Safety Effects of the SafetyEdgeSM

Technical Summary of Crash Modification Factors

FHWA Safety Program

http://safety.fhwa.dot.gov

1

INTRODUCTION

The objective of this study was to evaluate the safety effectiveness and to develop 

benefit-cost (B/C) ratios for the application of the SafetyEdgeSM treatment using 

scientifically rigorous crash-based analysis methods. More than half of all fatal crashes 

that occur annually in the United States involve a roadway departure, defined by 

FHWA as a crash which occurs after a vehicle crosses an edge line, a center line, or 

otherwise leaves the traveled way. Pavement edge drop-offs are known to contribute 

to some of the most severe roadway departure crashes. The typical scenario begins 

with one or more of the vehicle tires leaving the paved roadway. If the driver attempts 

to return to the paved surface without slowing, and a significant vertical edge dropoff is present, the sidewall of one or more tires will scrub against this edge and the 

driver may oversteer in an effort to return to the paved roadway. This may result in a 

driver losing control of the vehicle, contributing to a head-on collision in the opposing 

travel lane, a rollover, or a run-off-road (ROR) event on either side of the road. To 

mitigate roadway departure crashes, it is important to reduce the number of vehicles 

that encroach onto the roadside, and to minimize the consequences of a roadway 

departure event. One strategy to achieve the latter objective is to eliminate vertical 

pavement edges that may become drop-offs over the life of the pavement. 

The SafetyEdgeSM is constructed with a low-cost paver attachment that enables 

the pavement edge to be paved and compacted to a finished 30-degree angle 

to eliminate vertical edges and promote a smooth return to the travel lane after 

one or more wheels leave the pavement. Figure 1 illustrates typical cross-sections 

immediately after completing an asphalt pavement resurfacing project with and 

without the SafetyEdgeSM. The illustration on the left in Figure 1 shows the SafetyEdgeSM

with compacted backfill material that is graded flush with the paved road surface, 

while the illustration on the right in Figure 1 shows the backfill material graded flush 

with the paved roadway surface, but without the SafetyEdgeSM treatment beneath the 

graded backfill material. Over time, the material adjacent to the edge of pavement 

settles or erodes, exposing the edge, which is shown in Figure 2. The left panel in 

Figure 2 illustrates how the angled SafetyEdgeSM can easily be traversed by vehicles 

attempting to re-enter the roadway, while the panel on the right side of Figure 2 shows 

how the vertical or near-vertical pavement edge drop-off with traditional paving 

techniques can cause tire-scrubbing, which may lead to loss of control. 
2

Figure 1. Graphic. SafetyEdgeSM versus conventional paved edge immediately 

after repaving.

(Left panel: SafetyEdgeSM immediately after paving with backfill material graded flush with paved 

surface. Right panel: Conventional pavement overlay without the SafetyEdgeSM with backfill material 

graded flush with the paved surface)

Figure 2. Graphic. SafetyEdgeSM versus conventional paved edge after backfill 

material settles or erodes.

(Left panel: SafetyEdgeSM is exposed to traffic after backfill material settles or erodes. Right panel: 

Conventional pavement overlay without the SafetyEdgeSM after backfill material settles or erodes)

This study builds on past SafetyEdgeSM research by using a multi-State database and 

state-of-the-art analysis methodology. The large sample of repaving projects with 

and without the SafetyEdgeSM afforded the opportunity to develop crash modification 

factors (CMFs) for several crash types and to disaggregate the CMFs based on traffic 

volume and other roadway features. The CMFs were used to develop benefit-cost 

ratios for the SafetyEdgeSM treatment on two-lane rural highways.
3

METHODOLOGY

This research team compared the safety performance of two-lane rural highways 

with and without the SafetyEdgeSM treatment using an EB observational before-after 

study design (Hauer, 1997, Gross et al., 2010). The EB evaluation method accounts 

for regression-to-the-mean (if it exists), and for differences in traffic volume and for 

crash trends between the periods before and after the treatment was applied. The 

approach is comprised of the following three steps:

» Step 1: Predict what the safety performance of rural two-lane highways would 

have been in the after period had the SafetyEdgeSM not been implemented.

» Step 2: Estimate what the actual safety performance was in the after period with 

the SafetyEdgeSM. 

» Step 3: Compare the results of Step 1 and Step 2.

The study developed aggregate CMFs for several crash types, including one for dropoff-related crashes, the target crash type addressed by the SafetyEdgeSM. In addition, 

CMFs were developed for single-vehicle ROR and opposite direction crashes, two 

common crash outcomes when a vehicle encounters a vertical edge and the driver 

loses control. Finally, CMFs were developed for fatal and injury (FI) crashes and total 

crashes. Intersection-related and animal crashes were not included in the sample 

of any crash type used in the safety evaluation. The evaluation also considered 

average annual daily traffic, paved roadway width, presence of a horizontal curve, 

and the posted speed limit when investigating the disaggregate SafetyEdgeSM

treatment effects.

The economic evaluation of the SafetyEdgeSM treatment integrated the results of the 

safety evaluation. The expected annual benefit of the SafetyEdgeSM was estimated 

separately for ROR and FI crashes using the CMFs developed as part of this study. 

Additionally, this effort included manual field data collection for a sample of twolane highways repaved with and without the SafetyEdgeSM in Iowa, North Carolina, 

and Ohio in order to provide information on the characteristics of the treatment and 

reference sites. The data included measurements of the pavement edge shape and 

depth. 

3
4

SAFETY EVALUATION DATA SUMMARY

Sites that were re-paved with or without the SafetyEdgeSM in 2012 or earlier were 

included in the treatment and reference group samples. The analysis period 

ranged from 2005 through 2015, depending on the treatment installation dates and 

availability of data from the participating transportation agencies. At least three 

years of before and after period data were sought for each treatment and reference 

group site. Only re-paving projects that were completed during a single calendar 

year were included in the analysis, and the year of construction was excluded 

from the safety evaluation. Data from the States of Iowa, North Carolina, Ohio, and 

Pennsylvania, along with data from Marion County, Florida, were included in the 

evaluation.

Tables 1 through 5 provide a summary of the number of treatment and reference 

group sites, mile-years of data, average daily traffic volumes, and total crashes for 

the before and after periods. Detailed information regarding the roadway inventory 

and other crash types considered in the evaluation can be found in the final report.

Total, FI, ROR, and opposite direction crashes could be identified in all of the crash 

data files supplied by the five agencies included in the evaluation, although in some 

cases ROR data were limited to ROR to the right crashes. Drop-off-related crashes 

could be identified only in the Iowa, North Carolina, Ohio, and Pennsylvania data 

files. In Pennsylvania, the sample of drop-off-related crashes was too small to develop 

a CMF for this crash type. In Iowa, drop-off-related crashes represented 2.5 to 13.5 

percent of the total crashes during the before or after periods at the treatment or 

reference group sites. In North Carolina and Ohio, drop-off-related crashes ranged 

from 7.2 to 13.9 percent and 3.5 to 6.6 percent, respectively, of the total crashes.
5

Table 1. Safety evaluation data summary – Iowa.

Site Type Number 

of Sites Miles Before period mile years

After period 

mile years

Before period 

mean AADT

After period 

mean AADT

Before period 

total crashes

After period 

total crashes

Treatment 279 316.1 1,008 1,205 933 1,756 393 308

Reference 108 74.2 211 284 1,321 2,002 96 102

Table 2. Safety evaluation data summary – Marion County, Florida.

Site Type Number 

of Sites Miles Before period mile years

After period 

mile years

Before period 

mean AADT

After period 

mean AADT

Before period 

total crashes

After period 

total crashes

Treatment 40 36.4 165 127 6,250 5,675 130 162

Reference 51 21.3 99 71 3,616 3,546 101 104

Table 3. Safety evaluation data summary – North Carolina.

Site Type Number 

of Sites Miles Before period mile years

After period 

mile years

Before period 

mean AADT

After period 

mean AADT

Before period 

total crashes

After period 

total crashes

Treatment 727 369.1 2,375 948 3,285 3,065 2,496 1,139

Reference 573 196.8 1,263 502 2,059 2,275 847 397

Table 4. Safety evaluation data summary – Ohio.

Site Type Number 

of Sites Miles Before period mile years

After period 

mile years

Before period 

mean AADT

After period 

mean AADT

Before period 

total crashes

After period 

total crashes

Treatment 447 576.0 3,847 606 3,535 2,998 3,899 718

Reference 163 203.0 1,296 286 3,185 2,442 1,168 272

Table 5. Safety evaluation data summary – Pennsylvania.

Site Type Number 

of Sites Miles Before period mile years

After period 

mile years

Before period 

mean AADT

After period 

mean AADT

Before period 

total crashes

After period 

total crashes

Treatment 170 24.0 72 72 1,493 1,560 53 38

Reference 1,411 159.0 318 637 1,382 1,370 277 503
6

SAFETY EVALUATION RESULTS

The safety evaluation consisted of approximately 1,321 mi of rural two-lane highways 

repaved with the SafetyEdgeSM. The reference group included nearly 654 mi of 

two-lane rural highways repaved without the SafetyEdgeSM. The reference group 

sites were used to estimate safety performance functions (SPFs) for use in the EB 

methodology for total, FI, ROR, opposite direction, and drop-off-related crashes 

when the sample of crashes was adequate. SPFs were estimated separately using 

data from each transportation agency, producing agency-specific CMFs. CMFs are 

used to estimate the expected number of crashes after a safety countermeasure 

has been implemented—values exceeding 1.0 indicate that crashes are expected 

to increase after implementing the countermeasure, while values less than 1.0 

indicate a reduction in the expected number of crashes. The agency-specific CMFs 

developed in the safety evaluation were aggregated into an overall CMF for each 

crash type. 

CMF for Drop-off Related Crashes

Since drop-off related crashes are the specific subset of all crashes that the 

SafetyEdgeSM treatment is intended to mitigate, it is no surprise that the greatest 

potential crash reduction is for this crash type. Figure 3 shows the CMF point 

estimates for drop-off-related crashes and the associated 95th percentile 

confidence intervals. Data from the Iowa, North Carolina, and Ohio departments 

of transportation were used to develop the drop-off-related CMF. When combining 

these data across all States, the resulting “All” CMF for drop-off-related crashes 

is 0.655, which indicates that the SafetyEdgeSM is associated with a 34.5 percent 

reduction in drop-off-related crashes on two-lane rural highways. Only the combined 

“All” CMF and the North Carolina CMF were statistically significant at the 95thpercentile confidence level. The Ohio drop-off-related CMF for SafetyEdgeSM projects 

was statistically significant at the 90-percent confidence level. 
7

Figure 3. CMFs for drop‐off‐related crashes.

(*indicates CMF is statistically significant at 95‐percent confidence level)

0.681 0.628*

0.726

0.655*

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Iowa North Carolina Ohio All

Crash Modification Factor

Figure 3. Chart. CMFs for drop-off related crashes.

(*indicates CMF is statistically significant at 95-percent confidence level)

CMFs for ROR and Opposite Direction Crashes

ROR (including rollover) and head-on plus opposite direction sideswipes (referred 

to as opposite direction) crashes are two common outcomes when a driver loses 

control of the vehicle after encountering a vertical pavement edge. Figures 4 and 

5 show the ROR and opposite direction CMFs, respectively, developed from data 

from all five transportation agencies included in the safety evaluation, as well as an 

aggregate CMF by combining the results from all five transportation agencies.

The aggregated CMF for ROR crashes was 0.790. This result was statistically 

significant, with the 95-percent confidence interval ranging from 0.708 to 0.872. The 

individual ROR CMFs for Ohio and Pennsylvania were also statistically significant; 

however, the combined CMF is the most robust, representing a crash reduction of 

21 percent after application of the SafetyEdgeSM treatment to the sample of two-lane 

rural highways included in the present study.
8

For the opposite direction crash type, the aggregate CMF developed using data 

from all five transportation agencies included in the safety evaluation was 0.813, 

which was statistically significant at the 95th percentile confidence level. This 

indicates that, after implementing the SafetyEdgeSM treatment on the sample of 

two-lane rural highways included in the present safety evaluation, opposite direction 

crashes were expected to be 18.7 percent lower than along similar two-lane rural 

highways that were not repaved with the SafetyEdgeSM. The opposite direction CMFs 

for Marion County, Florida, North Carolina and Pennsylvania were also statistically 

significant based on the sample of data used in the safety evaluation. 

Figure 4. CMFs for run‐off‐road crashes.

(*indicates CMF is statistically significant at 95‐percent confidence level)

0.724

1.016

0.969

0.764*

0.115*

0.79*

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Iowa Marion

County,

Florida

North

Carolina

Ohio Pennsylvania All

Crash Modification Factor

Figure 4. Chart. CMFs for ROR crashes.

(*indicates CMF is statistically significant at 95-percent confidence level)
9

Figure 5. CMFs for opposite direction crashes.

(*indicates CMF is statistically significant at 95‐percent confidence level)

0.680

0.458*

0.751*

1.172

0.355*

0.813*

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Iowa Marion

County,

Florida

North

Carolina

Ohio Pennsylvania All

Crash Modification Factor

Figure 5. Chart. CMFs for opposite direction crashes.

(*indicates CMF is statistically significant at 95-percent confidence level)

CMF for Fatal and Injury Crashes

The FI CMF, shown for all five transportation agencies in Figure 6, was also statistically 

significant when aggregating the data in the study sample. The CMF was 0.892, 

which indicates that the SafetyEdgeSM is associated with a 10.8-percent reduction in 

FI crashes along two-lane rural highway segments included in the evaluation. The 

95th percentile confidence interval produces a CMF that ranges from 0.825 to 0.959. 

The FI CMFs developed for Marion County, Florida, and for Pennsylvania, were also 

statistically significant at the 95th percentile confidence level. 
100

Figure 6.  CMFs for fatal and injury crashes.

(*indicates CMF is statistically significant at 95‐percent confidence level)

0.842

0.602*

0.986 0.972

0.343*

0.892*

0

0.2

0.4

0.6

0.8

1

1.2

Iowa Marion

County,

Florida

North

Carolina

Ohio Pennsylvania All

Crash Modification Factor

Figure 6. Chart. CMFs for FI crashes.

(*indicates CMF is statistically significant at 95-percent confidence level)

CMF for Total Crashes

When aggregating the data across all five transportation agencies, the CMF for total 

crashes was 0.989, which was not statistically significant. This result was expected 

because the SafetyEdgeSM treatment is most likely associated with only a portion 

of roadway departure crashes on two-lane rural highways, which represents a low 

proportion of total crashes.
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CMFunction for ROR Crashes 

In addition to the analysis results presented above, the CMFs were disaggregated 

by the expected number of ROR crashes per mile-year before the SafetyEdgeSM was 

applied. A CMFunction was developed using a single independent variable, the 

expected crash frequency per mile-year before treatment, which logically captures 

the effects of the other variables investigated in the univariate categorical analysis. 

The CMFunction, which was estimated using linear regression, is:

CMFROR = 0.975 – (0.432 × Expected ROR crash frequency per mile-year)

Figure 7. Equation. CMFunction for ROR crashes.
122

ECONOMIC ANALYSIS

This economic analysis derived the safety benefits based on the CMFs from the ROR 

and FI crash types. The estimated CMFs were 0.790 for ROR and 0.892 for FI crashes 

(see Figures 4 and 6, respectively). The most recent FHWA mean comprehensive 

crash costs (developed in 2001) were used as a base (Council et al., 2005) 

and updated to 2016 dollars considering changes to the U.S. Department of 

Transportation (USDOT) value of a statistical life (VSL) from 2001 to 2016. The resulting 

aggregate 2016 unit costs for a ROR crash and an FI crash were calculated as 

$313,667 and $400,188, respectively. Using the annual crash reductions based on 

the aggregate CMFs from the present study, the analysis estimated annual benefits 

of $16,612,854 for ROR crashes and $20,538,582 for FI crashes.

The material considered in the construction cost was hot-mix asphalt with an 

average cost of $75.00 per ton for a hot-mix asphalt surface or wearing course from 

the five transportation agencies included in the evaluation. For an assumed 10-year 

service life, a discount rate of 7 percent based on the Office of Management and 

Budget Circular A-4 (OMB, 2016), and information on typical SafetyEdgeSM crosssections, the estimated annualized SafetyEdgeSM treatment cost ranged from $10.65 

per mile-year to $21.30 per mile-year for two-inch and four-inch hot-mix asphalt 

depths, respectively. For the approximately 1,320 mi of two-lane rural highways 

repaved with the SafetyEdgeSM included in the sample for the present study, the total 

annual costs were $14,075 and $28,151, respectively, for the two-inch and four-inch 

paving depths.

The resulting B/C ratios for ROR crashes, calculated as the ratio of the annual benefit 

to the annual cost, were 1,180 and 590 for the two-inch and four-inch depths, 

respectively. The analysis for FI crashes yielded B/C ratios of 1,460 and 730 for the 

two-inch and four-inch depths, respectively. These B/C values demonstrate that the 

SafetyEdgeSM is an effective countermeasure to reduce both ROR and FI crashes 

on two-lane rural highways. Additionally, applying the USDOT sensitivity analysis 

recommendations of 0.56 and 1.40 times the value of a statistical life (USDOT, 2016) 

directly to these B/C ratios yields values that, even with conservative assumptions, 

the SafetyEdgeSM can be applied cost effectively.
13

FIELD MEASUREMENT OF SAFETYEDGESM TREATMENT AND REFERENCE 

GROUP SITES

Pavement edge measurements were collected along approximately 50 mi of 

SafetyEdgeSM treatment sites in Iowa, 60 mi in North Carolina, and 55 mi in Ohio. 

Among these sites, 375 measurements of the SafetyEdgeSM shape were recorded. 

Pavement edge measurements were collected along approximately 25 mi of 

reference group sites in each of the three States. There were 210 reference group site 

measurements in the sample. Examples of the field measurements at treatment and 

reference group sites are shown in Figures 8 and 9, respectively. 

At the reference group and treatment sites in Ohio, the backfill material was nearly 

flush with the paved roadway surface and appeared be very stable under both 

weather and traffic conditions, so traffic along the majority of these sections had not 

yet been exposed to pavement edge drop-offs. The edges were dug out periodically 

at both treatment and reference sites to verify that the edge type was correctly 

identified in the data set. At the treatment sites (where the SafetyEdgeSM was used), 

the angle ranged from 9.8 to 39.8 degrees, with an average angle of 31.8 degrees 

and a standard deviation of 5.8 degrees. 

At the measured sites in North Carolina, the backfill material (often turf) covered 

much of the pavement edges. At several of the reference group sites in North 

Carolina, the edge of the pavement was raveling. Raveling was observed at only a 

few of the SafetyEdgeSM sites, providing preliminary evidence that sites treated with 

the SafetyEdgeSM may be more durable at the pavement edge than sites that are not 

paved with a SafetyEdgeSM. At the treatment sites in North Carolina, the SafetyEdgeSM

had an angle that ranged from 9.1 to 63.4 degrees, with an average angle of 29.7 

degrees and a standard deviation of 11.3 degrees. 

At the Iowa SafetyEdgeSM treatment and reference group sites, the backfill material 

at the pavement edge was no longer flush with the roadway surface at the time of 

the field review. At the Iowa treatment sites, the pavement edge angle ranged from 

21.8 to 42.3 degrees, with an average angle of 30.2 degrees. At the treatment sites in 

Iowa, the vertical dimension of the SafetyEdgeSM ranged from 1.1 to 3 inches. 
144

Figure 8. Photo. Example of Ohio SafetyEdgeSM Treatment Site (Coshocton County, 

State Route 83, Curve to the Right).

Figure 9. Photo. Example of Iowa Reference Group Site (Clinton County, State 

Route 136, Tangent Section).
15

SUMMARY

This study estimated CMFs for the SafetyEdgeSM treatment on two-lane rural highways 

using data from five transportation agencies. An EB observational before-after 

evaluation found statistically significant CMFs aggregated across the three States 

for which drop-off-related crash data were available. The drop-off-related CMF for 

the three States combined was 0.655. The analysis also found statistically significant 

CMFs aggregated across all five States for ROR crashes, opposite direction crashes, 

and FI crashes. The resulting CMFs for five States combined for ROR and opposite 

direction crashes were 0.790 and 0.813, respectively. These CMFs were consistent 

with past studies, indicating that the SafetyEdgeSM addresses these specific crash 

types. The resulting FI CMF of 0.892 is also reasonable based on past research, which 

indicates that drop-off-related crashes are often quite severe, although they are still 

only a relatively small proportion of crashes. The total crash CMF was not statistically 

significant at the 95-percent confidence level, which was somewhat expected 

because the effect of the SafetyEdgeSM is not likely to be seen amongst the large 

number of “property damage only” crashes in the total crash data sample. 

An economic evaluation found that the SafetyEdgeSM treatment can be applied 

cost-effectively. Even with conservative estimates of the pavement service life, VSL, 

and the depth of the pavement surface (i.e., construction costs), the B/C ratios 

ranged from 590 to 1,180 for ROR crashes, and from 730 to 1,460 for FI crashes. This 

large B/C ratio is the result of the nominal added cost to repave a roadway with the 

SafetyEdgeSM.

The pavement edge field data collection effort found the angle of the SafetyEdgeSM

averaged 32 degrees in Ohio, 30 degrees in North Carolina, and 30 degrees in Iowa, 

which is consistent with the FHWA design and construction guidance. 

In summary, this research indicates that the SafetyEdgeSM is a highly-effective safety 

countermeasure based on the aggregate CMFs developed in this multi-State study. 

The B/C ratios derived for these crash types underscore the cost-effectiveness of the 

SafetyEdgeSM on two-lane rural highways. 
16

REFERENCES

Council, F., E. Zolashnja, T. Miller, and B. Persaud. Crash Cost Estimates by Maximum 

Police-Reported Injury Severity Within Selected Crash Geometrics, FHWA-HRT05-051. 

2005.

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

of the SafetyEdgeSM Treatment. Report No. FHWA-HRT-11-024, Federal Highway 

Administration, McLean, VA, March 2011.

Gross F., B. Persaud and C. Lyon. A Guide to Developing Quality Crash Modification 

Factors. U.S. Department of Transportation, Federal Highway Administration Report. 

FHWA-SA-10-032, 2010.

Hallmark, S. L., A. Goswamy, and M. D. Pawlovich. Safety Edge Crash Modification 

Factors. Iowa State University Digital Repository, Tech Transfer Summaries. Iowa State 

University, Institute for Transportation, Ames, IA, August 2016.

Hauer, E. Observational Before-After Studies in Road Safety – Estimating the Effect of 

Highway and Traffic Engineering Measures on Road Safety. Elsevier Science, Inc., 

1997.

Office of Management and Budget. Circular A-4. Found at: https://www.whitehouse.

gov/omb/circulars_a004_a-4/. Accessed August 24, 2016.

Powell, R. B. NCAT Pavement Test Track: SafetyEdgeSM Experiences. Presentation at 

FHWA SafetyEdgeSM Webinar, November 10, 2016.

U.S. Department of Transportation. Guidance on Treatment of the Value of a 

Statistical Life (VSL) in U.S. Department of Transportation Analyses—2016 Adjustment. 

Memo, August 8, 2016. https://www.transportation.gov/sites/dot.gov/files/docs/

VSL%20Guidance%202016.pdf. Accessed December 19, 2016.

This document is a technical summary of the Federal Highway Administration 

report, Safety Effects of the SafetyEdgeSM, FHWA-HRT-17-081.

For More Information:

Visit https://safety.fhwa.dot.gov/roadway_dept/countermeasures/safe_recovery/ 

FHWA

Cathy Satterfield

Office of Safety

cathy.satterfield@dot.gov

Carol Tan

Office of Safety Research and Development

carol.tan@fhwa.dot.gov

Andy Mergenmeier

Resource Center Pavement and Materials Team

andy.mergenmeier@dot.gov

FHWA-SA-17-044