The Highway Safety Information System 

(HSIS) is a multi-State safety database 

that contains crash, roadway inventory, 

and traffic volume data for a select group 

of States. The current participating States—

California, Illinois, Maine, Minnesota, 

North Carolina, Ohio, and Washington—

were selected based on the quality of their 

data, the range of data available, and their 

ability to merge the data from the various 

files. The HSIS is used by FHWA staff, 

contractors, university researchers, and others 

to study current highway safety issues, direct 

research efforts, and evaluate the effectiveness 

of accident countermeasures.

Safety Evaluation of the 

Safety Edge Treatment

Summary Report

Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike . McLean, VA 22101-2296

When a vehicle leaves the traveled way and encounters a pavement-shoulder 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 more steering input. When the tire 

finally remounts the pavement, the larger steering angle may cause the vehicle to “slingshot” 

across the road. This can result in a head-on collision with other traffic or a loss of control 

and overturning of the vehicle on the roadway or roadside.

The safety edge is an innovative treatment intended to minimize drop-off-related crashes. With 

this treatment, the pavement edge is sloped at a 30-degree angle (see figure 1). This angle makes

 it easier for a driver to safely reenter the roadway after inadvertently driving onto the shoulder. 

Research conducted by 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.(1) Because drivers in the study were instructed to drive off the 

pavement edge, the TTI research has been criticized as not being representative of 

naïve drivers. Prior to this research, neither an actual field evaluation of the safety 

edge nor a formal effectiveness evaluation had been completed.

Evaluation Sites

Figure 1. Safety edge detail.

This evaluation was conducted as part of an eight-State Federal Highway 

Administration (FHWA) pooled-fund study. Three States—Georgia, Indiana, 

and New York—implemented the safety edge treatment and participated in a 

multiyear evaluation of the treatment. The evaluation considered the following 

types of study sites: 

• Treatment sites: sites that were resurfaced and treated with the safety edge.

• Comparison sites: sites that were resurfaced but not treated with the 

safety edge.

• Reference sites: sites that were similar to the treatment and comparison 

sites but were not resurfaced.
2

Treatment sites were selected by the participating States from among the sites considered for their 

normal resurfacing program for 2005. Comparison sites were selected from among projects that 

were resurfaced in 2005 but did not receive the safety edge treatment. 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 in the same geographical area 

of the State. Based on a preliminary review of the available resurfacing projects, it was decided 

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).

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

the study 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 were 

used to develop safety performance functions (SPFs) for an observational before-after 

empirical Bayes (EB) evaluation. The reference sites included the same roadway types as 

the treatment and comparison sites. Reference sites were chosen from the same highway 

districts as the treatment sites in the same geographical area of the State. 

Each resurfacing project was divided into smaller roadway segments so 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. The New York sites and the multilane sites in 

Georgia and Indiana are discussed in the project’s final report but are not discussed 

in this summary.(2) Table 1 provides descriptive statistics of the two-lane sites in 

Georgia and Indiana. Figure 2 and figure 3 show examples of typical study sites.

Table 1. Summary of traffic volume and crash data for two-lane roadways 

with paved and unpaved shoulders in Georgia and Indiana.

State Site type Number of sites Site length (mi)

Mean AADT before

resurfacing

Mean AADT after 

resurfacing Total crashes1

Fatal and injury 

crashes1

GA

Treatment 47 98.2 4,046 3,983 1,747 592

Comparison 50 119.7 4,929 6,104 1,718 603

Reference 132 412.0 4,118 4,122 4,594 1,748

Combined 229 629.9 4,182 4,285 8,059 2,943

IN

Treatment 30 83.5 6,584 6,561 419 68

Comparison 25 92.5 5,067 5,047 521 140

Reference 93 338.3 4,046 4,056 1,456 270

Combined 148 514.3 4,629 4,629 2,396 478

Combined 377 1,144.2 3,682 3,712 10,455 3,421

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

Notes: Georgia study periods were from 1999 to 2004 (before) and from 2006 to 2008 (after). Indiana study periods were from 2003 to 2004 (before) and from 2006 

to 2008 (after). Treatment sites were resurfaced with safety edge, comparison sites were resurfaced without safety edge, and reference sites were not resurfaced.

Safety Effectiveness Analysis

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

edge treatment while avoiding any potentially confounding safety effect due 

to pavement resurfacing. This was critical because the safety edge treatment 

is always implemented in conjunction with a resurfacing project. Previous 
3

research indicated that pavement resurfacing by itself may have a short-term negative 

effect on safety, increasing crashes because of increased speeds. One study found this 

effect to be statistically significant and to persist for 12 to 30 months after resurfacing.(3)

However, a more recent, larger study in National Cooperative Highway Research 

Program (NCHRP) Project 17-9(2) found inconsistent results.(4) Increases in 

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

frequency with resurfacing were found in others. Therefore, the safety effects of the 

pavement resurfacing and the safety effects of the safety edge treatment may be 

confounded, at least for some period of time.

The study design was developed to estimate the safety effect of the safety edge 

treatment despite the effect of pavement resurfacing. First, the study period 

after resurfacing was set at 3 years. This is long enough to extend beyond the 

duration of any short-term resurfacing effect. Second, resurfaced sites both with 

(treatment sites) and without (comparison sites) the safety edge treatment were 

considered. The ratio of safety between resurfaced sites with and without the 

safety edge treatment represents an effect of the safety edge treatment as long 

as the sites can be assumed comparable in all other respects. 

The evaluation approach was an observational before-after comparison using 

the EB technique, as formulated by Hauer.(5, 6) The specific version of the EB 

technique used in this evaluation was developed for the FHWA SafetyAnalyst 

software tools.(7) The primary objective of the before-after evaluation is 

to compare the actual observed number of crashes after the treatment is 

implemented to a calculated expected number of crashes in the after period 

had the treatment not been implemented. This comparison provides an 

estimate of the overall safety effectiveness of the treatment expressed as a 

percent change in crash frequency.

Figure 2. Georgia treatment site. Figure 3. Indiana treatment site.

Safety Effectiveness Results

The EB before-after observational evaluation procedure was programmed 

and executed in the SAS® software package and applied to the crash data 

before and after resurfacing with the safety edge treatment.(7) Safety 

effectiveness estimates and precision estimates (standard error), along 

with statistical significance, are presented in table 2.

The safety edge effect shown in table 2 is the ratio between the 

before-to-after change in crash frequency for the treatment sites 
4

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 1 and equation 2.

Table 2. Before-after EB evaluation results for two-lane roadways with paved and unpaved shoulders in Georgia and Indiana.

Shoulder 

type

Site

type

Number 

of sites

Percent

change Direction

Standard

error 

(Percent)

Safety edge effect

Effect

(Percent) Direction

Standard

error 

(Percent)

Statistically 

significant

Total crashes

Paved

T 39 10.027 Increase 6.262

C 26 21.556 Increase 8.203 9.485 Decrease 8.009 No

Unpaved

T 38 -7.657 Decrease 5.842

C 49 -1.221 Decrease 5.604 6.516 Decrease 7.910 No

Combined

T 77 1.546 Increase 4.293

C 75 7.654 Increase 4.662 5.674 Decrease 5.737 No

Fatal and injury crashes

Paved

T 39 14.685 Increase 11.337

C 26 37.393 Increase 14.199 16.528 Decrease 11.919 No

Unpaved

T 38 -0.961 Decrease 11.012

C 49 -6.884 Decrease 8.328 -6.361 Increase 15.147 No

Combined

T 77 7.328 Increase 7.93

C 75 9.148 Increase 7.341 1.667 Decrease 9.780 No

T = Treatment sites resurfaced with safety edge.

C = Comparison sites resurfaced without safety edge.

No = Not statistically significant at 10 percent significance level.

CMFResurfacing + Safety Edge = CMFResurfacingCMFSafety Edge (1)

CMF (2) Safety Edge = CMFResurfacing + Safety Edge

CMFResurfacing

CMFTreatment

CMFComparison

=

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 one. 

Similarly, the final CMF for the safety edge can be converted back to a percent 

change by subtracting the ratio from one 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 in the table as a positive value. 

The standard error of the safety edge effect is presented as a measure of its 

precision, along with an indication of its statistical significance.

The results in table 2 indicate that for all two-lane paved- and unpavedshoulder sites in Georgia and Indiana, the safety edge effect was 

5.7 percent for total crashes. This result was not statistically significant. 

Similar analyses found safety effectiveness estimates for the safety edge 

treatment of 6.3 percent for run-off-road crashes and 5.6 percent for 

drop-off-related crashes.(2) The results for run-off-road and drop-offrelated crashes were also not statistically significant. They vary more 

in sign and magnitude than the results for total crashes and generally 

appear less stable. Therefore, the 5.7 percent effectiveness estimate for 

reduction in total crashes appears to be the most reliable effectiveness 

estimate for the safety edge treatment.
5

Safety Edge Treatment Costs

Contract costs of resurfacing for both treatment and comparison sites were reviewed. Collectively, 

the cost of resurfacing with the safety edge treatment was found to be slightly less than without 

the safety edge treatment. The cost of resurfacing was found to be $96,000 per mi with the safety 

edge treatment and $110,000 per mi without the safety edge treatment. This difference is small 

enough that it could be due to project factors other than the safety edge treatment. 

Some advocates of the safety edge treatment maintain that incorporating it in resurfacing 

projects has little, if any, added cost because the asphalt is merely reformed to create the safety 

edge. The previous comparison could be interpreted as consistent with this hypothesis. 

However, construction practices vary between contractors and highway agencies, and while 

the amount of additional asphalt used for the safety edge treatment may be very small, 

it is unrealistic to assume that there is no additional cost to implement the treatment. 

Consequently, an alternative approach to estimating the cost per mile was developed. 

The alternative approach to determining the additional cost of the safety edge treatment 

was to compute the amount of asphalt used to provide the treatment and multiply this 

quantity by a typical bid cost per ton of hot-mix asphalt for a specific project. Using 

this method, 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.

Benefit-Cost Analysis

A benefit-cost analysis of the safety edge treatment based on the safety and cost 

results was conducted by calculating a benefit-cost ratio. The benefit-cost ratio 

for the safety edge treatment was determined using equation 3.

B/C = (3)

(NFIESECFI + NPDO ESECPDO ) (P/A, i%, n)

CCSE

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 property-damage-only crashes per mile per year 

before application of the safety edge treatment.

ESE = effectiveness (percentage reduction in crashes) for application 

of the safety edge treatment (5.7 percent reduction).

CFI = cost savings per crash for fatal and injury crashes reduced.

CPDO = cost savings per crash for property-damage-only 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 (7 years).

CCSE = cost for application of the safety edge treatment (in 

the range of $536–$2,145 per mi for both sides of the 

road combined).
6

The estimated crash costs were based on the latest published FHWA values, using the standard 

KABCO injury scale, as follows: 

• Fatal crash, $5,800,000.

• Incapacitating (A) injury crash, $402,000.

• Nonincapacitating (B) injury crash, $80,000.

• Possible (C) injury crash, $42,000.

• Property-damage-only crash, $4,000.(9)

The weighted average cost of a fatal or 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. 

Benefit-Cost Analysis Results

For each state and roadway type, benefit-cost analyses were performed for average 

annual daily traffic (AADT) volumes ranging from 1,000 to 20,000 vehicles per day. For 

two-lane highways with paved shoulders, application of the safety edge treatment had 

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 ranged from 3.7 to 62.8 for 

Georgia conditions and from 2.8 to 12.8 for Indiana conditions. The overall results of 

the minimum benefit-cost analysis are illustrated in figure 4. The maximum benefitcost ratios are four times the minimum benefit-cost ratios, as illustrated in figure 5.

These results suggest that the safety edge treatment is highly cost-effective 

under a broad range of conditions. Even though there is some uncertainty in 

the treatment effectiveness estimate, the safety edge treatment is likely to be 

a good safety investment in most situations and is especially so for roadways 

with higher volumes, where higher crash frequencies are expected.

Figure 4. AADT versus minimum benefit-cost ratio.

The overall results of the minimum benefit-cost analysis are illustrated in figure 4. The 

maximum benefit-cost ratios are four times the minimum benefit-cost ratios, as illustrated in 

figure 5. 

Figure 4. Graph. AADT vs. minimum benefit-cost ratio. 
7

Figure 5. AADT versus maximum benefit-cost ratio.

8 

Figure 4. Graph. AADT vs. minimum benefit-cost ratio. 

Figure 5. Graph. AADT vs. maximum benefit-cost ratio. 

The following conclusions were reached through this research:

• The results of the EB evaluation indicate that for all two-lane highway 

sites in two States the best estimate of the effectiveness of the safety edge

treatment is a reduction in total crashes of approximately 5.7 percent. 

While this result was not statistically significant, the results obtained 

for total crashes were nearly always in a positive direction. The 

results of evaluations for fatal and injury crashes were too variable 

to draw conclusions.

• The cost of adding the safety edge treatment to a resurfacing project 

is minimal. Overall project costs and the overall cost of asphalt 

resurfacing material did not increase for resurfacing projects with 

the safety edge compared to resurfacing projects without the safety 

edge. However, computations based on the volume of asphalt 

required to form the safety edge suggest that the cost of the 

safety edge treatment is approximately $536–$2,145 per mi for 

application to both sides of the roadway.

• Benefit-cost analysis based on the estimated 5.7 percent crash 

reduction effectiveness of the safety edge treatment found that 

this 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 a higher traffic volume and where the cost of 

installing the safety edge treatment is lower.

Conclusions
 for more information

FHWA-HRT-11-025

HRDS-20/01-11(1M)E

This research was conducted by Jerry Graham, Karen 

Richard, Mitchell O’Laughlin, and Doug Harwood of 

the Midwest Research Institute. The full final report 

by the same name can be downloaded from the FHWA 

Safety Research and Development publications Web site 

at http://www.fhwa.dot.gov/research/tfhrc/offices/safety/. 

For more information about HSIS, contact Carol Tan, HSIS 

program manager, HRDS, 202-493-3315, carol.tan@dot.gov.

Visit us on the Web at 

www.fhwa.dot.gov/research

The researchers offer the following recommendations as a result of this study:

• 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 was not statistically 

significant is not surprising given that the magnitude of that safety effect appears 

to be quite small (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 cost-effectiveness of the safety edge treatment increases with increasing 

traffic volumes. 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. For roads with higher traffic 

volumes, the safety edge treatment is highly cost-effective.

Recommendations

References

1. Zimmer, R.A., and Ivey, D.L. (1983). “Pavement Edges and Vehicle Stability—A 

Basis for Maintenance Guidelines,” Transportation Research Record 946, 48–56, 

Transportation Research Board, Washington, DC.

2. Graham, J. L., Richard, K.R., O’Laughlin, M. K, and Harwood, D.W. (2011). 

Safety Evaluation of the Safety Edge Treatment, Report No. FHWA-HRT-11-024, 

Federal Highway Administration, McLean, VA.

3. Hauer, E., Terry, D., and Griffith, M.S. (1994). “Effect of Resurfacing on 

Safety of Two-Lane Rural Roads in New York State,” Transportation Research 

Record 1467, 30–37, Transportation Research Board, Washington, DC.

4. Hughes, W.E., Prothe, L.M., and McGee, H.W. (2001). “Impacts of Resurfacing 

Projects With and Without Additional Safety Improvements,” NCHRP 

Research Results Digest 255, Transportation Research Board, Washington, DC.

5. Hauer, E. (1997). Observational Before-After Studies in Road Safety, 

Pergamon/Elsevier Science, Inc., Tarrytown, NY.

6. Hauer, E., Harwood, D.W., Council, F.M., and Griffith, M.S. (2002). 

“Estimating Safety by the Empirical Bayes Method: A Tutorial,” 

Transportation Research Record 1784, 126–131, Transportation 

Research Board, Washington, DC.

7. Harwood, D.W., Torbic, D.J., Richard, K.R., and Meyer, M.M. (2010). 

SafetyAnalyst: Software Tools for Safety Management of Specific 

Highway Sites, Report No. FHWA-HRT-10-063, Federal Highway 

Administration, McLean, VA.

8. SAS Institute, Inc. (2008). SAS/STAT® 9.2 User’s Guide, Cary, NC.

9. U.S. Department of Transportation. (2008). “Revised Departmental 

Guidance: Treatment of the Value of Preventing Fatalities and Injuries 

in Preparing Economic Analyses.” Accessed online: January 7, 

2011. (http://ostpxweb.dot.gov/policy/reports/080205.htm)