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CMF Study Expected to Bolster Support for Safety EdgeSM Treatment


Jason Giard, an operations engineer in FHWA’s Idaho Division, measures the height of a Safety EdgeSM treatment in Pocatello, Idaho.

Jason Giard, an operations engineer in FHWA’s Idaho Division, measures the height of a Safety EdgeSM treatment in Pocatello, Idaho.

It’s a dangerous situation.

A driver veers too far to the side of the road and his wheels slip off the shoulder. He tries to steer back onto the pavement, but the sides of his tires rub along the edge. He turns his wheels to achieve a greater angle, but overcorrects and the vehicle slingshots across the road. Will he regain control? Or will the vehicle overturn, or cross over into oncoming traffic? 

Adding a sloped pavement edge to roads is a treatment intended to minimize drop-off-related crashes. Known as Safety EdgeSM,the treatment creates a 30-degree slope that makes it easier for drivers to safely reenter the roadway after inadvertently driving onto the shoulder.

After conducting a 3-year study of Safety EdgeSM, researchers found that for two-lane highways in Georgia and Indiana the treatment could reduce crashes by about 5.7 percent. The study, which considered treated sites and untreated control sites, determined that the cost of Safety EdgeSM is minimal. Based on a theoretical quantity of additional asphalt material, the cost per mile for application on both sides of the road is approximately $536 for a 1.5 inch treatment depth and $2,145 for a 3.0 inch treatment depth.

Now the Federal Highway Administration (FHWA) is conducting a study to determine the crash modification factor (CMF) of Safety EdgeSM. This is expected to provide a better understanding of the potential crash reductions for various geometries and a greater justification for widespread implementation. A crash modification factor helps highway safety engineers and transportation researchers estimate the safety effectiveness of various countermeasures. It provides information that is used in the analysis of benefits and costs that help highway designers select an appropriate treatment to improve safety. 

“It helps in the selection, it helps in the prioritization, and it helps in the justification too,” says Carol Tan, Ph.D., who heads the Safety Management Team in FHWA’s Office of Safety Research and Development at Turner-Fairbank Highway Research Center.

Say there are two countermeasures designed to address the same type of crash and one has a higher CMF. “You’ll analyze the cost of each over its life-cycle compared to how many crashes they are expected to reduce over that lifetime before making a decision on which to use, or whether to use both,” explains Jerry Roche, a safety engineer in FHWA’s Iowa Division who helped in efforts that influenced Iowa’s decision to implement a policy to use Safety EdgeSM routinely on projects.

The University of North Carolina Highway Safety Research Center maintains a CMF Clearinghouse (www.cmfclearinghouse.org) funded by FHWA. Featuring an online repository of CMFs, the Web site has a searchable database and offers educational information about the application of CMFs. The Clearinghouse is regularly updated and provides summaries of published information about CMFs, including facts about development and statistical properties.

Each CMF included in the Clearinghouse has a star rating based on a scale from 1 to 5, where 5 is the highest or most reliable rating. This informs decisionmakers about the quality of the study that produced the CMF. “You really need to look at the study,” points out Tan. “If it wasn’t good in terms of the statistics, then you may not have a lot of confidence in the CMF.”

Safety EdgeSM involves minimal time and cost to implement. Installed during paving, it uses a commercially available shoe that attaches to existing equipment in just a few minutes. When constructing a 1.5 inch asphalt overlay, less than 1 percent of additional asphalt is needed to create the slope.

“Once you make the initial investment of a couple thousand dollars to buy the shoe, you can use it on multiple projects to pave hundreds of miles of roadway with Safety EdgeSM,” says FHWA Safety Engineer Frank Julian who was involved in developing the concept for Safety EdgeSM. “The cost is small over the lifetime of a paving project.”

In Iowa, as many as 18 percent of rural run-off-road crashes (on paved roadways with unpaved shoulders) may have been partially caused by the pavement edge, according to a study funded by FHWA and the AAA Foundation for Traffic Safety. FHWA loaned a Safety EdgeSM shoe to engineers for use on resurfacing projects in areas of Iowa that had a history of roadway departure crashes. After implementation, even large vehicles could traverse the edge without loss of control or damaging the edge. The Iowa Department of Transportation successfully used the shoe to mitigate drop-offs as high as 4 inches on one of its paving projects. These favorable results have influenced the State to adopt Safety EdgeSM as a standard practice.

“The drop-off problem doesn’t go away,” emphasizes Roche. “There is still the potential for an edge drop-off to appear as the adjacent unpaved material erodes. It’s just that Safety EdgeSM helps the vehicle get back onto the roadway. The transition is a lot smoother and it reduces the chance of head-on crashes or other severe crash types.”

Thanks to FHWA’s Every Day Counts (EDC) initiative—which aims to identify and deploy innovation that can shorten project delivery, enhance roadway safety, and protect the environment—awareness and implementation of Safety EdgeSM technology has increased significantly across the country.

“Because of EDC, we were able to get a lot of States to do demonstration projects and many have since adopted Safety EdgeSM for numerous paving projects,” observes Cathy Satterfield, who leads the Safety EdgeSM Team in FHWA’s Office of Safety. “We increased the sample size. Now, after a couple of years, we’re collecting that data and combining it with older data. So I think we’ll get a strong and statistically robust analysis from this CMF.”

The work plan for the study is set and over the next 3 years FHWA will receive regular summaries of data before the final CMF report is finished. “Many of the installations have occurred over the last 2 or 3 years, so we will need to wait for more data,” says Tan. “No two sites will be exactly the same, but because this is so cheap to implement, the benefit-cost ratio will most likely be very good.”

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