadway and 

shoulder type.

Indiana Crashes/Mile/Year

0

1

2

3

4

5

AADT (veh/day)

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000

F&I Accident SPF for Two-lane Roadway with Paved Shoulder

F&I Accident SPF for Two-lane Roadway with Unpaved Shoulder

Total Accident SPF for Two-lane Roadway with Paved Shoulder

Total Accident SPF for Two-lane Roadway with Unpaved Shoulder

Figure 4. Graph. Comparison of Indiana SPFs by crash severity and roadway and 

shoulder type. 
30

As noted earlier, the proportion of run-off-road and drop-off-related crashes (developed from 

reference sites) was sometimes needed to adjust total or fatal and injury SPFs for prediction of 

those crash types. Table 12 presents these proportions estimated from the reference site data.

Table 12. Run-off-road and drop-off-related crash frequencies as a proportion 

of total crashes.

State

Roadway 

type

Shoulder 

type

Crash 

severity 

Level

Proportion of 

run-off-road

crashes

Proportion of 

drop-off-related 

crashes

GA

Multilane Paved

Total 0.215 0.127

FI 0.295 0.194

PDO 0.162 0.084

Two-lane

Paved

Total 0.371 0.230

FI 0.511 0.368

PDO 0.298 0.158

Unpaved

Total 0.473 0.300

FI 0.574 0.410

PDO 0.398 0.219

IN Two-lane

Paved

Total 0.272 0.091

FI 0.566 0.225

PDO 0.199 0.058

Unpaved

Total 0.321 0.119

FI 0.645 0.248

PDO 0.253 0.091

FI = Fatal and injury crashes.

PDO = Property-damage-only crashes.

Additionally, yearly calibration factors were developed from the SPFs to provide a better yearly 

prediction in the methodology. These factors are needed because the SPFs are developed as an 

average of all years. The yearly calibration factor is determined as the ratio of the sum of observed

crashes for all sites for a specific roadway type/shoulder type combination to the sum of the predicted

crashes for the same sites using the AADT and crash count values for that year. These factors are 

presented in table 13 for Georgia and in table 14 for Indiana.
31

Table 13. Georgia SPF calibration factors.

Roadway 

type

Shoulder 

type

Crash 

severity 

level

Yearly calibration factors

2001 2002 2003 2004 2005 2006 2007 2008

Total crashes

Multilane Paved

Total 0.956 1.023 1.071 0.943 1.078 1.178 0.993 0.983

FI 0.908 1.091 0.950 1.153 1.168 1.170 0.959 0.942

PDO 0.998 1.005 1.155 0.849 1.049 1.203 1.031 1.021

Two-lane

Paved

Total 0.856 0.949 0.919 1.044 0.990 1.045 1.025 1.023

FI 0.926 0.979 0.996 1.114 1.139 1.167 1.115 1.075

PDO 0.823 0.933 0.873 0.998 0.905 0.977 0.969 0.990

Unpaved

Total 0.996 0.876 0.884 1.061 1.068 1.112 0.895 1.024

FI 1.056 0.999 0.840 1.106 1.318 1.202 1.031 1.167

PDO 0.964 0.804 0.910 1.036 0.922 1.062 0.817 0.943

Run-off-road crashes

Multilane Paved

Total 0.958 1.135 1.174 0.891 1.094 0.974 0.962 0.819

FI 1.167 1.268 1.267 1.267 1.425 1.216 1.048 1.064

PDO 0.928 1.168 1.241 0.731 0.987 0.917 0.999 0.747

Two-lane

Paved

Total 1.192 1.389 1.131 1.397 1.307 1.542 1.458 1.378

FI 1.302 1.188 1.226 1.502 1.481 1.688 1.474 1.416

PDO 1.110 1.581 1.058 1.318 1.168 1.430 1.451 1.355

Unpaved

Total 1.107 1.064 1.089 1.201 1.335 1.280 1.046 1.183

FI 1.150 1.167 0.828 1.241 1.405 1.232 1.114 1.265

PDO 1.003 0.905 1.282 1.095 1.194 1.256 0.923 1.036

Drop-off-related crashes

Multilane Paved

Total 1.040 1.102 1.134 1.101 1.034 1.003 1.156 0.774

FI 0.925 1.320 0.989 1.121 0.989 1.254 1.117 0.860

PDO 1.156 0.874 1.275 1.075 1.075 0.741 1.188 0.683

Two-lane

Paved

Total 1.203 1.410 1.144 1.385 1.364 1.609 1.491 1.511

FI 1.290 1.135 1.270 1.449 1.549 1.652 1.543 1.636

PDO 1.111 1.746 1.001 1.312 1.154 1.564 1.429 1.368

Unpaved

Total 1.129 1.035 1.133 1.240 1.397 1.409 1.194 1.303

FI 1.217 1.212 0.818 1.186 1.426 1.409 1.345 1.393

PDO 0.997 0.794 1.506 1.285 1.335 1.384 0.982 1.165

FI = Fatal and injury crashes.

PDO = Property-damage-only crashes.
32

Table 14. Indiana SPF calibration factors.

Roadway 

type

Shoulder 

type

Crash 

severity 

level

Yearly calibration factors

2003 2004 2005 2006 2007 2008

Total crashes

Two-lane

Paved

Total 0.932 0.944 0.579 0.605 0.320 0.384

FI 0.918 1.006 0.456 0.586 0.343 0.326

PDO 0.943 0.936 0.616 0.615 0.317 0.402

Unpaved

Total 1.268 1.011 0.629 0.556 0.365 0.265

FI 0.914 1.002 0.471 0.472 0.287 0.182

PDO 1.322 0.968 0.650 0.557 0.373 0.279

Run-off-road crashes

Two-lane

Paved

Total 1.092 0.936 0.607 0.551 0.304 0.497

FI 1.266 1.097 0.489 0.651 0.407 0.448

PDO 1.074 0.911 0.713 0.535 0.268 0.558

Unpaved

Total 1.002 0.863 0.479 0.363 0.279 0.177

FI 0.850 1.041 0.503 0.446 0.232 0.154

PDO 1.068 0.754 0.457 0.313 0.300 0.186

Drop-off-related crashes

Two-lane

Paved

Total 0.994 0.946 0.646 0.431 0.431 0.690

FI 0.729 0.722 0.362 0.434 0.290 0.290

PDO 1.016 0.929 0.777 0.310 0.467 0.934

Unpaved

Total 1.289 1.038 0.544 0.545 0.520 0.249

FI 1.265 0.989 0.661 0.441 0.385 0.220

PDO 1.298 1.066 0.459 0.613 0.610 0.267

FI = Fatal and injury crashes.

PDO = Property-damage-only crashes.

4.3 SAFETY EVALUATIONS

As previously discussed, four types of safety evaluations were performed as part of this study: 

(1) a safety comparison of treatment and comparison sites in the period before resurfacing; 

(2) an EB before-after evaluation; (3) a cross-sectional analysis; and (4) an analysis of shifts 

in the severity distribution from before to after resurfacing. The findings of these evaluations

are presented in this section.

4.3.1 Safety Comparison of Treatment and Comparison Sites in the Period Before Resurfacing

An evaluation was conducted to compare the safety performance of treatment and comparison sites

before resurfacing for specific States and roadway type/shoulder type combinations. This evaluation

is critical to the interpretation of the safety differences between the treatment and comparison sites

as an effect of the safety edge treatment. If the safety performance of the two types of sites differed

in the period before resurfacing, the comparison of treatment and comparison sites in the period 

after resurfacing may be influenced.

Initial comparisons were made by examination of scatter plots of crashes and traffic volumes

(crashes per mile per year versus lnAADT). Ideal plots would contain no discernable differences

between treatment and comparison sites nor any extreme points. Separation of the data points 
33

between the two groups may indicate a potential concern in the subsequent analyses. Furthermore,

if one group had systematically higher crash frequencies in the period before resurfacing, then the

analysis for the period after resurfacing might need to account for this difference. Finally, large 

variation in crash frequencies for the same AADT values could inhibit crash analysis of the 

treatment and comparison groups. Inspection of these plots with data from year 3 (see appendix C)

showed an improvement in the plots from year 1 and year 2.

Yearly total crash and target crash distributions were also presented in box plots to review data 

consistency from year to year. Ideal plots would have approximately the same distribution for 

crashes each year within a given site type and between site types. Additionally, potential concerns

for the crash analysis—specifically, a regression to the mean or resurfacing effect— may be 

identified if the period after resurfacing is included.

Since crash frequencies are known to experience random variation around the mean or regression 

to the mean, the average over several years for the period before resurfacing should be compared 

to the average of several years for the period after resurfacing. Therefore, if the after period data 

are within the range of yearly crash means but numerically higher than the before period average, 

then safety analyses might show an increase in crash frequency due to the treatment (provided 

AADT growth was minimal). Conversely, if the after implementation year data are lower than 

the before period average, then the treatment effect would be a decrease in crash frequency.

Examination of these graphs indicated that the after period data were almost always higher than 

the average of the before years but within the range of variation in yearly crash totals for both 

types of treated sites.

The apparent increase in crashes was examined to determine if it could be attributed to resurfacing.

A resurfacing effect occurs when the reference sites remain the same or decrease in crashes while the

treatment and comparison sites both increase. This effect was observed in nearly all of the plots.

One additional potential problem was found in this analysis. One treatment site on a two-lane 

highway with paved shoulders in Georgia site doubled in crash frequency from the before to the 

after period. Subsequent investigation found that this site was reconstructed during the second year

after resurfacing, and therefore, it was excluded from the safety analysis presented in this report.

Formal crash frequency comparisons of means between the treatment and comparison sites for the

period before resurfacing were conducted for each State/roadway type/shoulder type combination

and target crash type. Two types of comparisons were made, a comparison of EB-adjusted

expected crash frequencies and a comparison of observed crash frequencies. Both comparisons

were performed using PROC GENMOD, a generalized linear model procedure available in SAS®,

assuming a negative binomial crash distribution.(3) This procedure uses predictive modeling to 

test the means between the two treatment groups for statistical significance. 

The results of these analyses are presented in table 15 and table 16. For the EB-adjusted crash 

analysis, results are provided only for those roadway type/shoulder type combinations for which 

SPFs could be developed. However, all target crash types were considered as they can be estimated

by the EB technique. Regression coefficients with their standard errors are shown in the tables 

for each independent variable, including AADT and the treatment versus comparison site effect. 

The significance and p-value for each effect are also presented. Blank rows in the tables represent

models that did not converge.
34

Table 15. Evaluation of treatment versus comparison site effect for the period before resurfacing using EB-adjusted crash frequencies.

State

Roadway 

type

Shoulder 

type

Crash 

type and 

severity 

level

Number 

of siteyears Intercept

AADT effect Lane width effect Treatment versus comparison site effect

Dispersion 

parameter R2 Coefficient LR%

Standard 

error p-value

Statistically

significant?1 Coefficient

Standard 

error p-value

Statistically 

significant?1 Coefficient

Standard 

error p-value

Statistically 

significant?1

GA

Multilane Paved

TOT 102 -3.965 0.572 0.143 0.001 Yes -0.497 0.452 0.272 No 0.237 31.4

FI 102 -2.115 0.239 0.165 0.148 No -0.391 0.394 0.322 No 0.031 9.4

PDO 102 -5.926 0.740 0.172 0.001 Yes -0.531 0.525 0.312 No 0.282 35.3

rorTOT 102 -5.253 0.559 0.179 0.002 Yes -0.149 0.191 0.434 No 0.010 5.9

rorFI 102 -3.843 0.328 0.158 0.038 Yes -0.293 0.169 0.083 Yes 0.010 22.2

rorPDO 102 -7.761 0.757 0.189 0.001 Yes -0.043 0.239 0.856 No 0.010 8.1

doTOT 102 -4.265 0.430 0.127 0.001 Yes -0.095 0.132 0.469 No 0.010 21.2

doFI 102 -3.620 0.285 0.104 0.006 Yes -0.134 0.106 0.204 No 0.010 24.1

doPDO 102 -6.240 0.569 0.172 0.001 Yes -0.060 0.177 0.734 No 0.010 19.1

Two-lane

Paved

TOT 264 -12.086 1.475 0.089 0.000 Yes 0.154 0.177 0.384 No 0.010 56.9

FI 264 -11.367 1.306 0.099 0.000 Yes -0.104 0.129 0.420 No 0.010 33.5

PDO 264 -13.244 1.534 0.095 0.000 Yes 0.302 0.222 0.175 No 0.010 48.0

rorTOT 264 -5.358 1.133 0.107 0.000 Yes -0.361 0.093 0.001 Yes -0.259 0.141 0.067 Yes 0.010 25.3

rorFI 264 -4.377 0.973 0.132 0.001 Yes -0.381 0.070 0.001 Yes -0.338 0.126 0.007 Yes 0.010 5.2

rorPDO 264 -7.053 1.173 0.100 0.000 Yes -0.314 0.128 0.014 Yes -0.190 0.167 0.255 No 0.010 15.9

doTOT 264 -7.238 1.221 0.116 0.000 Yes -0.303 0.067 0.001 Yes -0.312 0.120 0.009 Yes 0.010 19.0

doFI 264 -6.870 1.207 0.124 0.000 Yes -0.366 0.102 0.000 Yes -0.369 0.158 0.020 Yes 0.010 1.7

doPDO 264 -10.155 1.290 0.105 0.000 Yes -0.189 0.107 0.078 Yes -0.169 0.123 0.171 No 0.010 12.1

Unpaved

TOT 318 -8.117 1.001 0.098 0.000 Yes 0.611 0.149 0.001 Yes 0.010 61.2

FI 318 -8.026 0.899 0.102 0.000 Yes 0.237 0.137 0.084 Yes 0.010 39.1

PDO 318 -9.051 1.031 0.079 0.000 Yes 0.902 0.180 0.001 Yes 0.010 57.8

rorTOT 318 -7.230 0.819 0.088 0.000 Yes 0.358 0.159 0.024 Yes 0.010 37.3

rorFI 318 -6.816 0.703 0.077 0.000 Yes 0.179 0.133 0.179 No 0.010 20.6

rorPDO 318 -8.895 0.909 0.071 0.000 Yes 0.574 0.208 0.006 Yes 0.010 30.0

doTOT 318 -7.444 0.801 0.084 0.000 Yes 0.271 0.142 0.056 Yes 0.010 29.7

doFI 318 -6.545 0.636 0.085 0.001 Yes 0.180 0.127 0.157 No 0.010 12.8

doPDO 318 -10.351 1.026 0.107 0.000 Yes 0.414 0.196 0.035 Yes 0.010 21.4

See notes at end of table.
35

Table 15. Evaluation of treatment versus comparison site effect for the period before resurfacing using EB-adjusted crash frequencies—

Continued.

State

Roadway 

type

Shoulder 

type

Crash 

type and 

severity 

level

Number 

of siteyears Intercept

AADT effect Lane width effect Treatment versus comparison site effect

Dispersion 

parameter R2 Coefficient LR%

Standard 

error p-value

Statistically 

significant?1 Coefficient

Standard 

error p-value

Statistically 

significant?1 Coefficient

Standard 

error p-value

Statistically 

significant?1

IN Two-lane

Paved

TOT 42 -10.904 1.409 0.056 0.000 Yes -0.517 0.307 0.092 Yes 0.150 7.3

FI 42 -21.302 2.546 0.222 0.000 Yes -1.076 0.338 0.001 Yes 0.010 9.3

PDO 42 -2.772 0.422 0.114 0.000 Yes -0.184 0.405 0.650 No 0.152 4.7

rorTOT 42 -2.431 0.208 0.283 0.463 No -0.054 0.208 0.793 No 0.010 23.8

rorFI 42 -4.735 0.361 0.061 0.001 Yes -0.073 0.078 0.352 No 0.010 34.0

rorPDO 42

doTOT 42

doFI 42 -5.918 0.391 0.059 0.001 Yes -0.269 0.159 0.090 Yes 0.010 35.2

doPDO 42

Unpaved

TOT 68 -0.578 0.787 0.231 0.001 Yes -0.506 0.117 0.001 Yes 0.097 0.273 0.723 No 0.137 43.8

FI 68 -1.688 0.470 0.141 0.001 Yes -0.312 0.061 0.001 Yes -0.063 0.165 0.701 No 0.010 15.9

PDO 68 -1.128 0.932 0.264 0.000 Yes -0.584 0.141 0.001 Yes 0.172 0.302 0.570 No 0.212 40.4

rorTOT 68 0.889 0.588 0.208 0.005 Yes -0.585 0.103 0.001 Yes -0.045 0.219 0.837 No 0.010 34.6

rorFI 68 -1.126 0.283 0.035 0.001 Yes -0.278 0.010 0.000 Yes -0.039 0.040 0.328 No 0.010 17.1

rorPDO 68 0.902 0.879 0.321 0.006 Yes -0.838 0.174 0.001 Yes -0.015 0.323 0.964 No 0.010 35.7

doTOT 68 -0.837 0.211 0.106 0.047 Yes -0.242 0.069 0.000 Yes -0.433 0.169 0.011 Yes 0.010 5.1

doFI 68 -1.842 0.139 0.056 0.013 Yes -0.190 0.036 0.001 Yes -0.246 0.092 0.008 Yes 0.010 21.4

doPDO 68 -1.212 0.259 0.161 0.108 Yes -0.285 0.107 0.008 Yes -0.565 0.258 0.028 Yes 0.010 3.5

1 At the 0.20 level.

TOT = total crashes (all severity levels combined).

Fl = fatal and injury crashes.

PDO = property–damage-only crashes.

ror = run-off-road crashes.

do = drop-off-related crashes.

Note: Blank cells represent models that did not converge.
36

Table 16. Evaluation of treatment versus comparison site effect for the period before resurfacing using observed crash frequencies.

State

Roadway

type

Shoulder 

type

Crash 

type and 

severity 

level

Number 

of siteyears Intercept

AADT effect Lane width effect Treatment versus comparison site effect

Dispersion 

parameter R2 Coefficient LR%

Standard

error p-value

Statistically 

significant?1 Coefficient

Standard 

error p-value

Statistically 

significant?1 Coefficient

Standard 

error p-value

Statistically 

significant?1

GA

Multilane Paved

TOT 102 -9.014 1.128 0.282 0.000 Yes -0.878 0.505 0.082 Yes 0.378 27.7

FI 102 -7.293 0.812 0.288 0.005 Yes -0.826 0.516 0.109 No 0.338 10.1

PDO 102 -10.881 1.286 0.321 0.000 Yes -0.885 0.527 0.093 Yes 0.505 27.3

rorTOT 102 -7.749 0.822 0.268 0.002 Yes -0.295 0.225 0.188 No 0.015 15.0

rorFI 102 -7.005 2.654 0.660 0.288 Yes -0.547 0.259 0.035 Yes 0.010 5.6

rorPDO 102 -9.207 3.372 0.911 0.353 Yes -0.119 0.282 0.672 No 0.010 16.2

doTOT 102 -8.374 2.175 0.844 0.229 Yes -0.355 0.180 0.048 Yes 0.010 11.9

doFI 102 -8.785 2.656 0.809 0.287 Yes -0.442 0.221 0.046 Yes 0.010 5.0

doPDO 102 -9.387 3.466 0.884 0.364 Yes -0.304 0.272 0.264 No 0.010 7.6

Two-lane

Paved

TOT 264 -8.045 0.982 0.130 0.000 Yes 0.222 0.176 0.207 No 0.247 35.7

FI 264 -7.646 0.852 0.143 0.000 Yes -0.121 0.152 0.426 No 0.018 21.3

PDO 264 -10.106 1.147 0.139 0.000 Yes 0.447 0.200 0.025 Yes 0.436 30.8

rorTOT 264 -3.346 0.666 0.121 0.000 Yes -0.220 0.161 0.172 Yes -0.231 0.206 0.261 No 0.166 13.9

rorFI 264 -1.188 0.465 0.156 0.003 Yes -0.303 0.161 0.059 Yes -0.593 0.229 0.010 Yes 0.073 7.2

rorPDO 264 -8.299 0.834 0.118 0.000 Yes 0.110 0.203 0.587 No 0.360 10.8

doTOT 264 -6.541 0.673 0.173 0.000 Yes -0.297 0.173 0.086 Yes 0.177 10.3

doFI 264 -1.063 0.414 0.202 0.040 Yes -0.300 0.170 0.077 Yes -0.752 0.249 0.003 Yes 0.173 5.7

doPDO 264 -10.600 1.038 0.204 0.000 Yes -0.016 0.223 0.942 No 0.156 9.9

Unpaved

TOT 318 -8.615 1.059 0.104 0.000 Yes 0.610 0.174 0.000 Yes 0.389 35.9

FI 318 -8.473 0.940 0.097 0.000 Yes 0.258 0.177 0.143 No 0.318 21.3

PDO 318 -9.950 1.148 0.119 0.000 Yes 0.864 0.197 0.000 Yes 0.419 34.3

rorTOT 318 -7.022 0.774 0.106 0.000 Yes 0.441 0.194 0.023 Yes 0.309 19.4

rorFI 318 -7.358 0.740 0.109 0.000 Yes 0.226 0.220 0.304 No 0.487 9.7

rorPDO 318 -8.611 0.874 0.150 0.000 Yes 0.653 0.228 0.004 Yes 0.385 16.8

doTOT 318 -7.106 0.736 0.132 0.000 Yes 0.397 0.212 0.061 Yes 0.247 15.6

doFI 318 -6.937 0.645 0.139 0.000 Yes 0.270 0.245 0.272 No 0.548 6.9

doPDO 318 -9.469 0.922 0.188 0.000 Yes 0.554 0.252 0.028 Yes 0.361 12.5

See notes at end of table.
37

Table 16. Evaluation of treatment versus comparison site effect for the period before resurfacing using observed crash frequencies—

Continued.

State

Roadway

type

Shoulder 

type

Crash 

type and 

severity 

level

Number 

of siteyears Intercept

AADT effect Lane width effect Treatment versus comparison site effect

Dispersion 

parameter R2 Coefficient LR%

Standard

error p-value

Statistically 

significant?1 Coefficient

Standard 

error p-value

Statistically 

significant?1 Coefficient

Standard 

error p-value

Statistically 

significant?1

IN Two-lane

Paved

TOT 42 -3.824 0.588 0.250 0.019 Yes -0.380 0.364 0.296 No 0.416 6.6

FI 42 -7.523 0.850 0.606 0.161 No -0.842 0.533 0.114 No 0.629 7.9

PDO 42 -3.076 0.465 0.235 0.048 Yes -0.205 0.356 0.565 No 0.369 4.1

rorTOT 42 -6.756 0.736 0.546 0.178 No -0.468 0.396 0.237 No 0.446 5.1

rorFI 42 -2.953 4.414 0.188 0.515 No -0.830 0.457 0.069 Yes 0.010 6.0

rorPDO 42 -8.070 0.815 0.589 0.167 No -0.092 0.432 0.831 No 0.461 5.0

doTOT 42 -13.860 1.420 1.212 0.241 No -0.996 0.503 0.048 Yes 0.478 6.0

doFI 42 0.0

doPDO 42 -23.901 2.478 0.959 0.010 Yes -0.477 0.529 0.368 No 0.010 11.9

Unpaved

TOT 68 -0.761 0.918 0.248 0.000 Yes -0.587 0.140 0.000 Yes 0.188 0.297 0.525 No 0.435 35.8

FI 68 -0.041 0.732 0.347 0.035 Yes -0.640 0.225 0.005 Yes -0.050 0.347 0.884 No 0.093 23.4

PDO 68 -1.612 0.998 0.270 0.000 Yes -0.594 0.155 0.000 Yes 0.269 0.304 0.377 No 0.527 31.5

rorTOT 68 1.418 0.806 0.316 0.011 Yes -0.783 0.200 0.000 Yes -0.068 0.331 0.838 No 0.221 34.7

rorFI 68 1.478 3.119 0.435 0.358 No -0.608 0.212 0.004 Yes -0.268 0.340 0.431 No 0.010 18.7

rorPDO 68 0.475 1.120 0.398 0.005 Yes -0.974 0.313 0.002 Yes 0.091 0.365 0.804 No 0.377 29.2

doTOT 68 1.029 0.101 0.386 0.794 No -0.312 0.246 0.204 No -1.107 0.596 0.063 Yes 0.010 21.7

doFI 68 0.0

doPDO 68 -4.194 0.270 0.560 0.630 No -1.090 0.699 0.119 No 0.584 6.4

NY Two-lane Paved

TOT 36 -5.328 0.674 0.085 0.000 Yes 0.127 0.182 0.484 No 0.486 24.9

FI 36 -6.943 0.766 0.113 0.000 Yes 0.308 0.172 0.074 Yes 0.674 19.3

PDO 36 -5.467 0.625 0.083 0.000 Yes -0.030 0.204 0.884 No 0.813 15.7

rorTOT 36 -4.846 0.480 0.085 0.000 Yes 0.577 0.140 0.000 Yes 0.243 19.6

rorFI 36 -5.333 0.486 0.122 0.000 Yes 0.643 0.175 0.000 Yes 0.410 14.4

rorPDO 36 -5.784 0.372 0.467 0.048 Yes 0.475 0.105 0.000 Yes 0.010 13.0

doTOT 36

doFI 36

doPDO 36

1 At the 0.20 significance level.

TOT = total crashes (all severity levels combined).

Fl = fatal and injury crashes.

PDO = property–damage-only crashes.

ror = run-off-road crashes.

do = drop-off-related crashes.

Note: Blank cells indicate models that did not converge.
38

Results from the analysis of EB-adjusted crash frequencies in table 15 show that there tended to be 

significant differences between treatment and comparison site crash frequencies for Georgia sites 

with unpaved shoulders in the period before resurfacing. Comparison sites that had unpaved 

shoulders had lower crash rates than treatment sites. There is also evidence of differences in

drop-off-related and run-off-road crashes for Georgia paved shoulder locations. Similarly, Indiana

unpaved shoulder locations differed for drop-off-related crashes. These locations had treatment 

sites with lower crash rates.

Results from the analysis of observed crash frequencies somewhat confirmed the results of the 

EB-adjusted crashes. However, there tended to be fewer significant results and poorer fit of the 

models in general. This was to be expected because EB-adjusted crashes are smoothed by the SPF

model predictions, causing smaller differences and less variation and leading to more significant

results. Differences between treatment and comparison sites were confirmed for Georgia unpaved

shoulder locations and drop-off-related crashes for paved shoulder locations. Additionally, 

New York locations, which were not tested by EB-adjusted crashes, showed differences for 

run-off-road crashes. All other significant differences were associated with poor models.

It was also desirable to confirm the existence of a cause-and-effect chain leading from the frequency

and height of pavement-edge drop-offs to the likelihood of crashes. The drop-off height analysis 

reported in chapter 3 indicated that two-lane highway sites with unpaved shoulders and the

multilane highway sites in Georgia did not have significant differences in the proportion of high 

drop-offs and therefore should have non-significant differences in crash frequency in the period 

before resurfacing. This expectation was not entirely supported by crash analysis results. However,

for cases in which there were significant differences, these differences were in the same direction

indicated in the drop-off analysis. That is, if drop-offs were more prevalent, then the sites had 

more crashes. Similarly, two-lane highway sites with paved shoulders in Georgia had comparison

sites with a significantly higher probability of high drop-offs, and the crash analysis showed the 

comparison sites had more crashes, although the result was not significant.

Results for Indiana sites on two-lane highways with paved shoulders were consistent with the 

analysis of drop-off measurements, but the results for Indiana sites on two-lane highways with 

unpaved shoulders were not consistent with the analysis of drop-off measurements. 

Overall, the treatment and comparison sites showed similar crash frequencies for paved shoulder 

sites in the period before resurfacing. By contrast, there were some statistically significant 

differences in crash frequencies between treatment and comparison sites for unpaved shoulders 

during the period before resurfacing. It should be noted that only 2 years of crash data were 

available for the period before resurfacing in Indiana, in comparison to 6 years for the period 

before resurfacing in Georgia. Thus, the variability of the Indiana crash frequencies was expected

to be higher. In most cases (with the single exception previously noted), the differences in crash 

frequencies between treatment and comparison sites were similar to the differences in proportions

of extreme drop-off heights for the period before resurfacing. 

4.3.2 Before-After Evaluation Using the EB Technique

An observational before-after evaluation was conducted using the EB technique to estimate the 

safety effectiveness of the safety edge treatment. Separate before-after evaluations were conducted
39

for resurfacing projects with the safety edge (treatment sites) and resurfacing projects without the 

safety edge (comparison sites). The ratio of these results was used to estimate the effect of the 

safety edge treatment.

All crash severity levels for total crashes, run-off-road crashes, and drop-off-related crashes were 

evaluated. The study period before resurfacing for these evaluations was the 4-year period from 

2001 to 2004. The study period after resurfacing was the 3-year period from 2006 to 2008. The 

entire year in which resurfacing was performed, 2005, was excluded from the evaluation. The 

rationale for excluding crashes during the construction year is that it takes time for drivers to adjust

to new driving conditions, and so the transition period is not necessarily representative of the longterm safety performance of the site. All of the crash data used in the evaluation were for complete

calendar years so that there would be no opportunity for seasonal biases to affect the results.

The EB procedure was programmed and executed in SAS®.

(3) Effectiveness estimates and their 

precision estimates, along with their statistical significance, are presented for specific crash types 

in table 17 through table 25.
40

Table 17. Before-after EB evaluation results for total crashes.

State

Roadway

type

Shoulder 

type

Site 

type

Number 

of 

sites

Odds 

ratio

Change in crash frequency 

from before to after 

resurfacing

Statistically 

significant? Safety edge effect

Percent 

change Direction

Standard 

error (%)

5% 

level

10% 

level Effect (%) Direction

Standard 

error (%) 5% level 10% level

GA Two-lane

Paved

T 25 1.804 13.123 Increase 7.276 No Yes

7.732 Decrease 9.596 No No

C 19 2.262 22.602 Increase 9.993 Yes Yes

Unpaved

T 22 2.246 -13.562 Decrease 6.038 Yes Yes

11.361 Decrease 8.467 No No

C 31 0.389 -2.483 Decrease 6.376 No No

Combined

T 47 0.143 -0.670 Decrease 4.697 No No

6.817 Decrease 6.459 No No

C 50 1.217 6.597 Increase 5.421 No No

IN Two-lane

Paved

T 14 0.047 0.567 Increase 12.167 No No

15.524 Decrease 14.422 No No

C 7 1.333 19.048 Increase 14.293 No No

Unpaved

T 16 1.524 29.925 Increase 19.639 No No -26.942 Increase 24.027 No No

C 18 0.201 2.350 Increase 11.691 No No

Combined

T 30 1.000 10.456 Increase 10.454 No No -0.235 Increase 12.622 No No

C 25 1.120 10.197 Increase 9.104 No No

GA 

& IN Two-lane

Paved

T 39 1.601 10.027 Increase 6.262 No No

9.485 Decrease 8.009 No No

C 26 2.628 21.556 Increase 8.203 Yes Yes

Unpaved

T 38 1.311 -7.657 Decrease 5.842 No No

6.516 Decrease 7.910 No No

C 49 0.218 -1.221 Decrease 5.604 No No

Combined

T 77 0.360 1.546 Increase 4.293 No No

5.674 Decrease 5.737 No No

C 75 1.642 7.654 Increase 4.662 No No

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.
41

Table 18. Before-after EB evaluation results for fatal and injury crashes.

State

Roadway 

type

Shoulder 

type

Site 

type

Number 

of

sites

Odds 

ratio

Change in crash frequency from 

before to after resurfacing

Statistically 

significant? Safety edge effect

Percent 

change Direction

Standard 

error (%)

5% 

level

10% 

level Effect (%) Direction

Standard 

error (%)

5% 

level

10% 

level

GA Two-lane

Paved

T 25 1.592 19.899 Increase 12.499 No No

10.959 Decrease 13.779 No No

C 19 2.244 34.656 Increase 15.446 Yes Yes

Unpaved

T 22 0.232 2.761 Increase 11.887 No No -15.555 Increase 17.687 No No

C 31 1.243 -11.072 Decrease 8.907 No No

Combined

T 47 1.346 11.647 Increase 8.651 No No -5.982 Increase 11.462 No No

C 50 0.676 5.345 Increase 7.905 No No

IN Two-lane

Paved

T 14 0.736 -18.579 Decrease 25.239 No No

44.993 Decrease 21.492 Yes Yes

C 7 1.359 48.020 Increase 35.335 No No

Unpaved

T 16 1.340 -35.945 Decrease 26.829 No No

43.548 Decrease 26.137 No No

C 18 0.598 13.469 Increase 22.521 No No

Combined

T 30 1.339 -24.947 Decrease 18.633 No No

40.939 Decrease 17.167 Yes Yes

C 25 1.403 27.078 Increase 19.294 No No

GA 

& IN Two-lane

Paved

T 39 1.295 14.685 Increase 11.337 No No

16.528 Decrease 11.919 No No

C 26 2.633 37.393 Increase 14.199 Yes Yes

Unpaved

T 38 0.087 -0.961 Decrease 11.012 No No -6.361 Increase 15.147 No No

C 49 0.827 -6.884 Decrease 8.328 No No

Combined

T 77 0.924 7.328 Increase 7.93 No No

1.667 Decrease 9.780 No No

C 75 1.246 9.148 Increase 7.341 No No

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.
42

Table 19. Before-after EB evaluation results for PDO crashes.

State

Roadway 

type

Shoulder 

type

Site 

type

Number 

of

sites

Odds 

ratio

Change in crash frequency from 

before to after resurfacing

Statistically 

significant? Safety edge effect

Percent 

change Direction

Standard 

error (%)

5% 

level

10% 

level Effect (%) Direction

Standard 

error (%)

5% 

level

10% 

level

GA Two-lane

Paved

T 25 0.975 9.276 Increase 9.511 No No

2.554 Decrease 15.183 No No

C 19 0.837 12.140 Increase 14.502 No No

Unpaved

T 22 2.886 -20.980 Decrease 7.271 Yes Yes

24.281 Decrease 10.078 Yes Yes

C 31 0.437 4.359 Increase 9.963 No No

Combined

T 47 1.150 -6.764 Decrease 5.881 No No

13.201 Decrease 8.607 No No

C 50 0.899 7.416 Increase 8.249 No No

IN Two-lane

Paved

T 14 0.287 4.045 Increase 14.108 No No

6.579 Decrease 18.450 No No

C 7 0.710 11.372 Increase 16.024 No No

Unpaved

T 16 1.898 47.501 Increase 25.033 No Yes -50.902 Increase 33.486 No No

C 18 0.160 -2.254 Decrease 14.046 No No

Combined

T 30 1.449 18.175 Increase 12.540 No No -12.894 Increase 16.531 No No

C 25 0.440 4.678 Increase 10.631 No No

GA 

& IN Two-lane

Paved

T 39 0.995 7.860 Increase 7.901 No No

3.845 Decrease 11.632 No No

C 26 1.127 12.173 Increase 10.803 No No

Unpaved

T 38 1.469 -10.599 Decrease 7.215 No No

12.795 Decrease 9.898 No No

C 49 0.309 2.518 Increase 8.150 No No

Combined

T 77 0.185 -0.995 Decrease 5.364 No No

7.100 Decrease 7.601 No No

C 75 1.007 6.572 Increase 6.529 No No

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.
43

Table 20. Before-after EB evaluation results for total run-off-road crashes.

State

Roadway 

type

Shoulder 

type

Site 

type

Number 

of 

sites

Odds 

ratio

Change in crash frequency from 

before to after resurfacing

Statistically 

significant? Safety edge effect

Percent 

change Direction

Standard 

error (%)

5% 

level

10% 

level Effect (%) Direction

Standard 

error (%) 5% level 10% level

GA Two-lane

Paved

T 25 3.375 27.777 Increase 8.230 Yes Yes

13.721 Decrease 9.010 No No

C 19 3.956 48.097 Increase 12.158 Yes Yes

Unpaved

T 22 0.102 0.718 Increase 7.049 No No

9.080 Decrease 8.652 No No

C 31 1.487 10.777 Increase 7.248 No No

Combined

T 47 2.594 14.006 Increase 5.400 Yes Yes

7.872 Decrease 6.406 No No

C 50 3.766 23.747 Increase 6.306 Yes Yes

IN Two-lane

Paved

T 14 3.152 63.675 Increase 20.201 Yes Yes

8.183 Decrease 15.844 No No

C 7 3.639 78.263 Increase 21.505 Yes Yes

Unpaved

T 16 3.408 105.593 Increase 30.982 Yes Yes -46.895 Increase 27.796 No Yes

C 18 2.502 39.959 Increase 15.968 Yes Yes

Combined

T 30 4.573 78.116 Increase 17.083 Yes Yes -13.484 Increase 14.389 No No

C 25 4.391 56.952 Increase 12.969 Yes Yes

GA 

& IN Two-lane

Paved

T 39 4.522 34.710 Increase 7.675 Yes Yes

14.177 Decrease 7.593 No Yes

C 26 5.351 56.962 Increase 10.645 Yes Yes

Unpaved

T 38 1.631 11.534 Increase 7.071 No No

4.786 Decrease 8.094 No No

C 49 2.581 17.140 Increase 6.640 Yes Yes

Combined

T 77 4.496 23.514 Increase 5.230 Yes Yes

6.315 Decrease 5.654 No No

C 75 5.576 31.840 Increase 5.710 Yes Yes

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.
44

Table 21. Before-after EB evaluation results for fatal and injury run-off-road crashes.

State

Roadway 

type

Shoulder 

type

Site 

type

Number 

of

sites

Odds 

ratio

Change in crash frequency from 

before to after resurfacing

Statistically 

significant? Safety edge effect

Percent 

change Direction

Standard 

error (%)

5% 

level

10% 

level Effect (%) Direction

Standard 

error (%) 5% level 10% level

GA Two-lane

Paved

T 25 3.036 46.712 Increase 15.384 Yes Yes

19.175 Decrease 12.659 No No

C 19 3.856 81.517 Increase 21.142 Yes Yes

Unpaved

T 22 1.891 28.134 Increase 14.877 No Yes -18.058 Increase 18.139 No No

C 31 0.784 8.535 Increase 10.892 No No

Combined

T 47 3.529 37.878 Increase 10.733 Yes Yes -3.609 Increase 11.192 No No

C 50 3.295 33.075 Increase 10.037 Yes Yes

IN Two-lane

Paved

T 14 0.086 -2.553 Decrease 29.761 No No

46.332 Decrease 20.632 Yes Yes

C 7 1.931 81.574 Increase 42.252 No Yes

Unpaved

T 16 0.140 6.109 Increase 43.637 No No -12.464 Increase 51.101 No No

C 18 0.309 -5.651 Decrease 18.313 No No

Combined

T 30 0.018 0.440 Increase 24.694 No No

16.402 Decrease 23.965 No No

C 25 1.137 20.146 Increase 17.714 No No

GA 

& IN Two-lane

Paved

T 39 2.877 39.851 Increase 13.853 Yes Yes

23.123 Decrease 11.053 Yes Yes

C 26 4.323 81.916 Increase 18.950 Yes Yes

Unpaved

T 38 1.877 26.499 Increase 14.120 No Yes -20.037 Increase 17.137 No No

C 49 0.573 5.383 Increase 9.388 No No

Combined

T 77 3.407 33.764 Increase 9.911 Yes Yes -2.622 Increase 10.228 No No

C 75 3.469 30.346 Increase 8.748 Yes Yes

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.
45

Table 22. Before-after EB evaluation results for PDO run-off-road crashes.

State

Roadway 

type

Shoulder 

type

Site 

type

Number 

of

sites

Odds 

ratio

Change in crash frequency from 

before to after resurfacing

Statistically 

significant? Safety edge effect

Percent 

change Direction

Standard 

error (%)

5% 

level

10% 

level Effect (%) Direction

Standard 

error (%) 5% level 10% level

GA Two-lane

Paved

T 25 1.821 18.575 Increase 10.200 No Yes

5.160 Decrease 14.498 No No

C 19 1.587 25.026 Increase 15.770 No No

Unpaved

T 22 1.286 -10.527 Decrease 8.185 No No

20.225 Decrease 10.451 No Yes

C 31 1.158 12.156 Increase 10.494 No No

Combined

T 47 0.535 3.462 Increase 6.472 No No

11.524 Decrease 8.673 No No

C 50 1.929 16.938 Increase 8.779 No Yes

IN Two-lane

Paved

T 14 3.154 82.392 Increase 26.120 Yes Yes -3.538 Increase 21.327 No No

C 7 2.922 76.160 Increase 26.061 Yes Yes

Unpaved

T 16 3.396 131.099 Increase 38.606 Yes Yes -31.229 Increase 30.365 No No

C 18 2.699 76.104 Increase 28.193 Yes Yes

Combined

T 30 4.549 99.719 Increase 21.920 Yes Yes -12.878 Increase 17.414 No No

C 25 3.999 76.934 Increase 19.240 Yes Yes

GA 

& IN Two-lane

Paved

T 39 3.323 32.288 Increase 9.718 Yes Yes

7.374 Decrease 11.235 No No

C 26 3.108 42.820 Increase 13.778 Yes Yes

Unpaved

T 38 0.657 5.574 Increase 8.485 No No

16.029 Decrease 9.550 No No

C 49 2.548 25.727 Increase 10.096 Yes Yes

Combined

T 77 2.976 19.201 Increase 6.452 Yes Yes

10.162 Decrease 7.404 No No

C 75 3.995 32.684 Increase 8.181 Yes Yes

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.
46

Table 23. Before-after EB evaluation results for total drop-off-related crashes.

State

Roadway 

type

Shoulder 

type

Site 

type

Number 

of

sites

Odds 

ratio

Change in crash frequency from 

before to after resurfacing

Statistically 

significant? Safety edge effect

Percent 

change Direction

Standard 

error (%)

5% 

level

10% 

level Effect (%) Direction

Standard 

error (%) 5% level 10% level

GA Two-lane

Paved

T 25 3.817 32.758 Increase 8.582 Yes Yes

10.293 Decrease 9.405 No No

C 19 3.930 47.991 Increase 12.210 Yes Yes

Unpaved

T 22 0.591 4.334 Increase 7.328 No No

9.130 Decrease 8.698 No No

C 31 1.970 14.817 Increase 7.522 No Yes

Combined

T 47 3.250 18.272 Increase 5.623 Yes Yes

6.603 Decrease 6.522 No No

C 50 4.115 26.633 Increase 6.472 Yes Yes

IN Two-lane

Paved

T 14 3.681 86.217 Increase 23.425 Yes Yes

8.759 Decrease 15.949 No No

C 7 4.202 104.093 Increase 24.772 Yes Yes

Unpaved

T 16 4.374 193.003 Increase 44.122 Yes Yes -79.564 Increase 34.036 Yes Yes

C 18 3.357 63.175 Increase 18.820 Yes Yes

Combined

T 30 5.509 117.38 Increase 21.305 Yes Yes -19.733 Increase 15.389 No No

C 25 5.386 81.554 Increase 15.141 Yes Yes

GA 

& IN Two-lane

Paved

T 39 5.191 42.320 Increase 8.153 Yes Yes

12.586 Decrease 7.803 No No

C 26 5.657 62.812 Increase 11.104 Yes Yes

Unpaved

T 38 2.489 18.854 Increase 7.574 Yes Yes

4.455 Decrease 8.189 No No

C 49 3.450 24.396 Increase 7.070 Yes Yes

Combined

T 77 5.564 31.039 Increase 5.578 Yes Yes

5.587 Decrease 5.739 No No

C 75 6.428 38.794 Increase 6.035 Yes Yes

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.
47

Table 24. Before-after EB evaluation results for fatal and injury drop-off-related crashes.

State

Roadway 

type

Shoulder 

type

Site 

type

Number 

of 

sites

Odds 

ratio

Change in crash frequency from 

before to after resurfacing

Statistically 

significant? Safety edge effect

Percent 

change Direction

Standard 

error (%)

5% 

level

10% 

level Effect (%) Direction

Standard 

error (%) 5% level 10% level

GA Two-lane

Paved

T 25 3.082 48.356 Increase 15.691 Yes Yes

15.135 Decrease 13.441 No No

C 19 3.639 74.815 Increase 20.560 Yes Yes

Unpaved

T 22 2.068 31.810 Increase 15.383 Yes Yes -18.357 Increase 18.246 No No

C 31 1.014 11.366 Increase 11.214 No No

Combined

T 47 3.683 40.587 Increase 11.019 Yes Yes -5.202 Increase 11.444 No No

C 50 3.317 33.635 Increase 10.139 Yes Yes

IN Two-lane

Paved

T 14 1.727 120.288 Increase 69.662 No Yes

42.488 Decrease 22.855 No Yes

C 7 3.073 283.030 Increase 92.115 Yes Yes

Unpaved

T 16 1.038 77.468 Increase 74.600 No No

38.503 Decrease 28.727 No No

C 18 3.208 188.581 Increase 58.789 Yes Yes

Combined

T 30 2.037 105.168 Increase 51.641 Yes Yes

36.972 Decrease 18.615 No Yes

C 25 4.473 225.519 Increase 50.414 Yes Yes

GA 

& IN Two-lane

Paved

T 39 3.473 53.546 Increase 15.419 Yes Yes

21.596 Decrease 11.453 No Yes

C 26 4.604 95.839 Increase 20.818 Yes Yes

Unpaved

T 38 2.255 34.086 Increase 15.115 Yes Yes -5.230 Increase 15.186 No No

C 49 2.381 27.422 Increase 11.516 Yes Yes

Combined

T 77 4.106 44.481 Increase 10.833 Yes Yes

4.676 Decrease 9.672 No No

C 75 4.980 51.568 Increase 10.356 Yes Yes

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.
48

Table 25. Before-after EB evaluation results for PDO drop-off-related crashes.

State

Roadway 

type

Shoulder 

type

Site 

type

Number 

of

sites

Odds 

ratio

Change in crash frequency from 

before to after resurfacing

Statistically

significant? Safety edge effect

Percent 

change Direction

Standard 

error (%)

5% 

level

10% 

level Effect (%) Direction

Standard 

error (%) 5% level 10% level

GA Two-lane

Paved

T 25 2.281 24.794 Increase 10.870 Yes Yes

2.550 Decrease 15.169 No No

C 19 1.698 28.059 Increase 16.524 No No

Unpaved

T 22 0.826 -7.063 Decrease 8.556 No No

20.631 Decrease 10.466 No Yes

C 31 1.550 17.095 Increase 11.032 No No

Combined

T 47 1.194 8.149 Increase 6.827 No No

10.829 Decrease 8.786 No No

C 50 2.307 21.283 Increase 9.224 Yes Yes

IN Two-lane

Paved

T 14 3.242 81.224 Increase 25.055 Yes Yes -2.553 Increase 20.262 No No

C 7 3.085 76.713 Increase 24.863 Yes Yes

Unpaved

T 16 4.122 216.618 Increase 52.557 Yes Yes -131.652 Increase 50.289 Yes Yes

C 18 1.922 36.678 Increase 19.087 No Yes

Combined

T 30 5.032 118.845 Increase 23.618 Yes Yes -40.967 Increase 20.594 No Yes

C 25 3.600 55.246 Increase 15.347 Yes Yes

GA 

& IN Two-lane

Paved

T 39 3.695 37.481 Increase 10.143 Yes Yes

5.319 Decrease 11.529 No No

C 26 3.217 45.205 Increase 14.052 Yes Yes

Unpaved

T 38 1.390 12.718 Increase 9.148 No No

7.890 Decrease 10.369 No No

C 49 2.330 22.373 Increase 9.601 Yes Yes

Combined

T 77 3.730 25.534 Increase 6.845 Yes Yes

4.435 Decrease 7.789 No No

C 75 3.926 31.360 Increase 7.988 Yes Yes

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.
49

The safety edge effect shown in the results tables is the ratio between the before-to-after change 

in crash frequency for the treatment sites and the before-to-after change in crash frequency for 

the comparison sites. This formulation of the safety effect was derived from the multiplicative 

nature of crash modification factors (CMFs), as shown in equation 3 and equation 4:

CMFResurfacing+SafetyEdge = CMFResurfacingCMFSafetyEdge (3)

or

CMF

CMF = CMF

CMF

CMF = Comparison

Treatment

Resurfacing

Resurfacing+SafetyEdge

SafetyEdge

(4)

The before-to-after percent change in crash frequency can be converted to a CMF for this calculation

by dividing by 100 and adding a value of 1. Similarly, the final CMF for the safety edge can be 

converted back to a percent change by subtracting the ratio from 1 and multiplying by 100. When

the increase in crashes with resurfacing was greater at the comparison sites than at the treatment 

sites, an indication that the safety edge treatment was effective, the safety edge effect is shown 

as a positive value. A precision estimate of the ratio was calculated and used to generate a 

confidence interval of the ratio. Confidence intervals excluding the value 1 indicate statistical 

significance. For instance, the safety edge effect for Georgia two-lane roadways with paved 

shoulders shown in table 17 is calculated by first converting the before-to-after changes to 

CMFs and then taking the ratio: (1+13.12/100)/(1+22.60/100) = 1.1312/1.2260 = 0.927. This 

CMF represents a (1-0.927)*100 = 7.73 percent decrease in crashes. Since both confidence intervals 

(i.e., 7.73±(9.60)*1.645 and 7.73±(9.60)*1.96, where 1.645 and 1.96 are critical confidence level 

values) contain the value 1, the estimate of the safety edge effect is not significant.

The EB results indicate that for all two-lane sites in Georgia and Indiana, the safety edge effect 

was 5.7 percent for total crashes, 6.3 percent for run-off-road crashes, and 5.6 percent for dropoff-related crashes. While none of these results is statistically significant, they do show a small,

consistent benefit of the provision of the safety edge on rural two-lane highways.

When the results are examined separately for the two shoulder types (sites with paved shoulders 

having widths of 4 ft or less and sites with unpaved shoulders) the use of the safety edge shows 

more benefit for paved shoulder sites than for unpaved shoulder sites. The safety edge effect for 

sites with paved shoulders is 9.5 percent for total crashes, 14.2 percent for run-off-road crashes, 

and 12.6 percent for drop-off-related crashes. The results for run-off road crashes are statistically 

significant at the 10 percent significance level, but the result for total crashes and drop-off related 

crashes are not statistically significant. For sites with unpaved shoulders, the safety edge effect 

was 6.5 percent for total crashes, 4.8 percent for run-off-road crashes, and 4.5 percent for dropoff-related crashes. None of these results is significant.

The expectation was that the use of the safety edge treatment would produce larger benefits on 

highways with unpaved shoulders, since potential drop-offs at such sites are closer to the travel 

lanes than on highways with paved shoulders and therefore are expected to be driven over more 

frequently. However, the sites with unpaved shoulders in both States had much lower ADT than 
50

the sites with paved shoulders, and the lower numbers of crashes in the before and after resurfacing 

periods undoubtedly affected the effectiveness estimates.

In considering the States individually, Georgia sites showed a safety edge effect of 6.8 percent 

for total crashes, 7.9 percent for run-off-road crashes, and 6.6 percent for drop-off-related crashes.

None of the results were statistically significant. Indiana sites had safety edge effects of -0.2 percent

for total crashes, -13.5 percent for run-off-road crashes, and -19.7 percent for drop-off-related 

crashes. The negative results are not statistically significant at the 10 percent significance level. 

The results for Indiana sites were affected by very low numbers of crashes in the before period.

Overall results for the EB evaluation are summarized in table 26 and compared to interim results 

obtained from analyses conducted 1 and 2 years after resurfacing. The analysis for data including

3 years after resurfacing, which are presented in this section of the report, includes additional 

comparisons because shoulder types and the two States were combined and compared. Fifty-six

of the 81 results for year 3 showed positive safety edge effects; however, only 11 of these 

positive safety effects were statistically significant. While 25 of the observed effects were

negative (e.g., comparison sites had fewer crashes than treatment sites), only 4 of these results 

were statistically significant.

Table 26. Summary of safety effects from year 3, year 2, and year 1 results 

for before-after EB safety evaluations.

Direction of 

safety effect

Statistically 

significant 

safety effect?

Number of cases

Year 3 

analysis results

Year 2 

analysis results

Year 1 

analysis results

Positive Yes 11 8 2

Positive No 45 14 13

Negative Yes 4 7 6

Negative No 21 7 15

Total 81 36 36

The magnitude of the effects also changed with the addition of the year 3 data. The safety effects 

from the year 3 evaluation were smaller and less variable than the year 1 or year 2 results. The 

overall impact of the safety edge was expected to be small, since drop-off-related crashes are 

usually only a small percentage of the total non-intersection crashes on rural roads. The year 3

results presented above follow this trend and therefore are considered more reliable than the earlier

results. However, in some cases the smaller magnitude of the safety edge effect makes it more 

difficult for the effect to be statistically significant.

Total crashes on all sites mainly increased; some of this increase may be due to a resurfacing 

effect that was very evident in the year 1 results but less so in later years.

The year 3 evaluation results presented above vary in magnitude and statistical significance. The 

overall evaluation results for total crashes in Georgia and Indiana combined show an average safety

edge treatment effect of 5.7 percent. In other words, the sites treated with the safety edge appear to

have lower crash frequencies after resurfacing than sites not treated with the safety edge. Although

not statistically significant, this seems to be the most appropriate overall effectiveness measure 

for the safety edge treatment from the EB evaluation. The lack of statistical significance for this 

result is not surprising given the small magnitude of the effect.
51

Two trends were evident in the EB analysis of run-off-road and drop-off-related crashes. First, 

the safety edge treatment generally appears to have had a positive effect on safety for all site 

types except for sites with unpaved shoulders in Indiana. This variability in results has not been 

fully explained. Second, the negative safety edge effects for Indiana sites with unpaved shoulders 

may be explained by low frequencies of drop-off-related crashes on comparison sites in the period

before resurfacing. The safety edge effect was statistically significant only for Indiana sites with 

unpaved shoulders (negative effect).

Georgia sites with paved shoulders showed safety edge treatment effects of approximately

14 percent for run-off-road crashes and 10 percent for drop-off-related crashes. Indiana sites with 

unpaved shoulders had safety edge effects of -31 to -47 percent for run-off-road crashes and -45

to -80 percent for drop-off-related crashes, and these effects were statistically significant. When 

data from both States were combined, the safety edge treatment effects for paved shoulders were 

14 percent for run-off-road crashes and 12.6 percent for drop-off-related crashes. The effect for 

run-off-road crashes was statistically significant at the 10 percent significance level. The treatment

effects for sites with unpaved shoulders were 5 percent for run-off-road crashes and 4.5 percent

for drop-off-related crashes. These small, non-significant effects are probably influenced strongly

by the Indiana sites with unpaved shoulders.

The effects for run-off-road and drop-off-related crashes are larger than the effects for total 

crashes in absolute magnitude but vary in sign and statistical significance. These evaluation 

results for run-off-road and drop-off-related crashes appear less stable, and thus less reliable,

than the results for total crashes. Although not statistically significant, the single most reliable 

estimate of the effectiveness of the safety edge treatment is the 5.7 percent reduction in total 

crashes observed for two-lane highways in the combined data for sites with both paved and 

unpaved shoulders in both Georgia and Indiana (see the last row in table 17).

There are several potential biases and limitations that may influence these results, including 

the following:

There were some observed differences between treatment and comparison sites for the period

before resurfacing which could confound the analysis results (see sections 3.1 and 4.3.1).

The sites with unpaved shoulders, where the safety edge treatment was expected to be most

effective, also had the lowest crash frequencies. This increased the variability in the data 

and made the statistical test less powerful.

4.3.3 Cross-Sectional Analysis

A cross-sectional analysis of the crash data for the period after resurfacing at the treatment and 

comparison sites was conducted to directly compare the safety performance. This cross-sectional 

analysis is analogous to the analysis of safety differences for the period before resurfacing reported

in section 4.3.1 but serves a different purpose. In this analysis, any observed difference in safety

performance between the treatment and comparison sites is interpreted as an effect of the safety 

edge treatment. This interpretation should be made cautiously because, as noted in sections 3 and 

4.3.1, there are other differences between the treatment and comparison sites that may affect the 

comparison.
52

The cross-sectional comparison of data for the period after resurfacing was conducted using 

analysis of covariance to assess the statistical significance of the treatment versus comparison 

site effects. This analysis was conducted for each State/roadway type/shoulder type combination 

with PROC GENMOD in SAS®.

(3) Traffic volume and site type (treatment versus comparison) 

were the main factors of interest in the analysis. Lane width was also considered but was not 

found to be statistically significant. The analysis was conducted with the same negative binomial 

modeling techniques described in the discussion of SPFs in section 4.2.

The safety edge treatment effect and its standard error were calculated for each target crash type

and adjusted for any covariates. The results are presented in table 27. The significance and p-value

for the treatment versus comparison site effects are also provided.

Where blank lines are shown in the table, the regression model did not converge, so no model could 

be developed. Table 27 shows that there were 44 models that converged for the final analysis. This 

is an improvement on the year 1 and year 2 analysis, for which only 20 and 35 models converged,

respectively. Thus, as additional years of data have become available, more models have been 

obtained in the cross-sectional analysis. Table 27 shows that the crash frequencies for the 

treatment sites after resurfacing were generally lower than for the comparison sites, indicating 

that the safety edge treatment was effective. Statistically significant results for the safety edge 

effect (treatment versus comparison sites) were obtained for 19 of the 44 models shown in the 

table. In 15 of these cases, the safety performance of the treatment sites was better than the 

comparison sites, indicating that the safety edge was effective. However, in four cases (three of 

which were on two-lane highways with unpaved shoulders in Georgia), the safety performance 

of the comparison sites was better than the treatment sites.

In summary, the cross-sectional analysis results are similar to the results of the EB analysis, 

suggesting that the safety edge treatment is effective in reducing crashes for sites with paved 

shoulders and for sites in Indiana with unpaved shoulders. However, results for sites in Georgia 

with unpaved shoulders did not show that the safety edge was effective in reducing crashes.

The potential biases and limitations of this analysis are as follows:

There were some observed differences between treatment and comparison sites for the period

before resurfacing which could confound the analysis results (see sections 3.1 and 4.3.1).

The sites with unpaved shoulders, where the safety edge treatment would be expected to 

be most effective, had the lowest crash frequencies, which increased the variability in the 

data and made the statistical test less powerful.

The cross-sectional approach does not explicitly compensate for regression to the mean.
53

Table 27. Cross-sectional analysis of safety edge treatment effect for the period after resurfacing.

State

Roadway 

type

Shoulder 

type

Crash 

type and 

severity 

level

Number 

of siteyears Intercept

AADT effect Treatment effect

Dispersion 

parameter R2

LR%

Safety 

edge 

effect2

Coefficient (%)

Standard 

error p-Value

Statistically 

significant?1 Coefficient

Standard 

error p-Value

Statistically 

significant?1

GA

Multilane Paved

TOT 51 -13.212 1.542 0.309 0.000 Yes -0.655 0.305 0.032 Yes 0.282 48.4 48.1

FI 51 -12.940 1.360 0.332 0.000 Yes -0.293 0.257 0.254 No 0.027 30.4 25.4

PDO 51 -14.627 1.656 0.372 0.000 Yes -0.703 0.345 0.042 Yes 0.404 44.0 50.5

rorTOT 51 -19.840 2.114 0.329 0.000 Yes -0.946 0.201 0.000 Yes 0.010 66.5 61.2

rorFI 51 -15.748 1.562 0.391 0.000 Yes -0.410 0.244 0.092 Yes 0.010 20.3 33.6

rorPDO 51 -23.547 2.462 0.308 0.000 Yes -1.280 0.240 0.000 Yes 0.010 61.8 72.2

doTOT 51 -19.432 2.029 0.446 0.000 Yes -0.882 0.219 0.000 Yes 0.010 48.8 58.6

doFI 51 -18.509 1.841 0.380 0.000 Yes -0.610 0.121 0.000 Yes 0.010 23.6 45.7

doPDO 51 -20.271 2.061 0.552 0.000 Yes -1.130 0.354 0.001 Yes 0.010 34.5 67.7

Two-lane

Paved

TOT 132 -8.695 1.104 0.121 0.000 Yes 0.111 0.183 0.545 No 0.178 51.0 -11.7

FI 132 -7.501 0.880 0.182 0.000 Yes -0.197 0.264 0.457 No 0.273 27.4 17.8

PDO 132 -11.162 1.306 0.125 0.000 Yes 0.414 0.193 0.032 Yes 0.136 49.6 -51.2

rorTOT 132 -7.654 0.902 0.161 0.000 Yes -0.120 0.260 0.645 No 0.279 30.3 11.3

rorFI 132 -5.201 0.546 0.197 0.006 Yes -0.497 0.355 0.161 No 0.453 11.5 39.2

rorPDO 132 -12.208 1.322 0.216 0.000 Yes 0.342 0.328 0.298 No 0.283 29.3 -40.8

doTOT 132 -8.244 0.927 0.163 0.000 Yes -0.153 0.301 0.611 No 0.287 25.9 14.2

doFI 132 -5.844 0.582 0.193 0.003 Yes -0.368 0.383 0.337 No 0.589 8.7 30.8

doPDO 132 -14.467 1.518 0.276 0.000 Yes 0.328 0.415 0.428 No 0.209 24.9 -38.9

Unpaved

TOT 159 -10.116 1.253 0.173 0.000 Yes 0.581 0.225 0.010 Yes 0.555 37.0 -78.7

FI 159 -8.599 0.959 0.182 0.000 Yes 0.415 0.226 0.066 Yes 0.267 24.3 -51.5

PDO 159 -12.683 1.498 0.199 0.000 Yes 0.594 0.274 0.030 Yes 0.683 34.0 -81.1

rorTOT 159 -7.229 0.799 0.169 0.000 Yes 0.341 0.222 0.125 No 0.270 17.9 -40.6

rorFI 159 -8.063 0.834 0.193 0.000 Yes 0.275 0.265 0.301 No 0.217 14.4 -31.6

rorPDO 159 -8.374 0.840 0.177 0.000 Yes 0.365 0.255 0.152 No 0.262 12.0 -44.0

doTOT 159 -7.422 0.773 0.196 0.000 Yes 0.379 0.262 0.149 No 0.301 14.2 -46.0

doFI 159 -8.725 0.882 0.202 0.000 Yes 0.297 0.257 0.248 No 0.128 14.0 -34.6

doPDO 159 -8.048 0.728 0.243 0.003 Yes 0.411 0.344 0.231 No 0.402 6.8 -50.9

See notes at end of table.
54

Table 27. Cross-sectional analysis of safety edge treatment effect for the period after resurfacing—Continued.

State

Roadway 

type

Shoulder 

type

Crash 

type and 

severity 

level

Number 

of siteyears Intercept

AADT effect Treatment effect

Dispersion 

parameter R2

LR%

Safety 

edge 

effect2

Coefficient (%)

Standard 

error p-Value

Statistically 

significant?1 Coefficient

Standard 

error p-Value

Statistically 

significant?1

IN Two-lane

Paved

TOT 63

FI 63 -1.982 0.117 0.647 0.856 No -0.819 0.582 0.159 No 0.853 4.2 55.9

PDO 63

rorTOT 63

rorFI 63

rorPDO 63

doTOT 63

doFI 63 -13.163 1.184 3.132 0.705 No -1.599 0.729 0.028 Yes 0.010 3.0 79.8

doPDO 63

Unpaved

TOT 102 -4.887 0.543 0.256 0.034 Yes -0.069 0.211 0.742 No 0.653 7.4 6.7

FI 102 -5.650 0.493 0.313 0.115 No -1.215 0.492 0.013 Yes 0.757 9.4 70.3

PDO 102 -5.657 0.594 0.269 0.027 Yes 0.206 0.231 0.373 No 0.697 6.1 -22.9

rorTOT 102 -3.429 0.273 0.396 0.491 No -0.864 0.394 0.028 Yes 0.527 8.7 57.9

rorFI 102 -3.926 0.219 0.399 0.583 No -1.689 0.610 0.006 Yes 0.247 10.2 81.5

rorPDO 102 -4.619 0.358 0.499 0.473 No -0.486 0.417 0.244 No 0.972 3.4 38.5

doTOT 102 -5.486 0.488 0.363 0.178 No -1.206 0.389 0.002 Yes 0.320 11.1 70.1

doFI 102 -9.490 0.869 0.355 0.014 Yes -1.970 1.029 0.056 Yes 0.010 9.3 86.0

doPDO 102 -4.672 0.327 0.525 0.534 No -0.990 0.407 0.015 Yes 0.718 5.4 62.8

NY Two-lane Paved

TOT 18 -3.595 0.510 0.186 0.006 Yes -0.278 0.273 0.307 No 0.117 28.4 24.3

FI 18 -9.373 1.040 0.134 0.000 Yes 0.092 0.144 0.525 No 0.010 57.9 -9.6

PDO 18 -2.241 0.311 0.281 0.268 No -0.440 0.405 0.277 No 0.214 16.9 35.6

rorTOT 18 -4.255 0.480 0.174 0.006 Yes -0.128 0.271 0.638 No 0.010 28.5 12.0

rorFI 18 -9.553 1.035 0.101 0.000 Yes 0.004 0.135 0.976 No 0.010 48.7 -0.4

rorPDO 18

doTOT 18

doFI 18

doPDO 18

1 At the 0.10 level.

2 Percent difference between treatment and comparison sites.

TOT = total crashes (all severity levels combined).

Fl = fatal–and–injury crashes.

PDO = property–damage-only crashes.

ror = run-off-road crashes.

do = drop-off-related crashes.

Note: Blank cells represent models that did not converge.
55

4.3.4 Analysis of Shifts in the Crash Severity Distribution

An analysis was conducted to assess whether the safety edge treatment affected the proportion of 

severe crashes for specific crash types. This analysis compared fatal and injury crashes as a

proportion of total crashes in the periods before and after resurfacing for each State/roadway type/

shoulder type combination. Results of this analysis are presented in table 28. The fatal and injury 

crash proportions were evaluated for run-off-road crashes, drop-off-related crashes, and all crash

types combined. These comparisons were made by estimating the mean difference in proportions 

and its confidence interval across all sites at a 10 percent significance level.

Table 28. Comparison of proportions of fatal and injury crashes before and after resurfacing.

Crash 

type State

Roadway 

type

Shoulder 

type

Site 

type

Average 

before 

proportion

Average 

after 

proportion

Estimated 

average 

difference

Number 

of sites

Estimated 

mean 

difference

Lower 90% 

confidence 

limit

Upper 90% 

confidence 

limit

Significant 

at the 

0.10 level?

TOT

GA

Multilane Paved T 0.362 0.397 0.035 10 0.088 –0.115 0.208 No

C 0.353 0.370 0.017 6 –0.024 –0.272 0.334 No

Two-lane

Paved T 0.276 0.246 –0.030 15 –0.030 –0.132 0.054 No

C 0.414 0.444 0.031 13 0.042 –0.167 0.296 No

Unpaved T 0.209 0.476 0.267 20 0.238 0.088 0.480 Yes

C 0.384 0.317 –0.067 24 –0.025 –0.151 0.085 No

All T 0.245 0.354 0.109 35 0.099 0.006 0.216 Yes

C 0.395 0.366 –0.030 37 –0.009 –0.122 0.095 No

IN Two-lane

Paved T 0.116 0.154 0.038 8 0.007 –0.172 0.286 No

C 0.222 0.165 –0.058 6 –0.034 –0.242 0.069 No

Unpaved T 0.111 0.088 –0.023 7 –0.166 –0.276 0.218 No

C 0.233 0.271 0.038 14 0.044 –0.042 0.165 No

All T 0.113 0.119 0.005 15 –0.047 –0.188 0.190 No

C 0.230 0.241 0.011 20 0.017 –0.072 0.106 No

NY Two-lane Paved T 0.507 0.334 –0.172 3 –0.181 No Test

C 0.407 0.219 –0.188 3 –0.188 No Test

All Two-lane

Paved T 0.239 0.221 –0.018 26 –0.044 –0.116 0.045 No

C 0.367 0.354 –0.013 22 –0.032 –0.150 0.108 No

Unpaved T 0.168 0.313 0.145 27 0.156 0.022 0.350 Yes

C 0.329 0.300 –0.028 38 0.008 –0.083 0.079 No

All T 0.205 0.265 0.059 53 No

C 0.343 0.320 –0.023 60 No

ROR GA

Multilane Paved T 0.340 0.459 0.119 9 0.168 –0.083 0.357 No

C 0.378 0.154 –0.224 6 –0.250 –0.400 –0.143 Yes

Two-lane

Paved T 0.331 0.234 –0.097 17 –0.148 –0.297 0.035 No

C 0.386 0.361 –0.025 11 –0.035 –0.467 0.321 No

Unpaved T 0.309 0.491 0.182 14 0.250 0.021 0.542 Yes

C 0.339 0.366 0.026 19 0.065 –0.126 0.250 No

All T 0.321 0.355 0.034 31 0.035 –0.125 0.200 No

C 0.357 0.364 0.007 30 0.024 –0.142 0.214 No

See notes at end of table.
56

Table 28. Comparison of proportions of fatal and injury crashes before and after resurfacing—

Continued.

Crash 

type State

Roadway 

type

Shoulder 

type

Site 

type

Average 

before 

proportion

Average 

after 

proportion

Estimated 

average 

difference

Number 

of sites

Estimated 

mean 

difference

Lower 90% 

confidence 

limit

Upper 90%

confidence 

limit

Significant 

at the

0.10 level?

ROR

IN Two-lane

Paved T 0.063 0.139 0.077 5 0.179 –0.171 0.750 No

C 0.486 0.193 –0.293 6 –0.317 –0.708 0.000 No

Unpaved T 0.207 0.096 –0.111 8 –0.333 –0.667 0.313 No

C 0.367 0.413 0.046 11 0.042 –0.294 0.387 No

All T 0.140 0.116 –0.024 13 –0.108 –0.333 0.333 No

C 0.400 0.351 –0.049 17 –0.113 –0.317 0.173 No

NY Two-lane Paved T 0.685 0.519 –0.166 3 –0.156 No Test

C 0.628 0.635 0.007 3 –0.023 No Test

All Two-lane

Paved T 0.267 0.223 –0.044 25 –0.097 –0.229 0.065 No

C 0.435 0.349 –0.086 20 –0.133 –0.367 0.100 No

Unpaved T 0.266 0.325 0.059 22 0.089 –0.110 0.333 No

C 0.350 0.383 0.034 30 0.061 –0.097 0.217 No

All T 0.267 0.271 0.005 47 –0.011 –0.134 0.122 No

C 0.381 0.370 –0.011 50 –0.008 –0.142 0.117 No

DO

GA

Multilane Paved T 0.410 0.526 0.116 9 0.167 –0.083 0.333 No

C 0.401 0.186 –0.216 6 –0.250 –0.458 –0.057 Yes

Two-lane

Paved T 0.416 0.313 –0.103 14 –0.200 –0.455 0.089 No

C 0.399 0.308 –0.091 12 –0.152 –0.500 0.250 No

Unpaved T 0.305 0.562 0.257 17 0.375 0.104 0.563 Yes

C 0.285 0.355 0.070 18 0.151 –0.089 0.333 No

All T 0.364 0.430 0.066 31 0.100 –0.075 0.292 No

C 0.328 0.337 0.009 30 0.000 –0.199 0.250 No

IN Two-lane

Paved T 0.000 0.071 0.071 1 1.000 No Test

C 0.238 0.097 –0.141 2 –0.492 No Test

Unpaved T 0.141 0.063 –0.078 4 –0.438 –1.000 1.000 No

C 0.435 0.289 –0.146 11 –0.338 –0.583 0.125 No

All T 0.075 0.067 –0.008 5 0.000 –1.000 1.000 No

C 0.380 0.236 –0.144 13 –0.375 –0.554 0.000 Yes

NY Two-lane Paved T 0.667 0.000 –0.667 2 –1.000 No Test

C 0.667 0.167 –0.500 2 –0.750 No Test

All Two-lane

Paved T 0.295 0.210 –0.085 17 –0.211 –0.472 0.078 No

C 0.388 0.243 –0.145 16 –0.300 –0.550 0.000 Yes

Unpaved T 0.236 0.352 0.116 21 0.250 0.000 0.508 No

C 0.340 0.331 –0.009 29 –0.003 –0.217 0.183 No

All T 0.267 0.277 0.010 38 0.000 –0.181 0.250 No

C 0.358 0.298 –0.060 45 –0.113 –0.289 0.028 No

TOT = Total crashes.

ROR = Run-off-road crashes.

DO = Drop-off-related crashes.

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.

Note: Blank cells indicate that the test of proportions could not be conducted or that the category had fewer than four sites.
57

These evaluations were performed with the Wilcoxon signed-rank test, a nonparametric test that 

does not require that the differences being considered follow a normal distribution. The Wilcoxon

signed-rank test was programmed in SAS® using the algorithm developed for the FHWA

SafetyAnalyst software.(8)The primary measures of interest presented in table 28 for differences 

in proportion of fatal and injury crashes are as follows:

Average proportion of fatal and injury crashes before resurfacing.

Average proportion of fatal and injury crashes after resurfacing.

Simple average difference in proportions (after-before).

Number of sites included in the analysis.

Estimated median before-after effect.

Lower confidence limit of median before-after effect.

Upper confidence limit of median before-after effect.

Summary of statistical significance.

The estimated average treatment effect is the difference between the proportions for the periods 

before and after resurfacing based only on those sites where the difference is not zero. Since the 

Wilcoxon signed-rank test uses only those sites with an observed non-zero change in the proportion

of fatal and injury crashes, it estimates the median rather than the mean. Consequently, the test 

results are less influenced by extreme changes in proportions. Cases in which the test of proportions

could not be conducted are left blank in the table.

A negative estimated median difference indicates that the proportion of fatal and injury crashes 

decreased. If the number of sites was less than four, no test was conducted. 

The proportion of severe crashes after resurfacing was lower than the proportion of severe crashes 

before resurfacing in 31 out of 58 cases shown in table 28. Thirteen of the 31 positive results were

for sites resurfaced with the safety edge treatment, and 18 were for sites resurfaced without the 

safety edge treatment. Only 4 of the 58 comparisons of severity proportions were statistically 

significant; all 4 of these cases were comparison sites. Overall, it appears that the proportion of

severe crashes was reduced from before to after resurfacing, but only a few of the results were 

statistically significant, and there is no apparent difference in the shift in severity distributions

between resurfacing with and without the safety edge treatment.

59

CHAPTER 5. ESTIMATED COST OF THE SAFETY EDGE TREATMENT

This chapter presents the analysis results for the cost of the safety edge treatment. Section 5.1 

discusses an analysis of costs for both treatment and comparison resurfacing contracts, and 

section 5.2 presents another method for determining the cost of the safety edge.

5.1 COMPARISON OF OVERALL COSTS OF RESURFACING PROJECTS

Since the safety edge treatment adds a wedge of asphalt to each edge of the roadway, it is expected

to add cost to a resurfacing project. Costs of resurfacing both treatment and comparison sites were

obtained from each of the participating States after the resurfacing project was completed and 

project accounts were finalized. The cost items obtained for each project included the engineer’s

estimate, the contract cost or price bid for the project by the winning bidder, and the cost per ton 

of the hot-mix asphalt (HMA) concrete used to resurface the roadway and form the safety edge.

The Georgia data set included 28 resurfacing projects (15 treatment and 13 comparison sites) and 

345 mi of roadway. A summary of the project costs for Georgia is shown in table 29. Costs per mile

of safety edge resurfacing versus non-safety edge resurfacing were found to be $110,000 versus

$140,000. 

Table 29. Summary of Georgia resurfacing project costs (2005).

Cost item

Weighted average cost Nonweighted average cost

Safety edge

sites

Comparison

sites

Safety edge

sites

Comparison

sites

Engineer’s estimate

($ million/mi) $2.650 $1.353 $3.222 $1.272

Contract cost 

($ million/mi) $1.306 $1.353 $1.183 $1.268

HMA resurfacing cost

($/ton) $45.730 $43.050 $49.210 $42.970

HMA resurfacing cost 

($ million/mi) — — $0.110 $0.140

— Not applicable.

The Indiana data set included 16 resurfacing projects (8 treatment and 8 comparison sites) and 

165 mi of roadway. A summary of the project costs for Indiana is shown in table 30. Costs per mile

of safety edge resurfacing versus non-safety edge resurfacing were found to be $140,000 versus

$150,000.

Table 30. Summary of Indiana resurfacing project costs (2005).

Cost item

Weighted average cost Nonweighted average cost

Safety edge

sites

Comparison

sites

Safety edge

sites

Comparison

sites

Engineer’s estimate 

($ million/mi) $1.878 $1.766 $1.748 $1.691

Contract cost 

($ million/mi) $1.505 $1.419 $1.407 $1.388

HMA resurfacing cost 

($/ton) $38.200 $35.510 $38.600 $35.650

HMA resurfacing cost 

($ million/mi) — — $0.140 $0.150

— Not applicable.
60

The New York data set included six resurfacing projects (three treatment and three comparison 

sites) and 25 mi of roadway. A summary of the project costs for New York is shown in table 31.

Costs per mile of safety edge resurfacing versus non-safety edge resurfacing were found to be 

$30,000 versus $40,000. Costs for New York projects are substantially less than Indiana and 

Georgia. The HMA costs were generally higher in Indiana and Georgia than in New York, but it 

is also possible that the New York projects differed in scope from those in Indiana and Georgia.

Table 31. Summary of New York resurfacing project costs (2005).

Cost item

Weighted average cost Nonweighted average cost

Safety edge

sites

Comparison

sites

Safety edge

sites

Comparison

sites

Engineer’s estimate 

($ million/mi) $0.368 $0.881 $0.354 $0.737

Contract cost 

($ million/mi) $0.106 $0.145 $0.108 $0.143

HMA resurfacing cost 

($/ton) $40.290 $49.180 $40.670 $51.710

HMA resurfacing cost 

($ million/mi) — — $0.030 $0.040

— Not applicable.

The cost analyses for resurfacing with the safety edge treatment as compared to resurfacing 

projects on similar roads without the safety edge treatment were reviewed collectively and 

individually. A summary of the resurfacing costs for all three States combined is shown in table 

32. Collectively, the cost of resurfacing with the safety edge treatment was found to be slightly 

less than without the safety edge treatment. Earlier analysis of the yield of coverage 

on safety edge and non-safety edge sites in Georgia found only a very small difference in the 

amount of area covered per ton of asphalt. 

Table 32. Summary of combined Georgia, Indiana, and New York 

resurfacing project costs (2005).

Cost item

Weighted average cost Nonweighted average cost

Safety edge

sites

Comparison

sites

Safety edge

sites

Comparison

sites

Engineer’s estimate 

($ million/mi) $1.632 $1.333 $1.775 $1.233

Contract cost 

($ million/mi) $0.973 $0.973 $0.899 $0.933

HMA resurfacing cost 

($/ton) $41.407 $42.578 $42.830 $43.445

HMA resurfacing cost 

($ million/mi) — — $0.096 $0.110

— Not applicable.

Some advocates of the safety edge treatment maintain that incorporating the treatment in resurfacing

projects has little, if any, added cost because the asphalt used is merely reformed to create the safety

edge treatment. The results summarized in table 32 can be interpreted as consistent with this

hypothesis. However, construction practices vary between contractors and highway agencies, and 

while the amount of asphalt used for the safety edge treatment may be very small, it is unrealistic 

to assume there is no additional cost to implement this treatment. The next section presents an 

alternative approach to estimating the additional cost per mile of the safety edge treatment. 
61

5.2 COST OF SAFETY EDGE TREATMENT BASED ON AMOUNT OF ASPHALT USED

An alternative method to determine the cost of the safety edge treatment is to compute the amount 

of asphalt used to provide the treatment and multiply this quantity by a typical bid cost per ton of 

HMA for that specific project.

Figure 5 shows a typical triangular cross section for the safety edge treatment. The safety edge 

treatment is shown with a cross slope of 30 degrees, which is consistent with current practice.

The cost per mile for the safety edge treatment on both sides of the road based on the cross 

section shown in figure 5 can be estimated using equation 5.

Figure 5. Diagram. Typical cross section for the safety edge treatment on one side of the road.

CCSE = (2000)

(A)(L)(D)(C) (5)

Where:

CCSE = cost for application of the safety edge treatment ($ per mi)

A = area of the safety edge treatment cross section (ft2

) [= 0.5 (h/12) 12

h / tan30° ]

h = height of safety edge treatment (inches)

L = length of safety edge treatment (ft)

D = HMA density (1b/ft3)

C = HMA cost ($/ton)

The height of the safety edge treatment (h) is estimated to range from 1.5 to 3.0 inches, based on 

the assumption that a 1.5-inch overlay will be placed and that the shoulder will be leveled between

0 and 1.5 inches below the elevation of the pavement existing before resurfacing.
62

The length of the safety edge treatment for a 1.0-mi road section would be 2.0 mi or 10,560 ft for 

both sides of the road combined.

The density of the HMA for the safety edge treatment is estimated to be 100 1b/ft3

. This is less 

than the maximum density of compacted asphalt because the safety edge treatment is not 

compacted as an overlay course would be.

The cost of HMA has increased since the 2005 costs shown in table 29 through table 32. HMA 

costs vary substantially between regions of the United States. Based on discussions with several 

highway agencies, a representative current price for HMA is $75 per ton.

Applying equation 5 to the values discussed above, the cost for a safety edge treatment 1.5 inches

high would be $536 per mi. The cost for a safety edge treatment 3.0 inches high would be $2,145 

per mi. Thus, a reasonable range of costs for the safety edge treatment is $536–2,145 per mi.
63

CHAPTER 6. BENEFIT-COST ANALYSIS

This chapter presents the results of a benefit-cost analysis of the safety edge treatment based on 

the results in this report. Section 6.1 presents the overall approach for determining benefit-cost 

estimates, section 6.2 documents the components of the analysis, and section 6.3 discusses the 

results of the benefit-cost analysis.

6.1 BENEFIT-COST ANALYSIS APPROACH

The benefit-cost ratio for the safety edge treatment has been determined according to equation 6:

B/C = 

SE

FI SE FI PDO SE PDO

CC

(N E C + N E C )(P / A,i%,n) (6)

Where:

B/C = benefit-cost ratio

NFI = number of fatal and injury crashes per mile per year before application of the safety 

edge treatment

NPDO = number of PDO crashes per mile per year before application of the safety edge 

treatment

ESE = effectiveness (percent reduction in crashes) for application of the safety edge treatment

CFI = cost savings per crash for fatal and injury crashes reduced

CPDO = cost savings per crash for PDO crashes reduced

(P/A, i, n) = uniform series present worth factor

i = minimum attractive rate of return (discount rate), expressed as a proportion (i.e., i = 0.04,

for a discount rate of 4 percent)

n = service life of safety edge treatment (years)

CCSE = cost for application of the safety edge treatment (dollars per mile)

6.2 COMPONENTS OF THE BENEFIT-COST ANALYSIS

The following sections document the components of the benefit-cost computation, including crash

frequencies, treatment effectiveness, crash costs, service life, minimum attractive rate of return,

uniform series present worth factor, and treatment cost.
64

6.2.1 Crash Frequencies

Crash frequencies per mile per year were estimated for the benefit-cost analysis using the SPFs

presented in section 4.2. Only two-lane highway sites were considered because no treatment

effectiveness measure was found for multilane highway sites. Both Georgia and Indiana SPFs 

were used because each State has an SPF and because using the individual State SPFs constitutes 

a sensitivity analysis of the results. The location of the SPFs used in the benefit-cost analysis are 

shown in table 33.

Table 33. SPFs used in benefit-cost analysis.

State Roadway type Shoulder type Crash type and security level Table

Georgia Two-lane highway Paved All crashes 10

Georgia Two-lane highway Paved F&I crashes 10

Georgia Two-lane highway Paved PDO crashes 10

Indiana Two-lane highway Paved All crashes 11

Indiana Two-lane highway Paved F&I crashes 11

Indiana Two-lane highway Paved PDO crashes 11

Georgia Two-lane highway Unpaved All crashes 10

Georgia Two-lane highway Unpaved F&I crashes 10

Georgia Two-lane highway Unpaved PDO crashes 10

Indiana Two-lane highway Unpaved All crashes 11

Indiana Two-lane highway Unpaved F&I crashes 11

Indiana Two-lane highway Unpaved PDO crashes 11

F&I = Fatal and injury. 

PDO = Property-damage-only.

The computation of crash frequencies was performed as illustrated in the following example of 

Georgia two-lane highways with paved shoulders. This example illustrates the computation of 

crash frequencies per mile per year for highways with a traffic volume of 1,000 vehicles per day.

SPF for total crashes from table 10:

NTOT = exp (-8.921 + 1.108 ln (1,000)) = 0.282 crashes per mi per year

SPF for fatal and injury crashes from table 10:

NFI = exp (-7.818 + 0.853 ln (1,000)) = 0.146 crashes per mi per year

SPF for PDO crashes from table 10:

NPDO = exp (-11.414 + 1.349 ln (1,000)) = 0.123 crashes per mi per year

Since the sum of NFI (0.146) and NPDO (0.123) is less than NTOT (0.282), the values of NFI and 

NPDO are adjusted so that this sum is equal to NTOT, as follows: 

NFI (adjusted) = 0.282 ÷

0.146+0.123

0.146

= 0.153 crashes per mi per year

NPDO (adjusted) = 0.282 ÷

0.146 + 0.123

0.123

= 0.129 crashes per mi per year
65

6.2.2 Treatment Effectiveness

Based on the results of the EB evaluation presented in section 4.3.2, the crash reduction effectiveness

of the safety edge treatment is 5.7 percent. Continuing the computational example for Georgia 

two-lane highways with paved shoulders and a traffic volume of 1,000 vehicles per day, the crash

reduction from the safety edge treatment would be estimated as follows:

For fatal and injury crashes:

0.153 (0.057) = 0.008721 crashes reduced per mi per year

For PDO crashes:

0.129 (0.057) = 0.007353 crashes reduced per mi per year

6.2.3 Crash Costs

The estimated crash costs used in this analysis are based on those currently used in SafetyAnalyst,

as follows:

Fatal crash = $5,800,000.

A injury crash = $402,000.

B injury crash = $80,000.

C injury crash = $42,000.

PDO crash = $4,000.

(8)

The costs are based on the latest published FHWA values.

(10)

The weighted average cost of a fatal

and injury crash (assuming 1 percent fatal crashes, 9 percent A injury crashes, 50 percent B injury

crashes, and 40 percent C injury crashes) is $150,980 per crash. Based on these crash costs, the 

estimated annual crash reduction benefits for the example presented above are as follows:

0.008721 (150,980) + (.007353) (4,000) = $1,346 per mi

6.2.4 Service Life

The service life of the safety edge treatment is estimated to be 7 years, the same as the service 

life of a typical pavement resurfacing project.

6.2.5 Minimum Attractive Rate of Return

The minimum attractive rate of return for this analysis is estimated to be 4 percent. This value is 

currently used in SafetyAnalyst and is representative of the real, long-term cost of capital (i.e., not

including inflation).(8)
66

6.2.6 Uniform Series Present Worth Factor

The uniform series present worth factor is applied to convert the annual crash reduction benefits 

to a present value. This factor is determined as shown in equation 7:

(P/A, i, n) = n

n

i i

i

(1 )

(1 ) 1

+

+

(7)

The uniform series present worth factor for a minimum attractive rate of return of 4 percent and a 

service life of 7 years is determined as follows:

(P/A, 4%, 7) = 7

7

0.04(1 0.04)

(1 0.04) 1

+

+

= 6.002

6.2.7 Treatment Cost

The cost of the safety edge treatment is estimated as falling in the range of $536 to 2,145 per mi 

for both sides of the road combined, as explained in section 5.2.

6.2.8 Benefit Cost Ratio

The value of the benefit-cost ratio is computed using equation 6. For the computational example 

previously presented, the maximum benefit-cost ratio (estimated for the minimum treatment cost 

of $536 per mi) is determined as follows:

B/C = 

536

(1,346)(6.002)

= 15.07

The minimum benefit-cost ratio for the same case (estimated for the maximum treatment cost of 

$2,145 per mi) is determined as follows:

B/C = 

2,145

(1,346)(6.002) = 3.77

The result indicates that the safety edge treatment provides at least $3 in benefits for each dollar 

spent on the treatment and possibly as much as $15 in benefits for each dollar spent on the treatment

depending on the thickness of the safety edge treatment provided. This example addresses sites

with a traffic volume of 1,000 vehicles per day. Larger benefit-cost ratios would be expected for 

sites with higher traffic volumes.

6.3 BENEFIT-COST ANALYSIS RESULTS

The results of the benefit-cost analysis are summarized in table 34 through table 37 for application 

of the safety edge treatment to four types of roadways.
67

Table 34. Benefit-cost analysis for application of safety edge treatment 

on Georgia two-lane roadways with paved shoulders.

AADT (vehicles/day) 1,000 5,000 10,000 15,000 20,000

Crash Frequencies

Total crashes per mile per year 0.282 1.675 3.611 5.659 7.784

F&I crashes per mile per year 0.146 0.575 1.039 1.469 1.877

PDO crashes per mile per year 0.123 1.079 2.748 4.748 6.999

F&I crashes per mile per year (adjusted) 0.153 0.583 0.991 1.337 1.646

PDO crashes per mile per year (adjusted) 0.129 1.093 2.620 4.322 6.138

Safety Benefits—Number of Crashes Reduced

F&I crashes reduced per mile per year 0.009 0.033 0.056 0.076 0.094

PDO crashes reduced per mile per year 0.007 0.062 0.149 0.246 0.350

Safety Benefits—Dollars

F&I crash reduction benefits per year ($) 1,314 5,015 8,528 11,505 14,165

PDO crash reduction benefits per year ($) 29 249 597 986 1,399

Total crash reduction benefits per year ($) 1,344 5,264 9,126 12,491 15,565

Present value of total benefits per year ($) 8,065 31,597 54,773 74,972 93,421

Treatment Cost

Minimum cost of safety edge treatment ($ per mile) 536 536 536 536 536

Maximum cost of safety edge treatment ($ per mile) 2,145 2,145 2,145 2,145 2,145

Benefit-Cost Ratio

Minimum benefit-cost ratio 3.8 14.7 25.5 35.0 43.6

Maximum benefit-cost ratio 15.0 59.0 102.2 139.9 174.3

F&I = Fatal and injury.

PDO = Property-damage-only.

Table 35. Benefit-cost analysis for application of safety edge treatment 

on Indiana two-lane roadways with paved shoulders.

AADT (veh/day) 1,000 5,000 10,000 15,000 20,000

Crash Frequencies

Total crashes per mile per year 0.664 2.175 3.626 4.888 6.043

F&I crashes per mile per year 0.158 0.444 0.694 0.900 1.082

PDO crashes per mile per year 0.542 1.722 2.832 3.789 4.659

F&I crashes per mile per year (adjusted) 0.150 0.446 0.713 0.938 1.139

PDO crashes per mile per year (adjusted) 0.514 1.729 2.912 3.950 4.904

Safety Benefits—Number of Crashes Reduced

F&I crashes reduced per mile per year 0.009 0.025 0.041 0.053 0.065

PDO crashes reduced per mile per year 0.029 0.099 0.166 0.225 0.280

Safety Benefits—Dollars

F&I crash reduction benefits ($) 1,291 3,841 6,138 8,072 9,804

PDO crash reduction benefits ($) 117 394 664 901 1,118

Total crash reduction benefits ($) 1,408 4,235 6,802 8,973 10,922

Present value of total benefits ($) 8,453 25,419 40,824 53,856 65,553

Treatment Cost

Minimum cost of safety edge treatment (per mile) 536 536 536 536 536

Maximum cost of safety edge treatment (per mile) 2,145 2,145 2,145 2,145 2,145

Benefit-Cost Ratio

Minimum benefit-cost ratio 3.9 11.9 19.0 25.1 30.6

Maximum benefit-cost ratio 15.8 47.4 76.2 100.5 122.3

F&I = Fatal and injury.

PDO = Property-damage-only.
68

Table 36. Benefit-cost analysis for application of safety edge treatment 

on Georgia two-lane roadways with unpaved shoulders.

AADT (veh/day) 1,000 5,000 10,000 15,000 20,000

Crash Frequencies

Total crashes per mile per year 0.377 1.822 3.588 5.335 7.068

F&I crashes per mile per year 0.144 0.673 1.307 1.927 2.538

PDO crashes per mile per year 0.226 1.151 2.320 3.496 4.676

F&I crashes per mile per year (adjusted) 0.147 0.672 1.293 1.896 2.487

PDO crashes per mile per year (adjusted) 0.231 1.150 2.296 3.439 4.581

Safety Benefits—Number of Crashes Reduced

F&I crashes reduced per mile per year 0.008 0.038 0.074 0.108 0.142

PDO crashes reduced per mile per year 0.013 0.066 0.131 0.196 0.261

Safety Benefits—Dollars

F&I crash reduction benefits ($) 1,263 5,782 11,126 16,314 21,403

PDO crash reduction benefits ($) 53 262 523 784 1,045

Total crash reduction benefits ($) 1,316 6,044 11,649 17,098 22,447

Present value of total benefits ($) 7,898 36,277 69,920 102,624 134,730

Treatment Cost

Minimum cost of safety edge treatment (per mile) 536 536 536 536 536

Maximum cost of safety edge treatment (per mile) 2,145 2,145 2,145 2,145 2,145

Benefit-Cost Ratio

Minimum benefit-cost ratio 3.7 16.9 32.5 47.8 62.8

Maximum benefit-cost ratio 14.7 67.7 130.4 191.5 251.4

F&I = Fatal and injury.

PDO = Property-damage-only.

Table 37. Benefit-cost analysis for application of safety edge treatment 

on Indiana two-lane roadways with unpaved shoulders.

AADT (veh/day) 1,000 5,000 10,000 15,000 20,000

Crash Frequencies

Total crashes per mile per year 0.409 1.263 2.053 2.728 3.338

F&I crashes per mile per year 0.118 0.235 0.317 0.376 0.426

PDO crashes per mile per year 0.336 1.027 1.662 2.202 2.689

F&I crashes per mile per year (adjusted) 0.106 0.236 0.329 0.398 0.456

PDO crashes per mile per year (adjusted) 0.302 1.028 1.725 2.330 2.882

Safety Benefits—Number of Crashes Reduced

F&I crashes reduced per mile per year 0.006 0.013 0.019 0.023 0.026

PDO crashes reduced per mile per year 0.017 0.059 0.098 0.133 0.164

Safety Benefits—Dollars

F&I crash reduction benefits ($) 916 2,027 2,827 3,428 3,926

PDO crash reduction benefits ($) 69 234 393 531 657

Total crash reduction benefits ($) 985 2,261 3,221 3,959 4,583

Present value of total benefits ($) 5,914 13,572 19,331 23,762 27,507

Treatment Cost

Minimum cost of safety edge treatment (per mile) 536 536 536 536 536

Maximum cost of safety edge treatment (per mile) 2,145 2,145 2,145 2,145 2,145

Benefit-Cost Ratio

Minimum benefit-cost ratio 2.8 6.3 9.0 11.1 12.8

Maximum benefit-cost ratio 11.0 25.3 36.1 44.3 51.3

F&I = Fatal and injury.

PDO = Property-damage-only.
69

For each State and roadway type, benefit-cost analyses were performed for traffic volumes ranging

from 1,000 to 20,000 vehicles per day. The overall results of the benefit-cost analysis are shown 

in figure 6 and figure 7.

Figure 6. Graph. Minimum benefit-cost ratios for the safety edge treatment 

as a function of AADT.

Figure 7. Graph. Maximum benefit-cost ratios for the safety edge treatment 

as a function of AADT.
70

For two-lane highways with paved shoulders, application of the safety edge treatment has minimum

benefit-cost ratios ranging from 3.8 to 43.6 for Georgia conditions and from 3.9 to 30.6 for Indiana

conditions. For two-lane highways with unpaved shoulders, the minimum benefit-cost ratios for 

the safety edge treatment range from 3.7 to 62.8 for Georgia conditions and 2.8 to 12.8 for Indiana

conditions. In all these cases, the maximum benefit-cost ratios are at least four times the minimum

benefit-cost ratios.

These results suggest that the safety edge treatment is highly cost-effective under a broad range 

of conditions. Even though there is uncertainty in the treatment effectiveness estimate, the safety 

edge treatment is likely to be a good safety investment in most situations, especially for roadways

with higher volume levels, where higher crash frequencies are expected.
71

CHAPTER 7. CONCLUSIONS

Conclusions from the analysis of pavement-edge drop-off field measurements and crash data, 

based on 3 years of data for the period after resurfacing and installation of the safety edge treatment,

are as follows:

The EB evaluation for the safety edge treatment with 3 years of crash data for the period 

after resurfacing found that 56 of the 81 comparisons showed a positive safety effect for 

the safety edge treatment. However, only 11 of these comparisons were statistically 

significant, which may be due in part to the small magnitude of the safety edge effect.

The EB evaluation results indicated that the best estimate of effectiveness of the safety

edge treatment for all two-lane highway sites in two States is an approximately 5.7 percent

reduction in total crashes. While this result was not statistically significant, the evaluation 

results obtained for total crashes were nearly always in the positive direction. The results 

of separate evaluations for fatal and injury crashes and PDO crashes were too variable to 

draw conclusions.

Benefit-cost analysis based on the estimated 5.7 percent crash reduction effectiveness found

that the safety edge treatment is so inexpensive that it is highly cost-effective for application 

in a broad range of conditions on two-lane highways. Computed minimum values for

benefit-cost ratios ranged from 4 to 44 for two-lane highways with paved shoulders and 

from 4 to 63 for two-lane highways with unpaved shoulders. The benefit-cost ratios are 

generally higher with increasing traffic volume and where the cost of installing the safety 

edge treatment is lower.

The cost of adding the safety edge treatment to a resurfacing project is minimal. Comparisons

of overall project costs and overall costs of HMA resurfacing material did not show an 

increase for resurfacing projects with the safety edge when compared to normal resurfacing

projects without the safety edge. However, computations based on the volume of asphalt 

required to form the safety edge suggested that the cost of the safety edge treatment is 

approximately $536–2,145 per mi for application to both sides of the roadway.

Resurfacing with or without the safety edge treatment was found to decrease the proportion

of drop-off-heights exceeding 2 inches, at least in the short term. However, there is little

evidence that resurfacing with the safety edge treatment creates more high drop-offs than 

resurfacing without the safety edge treatment. Data for drop-off heights showed that the 

proportion of drop-offs on both treatment and comparison sites increased in the second and

third years after resurfacing. There is no evidence that the safety edge treatment sites have

more high drop-offs than comparison sites that did not have the safety edge treatment.

Evaluation results for the effect of the safety edge treatment on run-off-road crashes and 

drop-off-related crashes on two-lane highways were variable and inconsistent. More sites 

and higher crash frequencies are needed to obtain consistent, statistically significant results. 

Two trends were evident in the EB analysis of run-off-road and drop-off-related crashes. 

First, the safety edge treatment generally appears to have a positive effect on safety for all 
72

site types except for sites with unpaved shoulders in Indiana. This variability in results 

has not yet been fully explained. Second, however, the negative safety edge effects for 

Indiana sites with unpaved shoulders may be explained by low frequencies of drop-offrelated crashes on comparison sites in the period before resurfacing.

There were not enough sites at which the safety edge treatment was applied on rural 

multilane highways to obtain meaningful evaluation results. However, the physical role of

the safety edge treatment is no different on multilane highways than on two-lane highways.

Results of the cross-sectional analysis, while not definitive, suggested that the safety edge 

treatment is effective on multilane highways.

An increase in total crashes for the first 12–30 months after resurfacing has been noted in 

previous studies of the effect of resurfacing on crashes.

(4)

The observed increase in crash

frequency for the period immediately after resurfacing may have resulted from this effect. 

The use of 3 years of crash data after resurfacing resulted in more realistic estimates of the

safety effectiveness of the safety edge than analysis using 1–2 years of data. 

A test of the proportion of fatal and injury crashes after resurfacing indicated that the 

proportion of fatal and injury crashes decreased significantly after resurfacing. However, 

there was no apparent shift in crash severity distributions between sites that were resurfaced

with and without the safety edge treatment.

Resurfacing appears to increase crash frequencies, at least in the short term, and to reduce 

crash severities. Incorporating the safety edge treatment in a resurfacing project appears 

to reduce crash frequencies slightly but to have no effect on crash severities.
73

CHAPTER 8. RECOMMENDATIONS

The following recommendations can be made based on the results of the research presented in 

this report: 

The safety edge treatment is suitable for use by highway agencies under a broad range of 

conditions on two-lane highways. While the evaluation results for total crashes were not 

statistically significant, there is no indication that the effect of the safety edge treatment 

on total crashes is other than positive.

That the overall effectiveness of the safety edge treatment found in this study was not

statistically significant is not surprising given that the magnitude of that safety effect appears

to be small (i.e., approximately 5.7 percent). However, the safety edge treatment is so

inexpensive that its application under most conditions appears to be highly cost-effective. 

The effect of the safety edge treatment would be cost-effective for two-lane highways with

traffic volumes over 1,000 vehicles per day even if its effectiveness were 2 percent rather 

than 5.7 percent.

The cost-effectiveness of the safety edge treatment increases with increasing traffic volumes.

For roads with higher traffic volumes, the safety edge treatment is highly cost-effective.

75

APPENDIX A. IDENTIFICATION OF DROP-OFF-RELATED CRASHES

All crashes obtained from participating agencies were screened, and crashes that were not relevant

to the study were excluded. All remaining crashes were then classified into whether one or more 

of the involved vehicles ran off the road. Then, each run-off-road crash was classified as to whether

it was potentially related to a pavement-edge drop-off. Differences in accident reporting between 

agencies led to individualized classification criteria for each agency. The classification criteria

and data elements used for each agency are described in table 38.

Table 38. Classification criteria for crashes.

Classification Georgia Indiana New York

Excluded 

crashes

Intersection and 

intersection-related

Intersection and 

intersection-related

Intersection and 

intersection-related

and

Non-reportable crashes and 

non-injury crashes (with less 

than $1,000 in property 

damage to any vehicle) since 

these crashes were not 

available for all years

Run-off-road 

crashes

If harmful event included a 

roadside object

or 

If location of impact was off 

the roadway 

If any vehicle collided with a 

roadside object

or

If manner of collision was ranoff-road

or

If primary factor was ran-offroad right or ran-off-road left

If accident type involved a 

roadside object

or

If location of first harmful 

event was off the roadway 

or

If second event for any vehicle 

involved a roadside object

Drop-offrelated 

crashes

If crash road type was 

defective shoulders or 

“holes, deep ruts, bumps”

or

If driver contributing factor 

indicated driver lost control

If primary factor was 

overcorrecting/over-steering

If contributing factor for any 

involved vehicle was defective 

shoulder

77

APPENDIX B. PAVEMENT-EDGE DROP-OFF DATA COLLECTION METHODOLOGY

This appendix presents the methodology used to collect field measurements for pavement-edge 

drop-offs. 

SELECTION OF DATA COLLECTION LOCATIONS

Several data collection locations were selected within each resurfacing project site to obtain field 

measurements of pavement-edge drop-offs. Data collection locations were generally 2–4 mi apart.

There were typically three to four data collection locations within each site, depending on the 

overall site length. 

Each data collection location was predefined as being a specified distance, in whole miles, from the

start of the site. Then, to remove bias from the data collection process, a random offset was added

to the predefined distance. This random offset, selected separately for each data location, was 

0.1–0.9 mi, in increments of 0.1 mi. The location defined by the predefined distance plus the offset

was used as the starting point for data collection. Field data collection personnel were given

discretion to move the starting point if the measurement location was clearly not representative 

of the roadway as a whole or if sight distance was too limited for measurements to be made safely.

Data were not collected at a selected location if recent maintenance had occurred or if weather 

did not permit data to be collected safely or accurately. 

FIELD MEASUREMENTS

Roadway characteristics were recorded at the selected starting point, and pavement-edge 

drop-off height was measured every 52 ft on both sides of the roadway over a 0.1-mi interval 

beginning at the starting point. A field data collection form is illustrated in figure 8. The data 

collection intervals are illustrated in figure 9. The measurements illustrated in the figure were

repeated at intervals of 2–4 mi along the roadway, as previously described. 

The roadway characteristics recorded at each data collection starting point included the following:

Speed limit.

Pavement type.

Shoulder type.

Shoulder grade.

Shoulder width.

Lane cross-slope.

Lane width.

Pavement edge drop-off shape.

Grade.
78

Figure 8. Illustration. Sample data collection form.
79

Figure 9. Illustration. Data collection intervals.

SHOULDER TYPE AND WIDTH

Shoulder types were generally recorded as paved, gravel, or earth. When a mixture of shoulder 

types was found (i.e., a composite shoulder), the width of paved shoulder beyond the edge of the 

traveled way was recorded and the presence of the other shoulder type was noted. 

DROP-OFF SHAPE

Drop-off shapes are shown in the data collection form in figure 8. Shapes A, B, and C are defined

in other literature.

(2) Most shapes correspond to A, B, or C. Shape A typically corresponds to the

edge of concrete pavement. The likely cause of such drop-offs is settling of the concrete pavement.

Shape A may also occur when asphalt pavement breaks. Shape B is the most common shape for 

drop-offs at the edge of an asphalt pavement. It is the shape that occurs from a typical overlay. 

Shape C corresponds to the safety wedge. It is recorded when the edge shape is angled at

approximately 45 degrees and appears to be intentionally shaped at that angle. Other drop-off 

shapes were also recorded when present.

LANE WIDTH AND PAVEMENT WIDTH

Both pavement widths (i.e., traveled way width) and lane widths were measured. Lane widths

were measured from the edge of the lane to the painted centerline of the roadway. Where no 

centerline was present, the lane width was calculated as half of the total pavement width. Where 

pavement extended less than 4 inches beyond the pavement edge line, it was included in the lane 

width. Where pavement extended more than 4 inches beyond the pavement edge line, it was 

treated as a paved shoulder.

DROP-OFF HEIGHT

Drop-off height was measured to the nearest 0.125 inch since most measuring tools measure in 

0.125-inch increments. Additionally, measurement tools marked with 0.125-inch increments are 

easier to read consistently than those marked with 0.1-inch increments. It is assumed that a tire 
80

could catch on just a few inches of drop-off, even if shoulder material is at grade beyond that 

distance. Therefore, drop-off height was measured approximately 4 inches from the edge of the 

pavement for shape A and 4 inches from the base of the pavement for shapes B and C (see figure 10).

Figure 10. Illustration. Measurement of drop-off perpendicular to pavement surface.

Drop-off height was measured by placing a level across the top of the pavement surface so that it 

hung over the shoulder. A ruler was then used to measure the vertical distance between the shoulder

and the level at the appropriate location. Drop-off height was measured from the ground to the 

base of the level, as shown in figure 11.

Figure 11. Photo. Measurement of pavement-edge drop-off height.

Pavement-edge drop-off height was not measured at driveways or minor intersections if they

coincided with a planned data collection point. If a driveway or intersection was located at a data 

collection point along a segment, data collectors recorded that information and moved to the next 

data collection point.
81

APPENDIX C. SCATTER PLOTS OF ACCIDENTS AND AADT

Figure 12 through figure 16 are scatter plots of crashes (per mile per year) and traffic volume 

(log basis) that were used to determine the appropriateness of modeling assumptions. In general, 

these plots show a positive relationship between crashes and traffic volume (i.e., crashes increase 

with increasing volume). Also, distributions for comparison, treatment, and reference sites 

appropriately overlap each other and do not contain extreme outliers.
82

Total Crashes/Mile/Year

0

1

2

3

4

5

6

7

8

9

10

11

12

LnAADT

8.4 8.5 8.6 8.7 8.8 8.9 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 10.0 10.1

Site Type Comparison Reference Treatment

Figure 12. Graph. Georgia multilane roadway with paved shoulder.
83

Total Crashes/Mile/Year

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

LnAADT

6 7 8 9 10

Site Type Comparison Reference Treatment

Figure 13. Graph. Georgia two-lane roadway with paved shoulder.
84

Total Crashes/Mile/Year

0

1

2

3

4

5

6

7

8

LnAADT

6 7 8 9 10

Site Type Comparison Reference Treatment

Figure 14. Graph. Georgia two-lane roadway with unpaved shoulder.
85

Total Crashes/Mile/Year

0

1

2

3

4

5

6

LnAADT

7 8 9 10

Site Type Comparison Reference Treatment

Figure 15. Graph. Indiana two-lane roadway with paved shoulder.
86

Total Crashes/Mile/Year

0

1

2

3

4

LnAADT

5 6 7 8 9 10

Site Type Comparison Reference Treatment

Figure 16. Graph. Indiana two-lane roadway with unpaved shoulder.
87

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Accessed online: March 1, 2011. (http://ostpxweb.dot.gov/policy/reports/080205.htm)


HRDS-20/04-11(300)E Recycled

Recyclable