Steep pavement-edge drop-offs are more common on rural two-lane roads with unpaved shoulders, but also occur on roads with paved shoulders.
Typically, the unbound material at the pavement’s edge settles and erodes over time, creating a drop-off and increasing the difficulty drivers face in getting their tires back on the pavement after they slip off. The steeper and deeper the drop-off, research shows, the more likely it is that a driver will lose control of the vehicle while trying to get back on the pavement. This can make the vehicle careen into the oncoming lane. Sometimes drivers cannot regain control of the vehicle and roll over or crash on the other side of the road, plunging into ditches, posts or vegetation along the shoulder.
Although hundreds or even thousands of such crashes are thought to occur on America’s rural roads every year, empirical, statistical data proving that the drop-off is a factor is not easily found. Indeed, a recent crash on U.S. 30 in Iowa is a case in point. Four accidents in the same place were traced to the same cause, offering convincing proof of the problem, yet the statistical frequency of those crashes was calculated at less than 0.001% (less than one in a million) of the vehicles traveling that roadway in the three-month period leading up to the accident. On many lightly traveled rural roads, accidents involving pavement-edge drop-off occur far more randomly and are only evident to accident investigators who are aware of how drop-off can affect driving safety.
Thus, even though highway safety researchers have known since the 1980s that deep drop-offs created dangerous control issues for drivers in performance tests, without a practical method for providing a durable beveled edge, many agencies tried to control the drop-offs solely through maintenance. Thus the complicity of drop-offs to thousands of rural-road fatalities each year has persisted when edges were exposed.
Sloped solution
Today, the issue of pavement-edge drop-off is being aggressively addressed by highway engineers and road managers at the federal, state and local levels, thanks to the Safety Edge initiative of the Federal Highway Administration (FHWA).
Safety Edge is both a promotional program and a pavement detail. The safety edge shape is constructed with an attachment that bolts to the end of a paver screed and an attachment that modifies the paver end gate. These attachments extrude a 30° to 35° slope on the edge of an asphalt lift, thereby consolidating the asphalt material in the slope. This is vital to the longevity of the edge. Various devices for constructing the safety edge are available from a number of manufacturers, and it is expected that as use of the Safety Edge becomes more widespread, there will be other methods and equipment to construct it.
The sloped edge radically reduces control issues for drivers trying to recover when their tires slip over the pavement’s edge. The effectiveness of this slope is a phenomenon first observed in driving tests conducted by Zimmer and Ivey in 1982.
More recently, a state-of-the-art empirical study of accidents on safety-edged road sections in Indiana, New York and Georgia offered definitive proof of the efficacy of the Safety Edge solution. Over a three-year observation period, there was a 5.7% reduction in the frequency of all types of crashes on these roads.
“We expect that compaction with the proven devices is similar to that of the screed, in the 80 to 85% range,” estimated Frank Julian, an FHWA safety engineer who, along with materials engineer Chris Wagner, led the federal initiative to deal with the drop-off problem. “But the pavement wedge doesn’t bear frequent loads, and that amount of consolidation extends the service life of the edge.” Julian said early projects now seven years old still have their beveled edges intact.
For a 2-in. overlay, only a tiny amount of extra asphalt-paving material is required for the safety-edge extension of the lane. Estimates from contractors and highway engineers place the extra amount at less than 1%. The safety-edge solution for concrete roads requires somewhat more materials.
Edge of excitement
Asphalt contractors familiar with the technology and process tend to be enthusiastic about it.
Craig Parker, vice president of Silver Star Construction in Oklahoma, came away from a demonstration project impressed. Already personally familiar with the challenges of steering back onto pavement from a low shoulder, Parker liked the simplicity of the solution and its prospects for reducing agency and contractor liabilities as well as enhancing driver safety.
In Iowa, contractor enthusiasm is especially high because the safety edge can reduce costs. Paving contractors are generally required to build a temporary aggregate fill next to new pavement before it can be opened to traffic, explained Bill Rosener, executive vice president of the Asphalt Paving Association of Iowa. With the Safety Edge, that requirement is generally waived, freeing contractors to perform shouldering operations over long distances at the end of a job rather than at the end of each day.
Safety-edge paving is being adopted in Iowa with great velocity.
“In this [2010] construction season,” noted Jerry Roche, an Iowa-based FHWA safety engineer who has promoted the program vigorously in the state, “Iowa contractors installed more roads with safety edges than projects in all the other states combined.” The Iowa DOT has adopted the program, and Roche has worked extensively with Iowa counties and contractors to promote an even broader-based acceptance of Safety Edge. Roche even provides free temporary use of safety-edge devices to contractors and agencies new to the process.
But the trend to putting a safety edge on newly paved rural highways stems far beyond Iowa. When Julian and Wagner began looking for solutions to pavement-edge drop-off crashes, one of their first allies was the Georgia DOT. Lynn Bean, now assistant construction engineer for Georgia DOT District 2, was fabricating the forerunner of today’s safety-edge devices when Julian and Wagner came calling, and he and the department embraced the FHWA initiative immediately.
“We had a meeting with engineers from the Georgia DOT and contractors,” recalled Julian, “and we asked an equipment manufacturer that already made a wedge device for paving joints to attend.” The invitation turned out to be a key to popularizing the safety-edge concept. “The Georgia Shoe, as we called it, worked,” explained Julian, “But it was a local solution. The DOT wasn’t going to go to other states and sell it.”
The manufacturer did. After crafting a mechanism specifically to shape and consolidate a 30° asphalt-pavement edge, the supplier became another advocate for the Safety Edge, and ultimately two more manufacturers developed competing products and joined the effort.
Many other states are employing the safety-edge pavement design, and dozens more are being exposed to it through FHWA’s “Every Day Counts” program, which presents Safety Edge as an initiative that can be adopted quickly because it has no downside risk and because it can begin preventing crashes immediately. The program is backed by an information-rich website at www.fhwa.dot.gov/everydaycounts/-technology/-safetyedge/casestudies.cfm. It also features teams of FHWA engineers hosting informational meetings all across the country for state and local agency personnel and contractors.
With the weight of the FHWA behind it, the Safety Edge program is often incorporated into standard use by agencies with unusual speed. One contractor was both delighted and concerned with how quickly his state DOT adopted the Safety Edge standard. “It’s great to see an agency be so decisive,” he said, “But at the same time, I’d be more comfortable if we’d had a period for comments and questions.”
Still have questions
That contractor’s reservations specifically called into question how the beveled edge of the pavement would hold up over time, since in some applications it is placed on top of an unbound material that is less stable than the base under the main pavement.
That was one of the answers provided by the soon-to-be released three-year study of safety-edge pavements.
Other questions about Safety Edge—from how many crashes involve pavement-edge drop-off to how effective a 30° edge can be in mitigating the control issues posed by the drop-off—have been answered in a considerable library of research dating back to the late 1970s.
One of the most useful recent documents in this library is the 2006 paper “Safety Impacts of Pavement Edge Drop-offs” authored by a team of researchers from three different institutions: the Iowa State Center for Transportation Research, the Midwest Research Institute and the University of North Carolina (Chapel Hill) Highway Safety Research Center. Prepared in cooperation with FHWA for the AAA Foundation for Public Safety, the 144-page document summarizes key research that substantiates the significant hazard posed by pavement-edge drop-off and the efficacy of a 30° edge in correcting the problem (download free at www.aaafoundation.org/pdf/PEDO_report.pdf).
While Safety Edge is almost certainly coming to a rural road near you in the near future, this is no passing fad. The concept and the science behind Safety Edge have been evolving for several decades, and today’s program fulfills the dreams of two generations of dedicated highway-safety professionals who have advanced the cause. AT
