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|Federal Highway Administration > Publications > Public Roads > Vol. 72 · No. 1 > Gaining Traction In Roadway Safety|
Publication Number: FHWA-HRT-08-005
Gaining Traction In Roadway Safety
by Frank Julian and Steve Moler
High-friction surfacing systems show promise in helping transportation agencies improve skid resistance on wet pavement and hazardous curves and grades.
According to the National Highway Traffic Safety Administration’s Fatality Analysis Reporting System (FARS), since the 1970s annual highway fatalities in the United States have held steady at about 40,000 people. Over the last three decades, the total comes to roughly 1.2 million fatalities, the equivalent of the population of San Diego, CA. Further, a March 2008 report by AAA, Crashes vs. Congestion: What’s the Cost to Society?, finds that automobile crashes cost U.S. motorists more than $164 billion per year, or about $5 trillion over the past three decades, taking into account property damage, lost earnings, medical costs, emergency services, legal costs, and travel delays.
Transportation officials at every level continue to invest time, energy, and resources into researching the causes of automobile crashes and developing measures to prevent crashes from happening in the first place. Two common crash types receiving considerable attention are run-off-the-road and wet-weather crashes. About 25 percent of all crashes and 14 percent of all fatal crashes occur on wet pavement, according to the American Association of State Highway and Transportation Officials (AASHTO). In 2003, more than 25,000, or 59 percent, of the highway fatalities occurred when a vehicle left its lane or ran off the road.
For sharp curves and wet pavement, a promising new approach is emerging. Engineers at the Federal Highway Administration (FHWA), State departments of transportation (DOTs), and in the private sector are devising and refining high-friction surfacing systems. These overlays consist of resins and polymers with a binder topped with small, hard aggregate that helps vehicles stay on the road. The surfacing systems also are especially resistant to wear and tear. Demonstration projects at locations across the country over the last 10 years are beginning to show promising results for reducing crashes.
Focus on Curves and Wet Weather
AASHTO’s Strategic Highway Safety Plan identifies 22 goals for reducing fatal highway crashes. Two of the goals include keeping vehicles on the roadway and minimizing the consequences of leaving the road. One strategy for keeping vehicles on the roadway and thereby reducing the number of people killed by hitting fixed objects on the roadside is to use skid-resistant pavement surfaces.
Sharp horizontal curves are often the sites of run-off-the-road and wet-weather crashes. In fact, about 25 percent of the Nation’s fatal crashes in 2006 occurred along sharp horizontal curves, mostly on two-lane rural highways. About 76 percent of curve-related fatal crashes were single-vehicle incidents where the vehicle left the roadway and struck a fixed object or overturned. The average crash rate for horizontal curves is about three times that of other highway segments, according to FARS data.
FHWA and AASHTO agree that existing design criteria provide an adequate safety margin against vehicles skidding off the roadway. But such criteria hold true only if drivers do not exceed the road’s design speed, are able to maintain their lane position, and follow the curve radius. Too often, however, drivers violate these conditions by speeding, driving aggressively, driving under the influence of alcohol or drugs, being inattentive, or becoming distracted. Hazardous driver behaviors underscore the need for enhancing pavement friction, especially on sharp horizontal curves and steep grades, and in wet weather.
Providing motorists more traction is one solution. Traffic safety experts are working to improve understanding of friction on asphalt and concrete pavement surfaces, especially in wet weather. According to the FHWA report Surface Finishing of Portland Cement Concrete Pavements (FHWA-SA-96-068), surface treatments using epoxy resin and calcined bauxite can improve the friction of existing portland cement concrete surfaces on roadways with high numbers of wet-weather crashes. FHWA’s Technical Advisories T5040.36, dated June 17, 2005, and T5040.31, dated December 26, 1990, also provide information and guidance on high-friction surfaces.
Studies show that improving the friction of pavement surfaces reduces wet-road and total crashes. New York State, for example, implemented a program that identifies sites statewide that have low pavement friction. The State treats them with overlays and microsurfacing as part of its maintenance program. Between 1995 and 1997, the New York State DOT treated 36 sites on Long Island, resulting in a reduction of more than 800 annual wet-road crashes. These results showed that improving pavement friction at locations with high wet-weather crash rates reduces wet-road crashes by 50 percent and total crashes by 20 percent.
Two types of surface texture affect wet pavement friction: microtexture and macrotexture. Microtexture, with wavelengths of 1.0 micron, µm to 0.5 millimeter, mm (0.00004 inch to 0.01969 inch), is generally provided in asphalt pavements by the relative roughness of the aggregate particles and in concrete surfaces by the fine aggregate. Macrotexture, with wavelengths of 0.5 mm to 50.0 mm (0.01969 inch to 1.969 inches), is generally provided in asphalt pavement by proper aggregate gradation and in concrete surfaces by a supplemental treatment such as tining, grinding, or turf drag. How these two properties are combined creates pavement friction or skid resistance.
How can transportation officials increase pavement friction beyond what is attainable through traditional techniques? One way is through new high-friction surfacing systems, which consist of combining resins and polymers — usually urethane, silicon, or epoxy — with a binder topped with a natural or synthetic hard aggregate.
What distinguishes these overlays from standard asphalt and concrete pavement surfaces is the microtexture, macrotexture, and the durability of that texture. High-friction surfacing systems typically use much smaller and harder aggregates, such as calcined bauxite, slag, or other synthetic aggregates. These aggregates are generally less than 6.0 mm (0.23 inch) in diameter and have high skid resistance.
The small and hard aggregate makes the overlay much more resistant to wear and polishing. The resin or polymer binder combination locks the aggregate firmly in place, creating an extremely rough, hard, durable surface capable of withstanding everyday roadway demands such as heavy braking and snowplowing. The rougher texture and greater surface area increase the pavement’s friction. Road crew workers can apply high-friction surfacing treatments on top of most road surfaces, including asphalt, concrete, steel, and wood. The crew applies the high-friction pavement surface as a thin overlay using a multistep process. After establishing traffic control, the crew sweeps and moisture-dries the pavement surface where necessary. Next, the workers tape over any pavement markings. At the same time, a crew mixes the binder and then spreads it over the surface area using squeegees or a mechanical spreader.
Then, the workers spread the aggregate over the binder, sweeping away any excess using brooms or a mechanical sweeper. Most high-friction surfaces cure in 3–4 hours, as long as ambient temperatures meet manufacturers’ specifications, typically 10–15 degrees Celsius (50–60 degrees Fahrenheit). On a typical high-friction overlay (a distance of several hundred feet or less), the entire process can usually be completed in half a day.
Because of the relatively short distances involved in the typical high-friction overlay — usually 30 to 90 meters (100 to 300 feet) — pavement smoothness, tire wear, and fuel consumption are not significant factors.
High-friction surfacing systems developed in Europe in the early 1960s and arrived in the United States in the late 1970s. Transportation agencies initially used high-friction surfacing systems to seal and improve the safety of bridge decks and to reduce skidding and shorten stopping distances on approaches to intersections and tollbooths. Brightly colored or pigmented high-friction surfaces are common in Europe — and to a lesser extent in the United States — as traffic calmers and delineators, especially at crosswalks and roundabouts. European transportation agencies also use these surfaces to mark bicycle and bus lanes.
In the late 1980s, U.S. researchers began to investigate the effectiveness of applying high-friction surfacing systems on horizontal curves to reduce run-off-the-road crashes. A 1989 study for FHWA by the University of Michigan Transportation Research Institute evaluated 15 troublesome freeway ramps at 11 interchanges in 5 States. The study found that truck crashes were all “clearly related to geometry and vehicle dynamics.” In general, tight-radius curves on ramps and short acceleration and deceleration lanes cause problems for heavy trucks, the report said.
Another of the study’s major findings was that friction levels on high-speed ramps can be dangerously low in certain conditions. Hydroplaning can occur in wet weather at sites with poor pavement texture, making trucks particularly vulnerable on tight-radius curves. According to the report, “One proven countermeasure is to resurface ramps with high-friction overlays.”
Toward that end, FHWA’s Office of Pavement Technology and Office of Safety Design are leading an effort to demonstrate and evaluate the effectiveness of high-friction surfacing systems at horizontal curves with high crash rates. In this project, FHWA provides technical assistance and covers the cost of the overlay, while States handle traffic control and pre- and post-project traffic studies. FHWA’s Innovative Bridge Research and Construction (IBRC) Program also sponsors some demonstration projects.
“We’re trying to look at what we can do to reduce run-off-the-road crashes,” says Mark Swanlund, a senior pavement design engineer in the FHWA Office of Pavement Technology. “These low-cost treatments on horizontal curves have the potential to improve roadway safety at high-crash locations. We recognized the possible benefits in evaluating this technology.”
Numerous State, county, and city transportation agencies across the country are involved in demonstration projects or full implementation of high-friction surfacing systems to improve safety. Although many of these projects are too recent to obtain adequate long-term crash data to substantiate overall effectiveness, preliminary data and observations suggest this technology could indeed be an effective countermeasure for wet pavement and on dangerous curves and steep grades.
Freeway Ramp Improvements in Florida
The Florida Department of Transportation (FDOT) recently launched a demonstration project to evaluate installation of an epoxy-resin, high-friction, thin overlay treatment on a crash-prone freeway loop ramp near Fort Lauderdale. Despite an advisory speed limit of 40 kilometers (25 miles) per hour and adequate curve warning and chevron signs, the ramp from eastbound Palm Royal Boulevard to northbound I–75 in Weston recorded 12 run-off-the-road crashes in 2002–2004, with 83 percent of those crashes occurring when the road was wet.
FDOT applied the high-friction surface over a 91-meter (300-foot)-long section within the area on I–75 where most of the crashes had occurred. The treatment consisted of an epoxy-resin binder topped with a calcined bauxite aggregate. Workers applied the treatment on May 15, 2006, using buckets and squeegees during a night operation starting at 10 p.m. FDOT opened the ramp to traffic by 5 a.m. the next morning.
A traffic consulting firm hired by FDOT to conduct before-and-after studies in 2007 found that only two run-off-the-road crashes occurred on the ramp within 1 year of the treatment’s application. Also, the proportion of drivers encroaching on the outer and inner shoulders decreased significantly under wet conditions after installation of the high-friction overlay. The study revealed no significant differences measured in dry conditions.
Skid tests conducted after the treatment showed a substantial increase in pavement friction. “I’ve been really impressed with the results,” says Gilbert Soles, a safety program manager for FDOT’s District 4 who supervised the demonstration project. “It’s made a difference. We’ve seen fewer crashes as a result of the treatment.”
The consultant’s final report, Evaluation of Innovative Safety Treatments, concluded, “Overall, the [treatment] was found to be effective in increasing the friction between the roadway and vehicle tires. The treatment also was found to assist motorists in maintaining their lane positions under wet pavement conditions. In addition, drivers tended to slow down when traveling over the treated section of the ramp. It appears the use of [the treatment] may be a practical countermeasure for improving safety at locations that are prone to run-off-the-road crashes, particularly sharp curves and entry/exit ramps.”
FDOT is planning two more demonstration projects involving similar high-friction surfacing systems. One project involves a sharp curve on an interstate off-ramp with numerous guardrail hits. The other will take place at a downgrade from a local street bridge to a signalized intersection where rear-end crashes have occurred.
Improving Safety At Intersections
The city of Bellevue, WA, near Seattle, applied a similar treatment on an approach to one of its more crash-prone intersections. Drivers heading east on Forest Drive just before reaching Cold Creek Parkway must simultaneously negotiate an extremely steep (14 percent) downgrade and a sharp 33.5-meter (110-foot)-radius left curve before reaching a signalized T-intersection.
From 1997 to 2002, the steep approach experienced 21 crashes, some serious, including four rollovers. Nearly all the crashes were rear-enders that occurred in wet or icy conditions where excess speed was a factor. Bellevue tried several countermeasures, including installing a large flashing warning sign at the bottom of the grade, additional road markers, and raised pavement buttons.
In October 2004, the city installed a high-friction, epoxy-resin overlay similar to that used in Florida on the steepest and curviest section of Forest Drive just before the intersection. Crews first installed the treatment on the downhill lane, then completed the uphill lane the next day. Each lane was opened to traffic within 4–6 hours.
After the installation, the city recorded significant increases in pavement friction along the treated section. Between October 2004 and June 2008, only two crashes occurred on the steep approach after the treatment — one the result of brake failure, the other due to an inattentive driver. Increasing the friction helped create a surface that even in wet and icy conditions acts similarly to dry pavement. “We’ve received a lot of good comments from the public,” says Judy Johnson, the Bellevue street maintenance superintendent. “We’re not having any maintenance issues and the treatment has proven to be very durable.”
Facing a similar situation with a steep grade ending at a busy urban intersection, the city of Lincoln, NE, completed a high-friction surfacing demonstration project in October 2007. Rosa Parks Way makes a steep descent from a bridge down to a major downtown intersection at Ninth Street. The downgrade was prone to icing, with heavy trucks in particular having difficulty braking for the intersection despite variable message signs warning drivers of the steep grade. The city expects to complete its before-and-after traffic study by the end of 2008.
Applications on Two-Lane Rural Roads
The Pennsylvania Department of Transportation (PennDOT) recently embarked on a demonstration project involving a high-friction overlay along a high-crash section of State Route 611 in Northampton County, adjacent to the Delaware Canal. This section of two-lane rural highway included a sharp horizontal curve, a narrow 2.7-meter (9-foot) lane width, and no shoulders. On one side is a steep cliff, on the other a steep dropoff to the canal below. From 1997 to 2006, this curve segment recorded 22 crashes. All occurred in the southbound lane, and all were single vehicle run-off-the-road or head-on crashes, according to PennDOT.
No crashes yet have occurred after PennDOT applied the treatment in June 2007, according to Stephen P. Pohowsky, a PennDOT safety program specialist who supervised the project. To obtain more data, PennDOT plans to install high-friction surface treatments in its Engineering District 5-0 at two additional high-traffic, crash-prone sites with sharp horizontal curves.
The Tennessee Department of Transportation (TDOT) tried the same epoxy-resin calcined aggregate overlay on a 91-meter (300-foot) section of a cloverleaf ramp from Conference Drive to Vietnam Veterans Boulevard near Hendersonville in August 2004. The ramp’s concrete pavement had become noticeably polished due to heavy traffic. The treatment significantly improved the ramp’s friction, according to TDOT.
“I’ve been around this business for 28 years, and I can tell you that this is one of the best solutions I’ve seen for preventing run-off-the-road crashes,” says Danny Lane, an operations specialist with TDOT’s Materials and Tests Division. “The fact that you can get these friction numbers way up means you’re going to have fewer skidding problems, and that’s going to save lives.”
Anti-Icing, High-Friction Surfacing Systems
Another, more recently developed, high-friction surfacing system, which uses a different technology than the epoxy-resin, bauxite aggregate products, is in place at 57 sites in 23 States, most in the North and Northeast. This system combines a patented epoxy-resin topped with a specially designed hard aggregate to create a rough, rigid, spongelike surface, which holds anti-icing liquid near the pavement surface and then slowly releases it over several days. Studies in Iowa, Minnesota, and Ohio show the treatment minimizes or even prevents frost, snow, and ice from accumulating on the road surface over multiple winter storms.
The Ohio Turnpike Commission is evaluating this high-friction surfacing treatment at the I–80 eastbound exit 173 near Brecksville. The ramp features an incline with a sharp curve. The ramp was the site of 49 mostly wet-weather crashes in the 2 years prior to installation of the treatment, according to the commission. There has been a significant decrease in weather-related and run-off-the-road crashes on the ramp since the commission installed the overlay in November 2005.
“It is evident based on anecdotal information from the site that there is a marked improvement in skid resistance over the previous surface in all weather conditions,” says Tim Ujvari, chief of the commission’s regional maintenance department that carried out the project.
The Minnesota Department of Transportation applied the same high-friction deicing surfacing system on the southbound lanes of the Highway 169 Mitchell Bridge near Hibbing in July 2006. The northbound lanes were not treated. As part of a research effort at the Northland Advanced Transportation Systems Research Laboratories, a study by John Evans of the chemistry department at the University of Minnesota Duluth showed no crashes were reported on the southbound lanes in the winter following the installation, compared with eight crashes on the untreated northbound lanes during the same period. Three of those crashes were weather-related, three were related to unsafe or illegal speed, and two were due to unknown factors.
“While one would be cautious of over-interpreting such small data sets, [the evidence] strongly suggests that this overlay treatment is…contributing to [crash] reduction,” Evans noted in his study.
Wilfrid Nixon, a professor of civil and environmental engineering at The University of Iowa, evaluated nine projects where the anti-icing and antiskid treatment was installed. All the projects occurred on bridges or approaches to bridges, including on- and off-ramps. Eight of the overlays were installed by the respective State DOT (with technical assistance from the manufacturer) prior to the winter of 2005–2006, and one was installed in summer 2003. The latter project provided lengthier testing data.
Nixon, who specializes in snow and ice removal from roads, concluded in his study that test sections remained clear of snow and ice when the precipitation was accumulating on the control sections. When accumulations did occur on the test sections, the researchers observed no bonding of snow and ice to the pavement. The accumulations could be controlled by plowing and deicing chemicals.
Furthermore, the study determined that the amount of chemicals needed to obtain safe driving conditions was less on the test sections than on the control sections. The treated segments maintained mobility longer and could be returned to full traffic levels more easily than nontreated sections. The study reported that maintenance crews could maintain bare pavement conditions on the test sections with about half the chemicals they applied to control sections.
In his 2006–2007 followup report on the demonstration projects, Nixon concluded that, after 2 years of consistent evidence, “improved performance [of the anti-icing overlay] under winter conditions…does indeed translate into safety improvements for the traveling public.”
Setting the Stage
Although it might be too soon to know conclusively whether high-friction surfacing systems are indeed an effective countermeasure to run-off-the-road and wet-weather crashes, researchers continue seeking the answer through ongoing demonstration projects. Current anecdotal and other evidence indicates that high-friction surfaces are life-saving treatments. Therefore, the stage is set for more comprehensive and conclusive evaluations to take place in locations where this treatment can work.
Frank Julian is a safety engineer at the FHWA Resource Center office in Atlanta. He has been with FHWA for 22 years, serving in a variety of positions, including highway safety team leader in the former FHWA Southern Resource Center, regional safety engineer for the former FHWA Region 4, area engineer, engineering programs coordinator and safety management engineer in the Georgia Division Office, and highway engineer in the Florida Division Office. He holds a B.S. in civil engineering from Auburn University.
Steve Moler is a public affairs specialist at FHWA’s Resource Center office in San Francisco. He has been with FHWA for 7 years, assisting the agency’s field offices and partners with media relations, public relations, and public involvement communications. He has a B.S. in journalism from the University of Colorado at Boulder.
For more information about FHWA’s Road Departure Safety program, contact Mary L. McDonough at 202–366–2175 or firstname.lastname@example.org.
This article represents the views of the authors and not the position or policies of FHWA.
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