Low-Cost Safety Improvements for Horizontal Curves
A new FHWA publication highlights proven, cost-effective treatments that help reduce run-off-road and roadway departure crashes on rural roads.
In a typical year, more than one-quarter of all fatal highway crashes in the United States — 10,427 in 2007, for example — occur on curves. The vast majority of these horizontal curve crashes are roadway departures, accounting for 83 percent of horizontal curve fatalities in 2007. Many countermeasures to address roadway departures are very effective when placed appropriately to address horizontal curves. Therefore, the Federal Highway Administration's (FHWA) focus on reducing roadway departures goes hand in hand with preventing crashes at horizontal curves.
Most of the fatalities attributed to roadway departures and crashes at horizontal curves occur on rural roads, especially two-lane roads. These roads tend to have unforgiving roadsides and shoulders, and less access to emergency services. Also, motorists are likely to travel on them at higher speeds than the roads are designed to accommodate safely. The magnitude of the rural two-lane highway system and relatively low traffic volumes make significant expenditures for improvements unlikely and pose safety challenges to the agencies that repair and maintain them. Focusing on the horizontal curves on these roadways can help make that process more manageable.
"The bottom line is that 28 percent of the 41,059 people who died on our highways in 2007 — more than 11,000 of our family members, friends, and neighbors — died in curve-related crashes," says Division Administrator Tom Smith, of the Federal Highway Administration's (FHWA) West Virginia Division Office, whose State struggles with crashes on horizontal curves.
To reduce the number of deaths and injuries on public roads, FHWA is deploying a variety of low-cost safety improvements, including sign enhancements, barrier delineation, and rumble strips, that can reduce the risk posed by horizontal curves. Many of these low-cost treatments are equally effective in both rural and urban environments. A new publication, Low-Cost Treatments for Horizontal Curve Safety (FHWA-SA-07-002), provides practical information regarding numerous safety treatments that State departments of transportation (DOTs) and local road agencies can deploy without major investments.
Focusing on Horizontal Curve Safety
"If you are going to solve the roadway departure problem, you have to solve the horizontal curve problem," says Frank Julian, a safety engineer on the FHWA Resource Center's Safety and Design Technical Service Team. "The most cost-effective approach to solving roadway departure crashes is to focus on horizontal curves because they make up a small percentage of the road miles but account for one-quarter of all highway fatalities."
Roadway Departure DefinedRoadway departure is one of four major focus areas for FHWA in the safety arena. For the purposes of this article, roadway departure refers to single-vehicle, run-off-road crashes (where vehicles strike trees, utility poles, rocks, or other fixed objects, or overturn) as well as head-on collisions and other opposite direction crashes that commonly occur when vehicles drift into opposing lanes. |
Improving safety at horizontal curves helps achieve one of FHWA's key lifesaving strategies: reducing roadway departures. One approach to implementing this strategy is to provide local agencies, which bear the greatest responsibility for the Nation's vast network of two-lane rural roads, with tools to implement positive changes. "States and local governments should be attacking the roadway departure problem strategically and systematically to treat large numbers of horizontal curves with low-cost improvements," Julian says.
FHWA's emphasis on improving safety at horizontal curves is a direct result of the American Association of State Highway and Transportation Officials' (AASHTO) Strategic Highway Safety Plan (SHSP), adopted in 1997 and updated in 2004. To help implement the SHSP, the National Cooperative Highway Research Program (NCHRP) developed a series of guides to help State and local DOTs identify ways to reduce injuries and fatalities in targeted areas, including on horizontal curves. Several goals of the plan — keeping vehicles on the roadway, minimizing the consequences of roadway departures, and reducing head-on collisions — are addressed in volume 7 of NCHRP Report 500: A Guide for Reducing Collisions on Horizontal Curves.
Some States, such as Minnesota, already are taking steps to zero in on horizontal curves, with FHWA's help. "Freeborn County has benefited from the partnerships with our State transportation department and the FHWA Minnesota Division Office," says County Engineer Susan G. Miller. "Through various grants, we have been able to assess our crash data, which indicate that Freeborn County should target horizontal curves for safety strategies. From there, we developed programs for appropriate countermeasures, such as curve enhancement signage [chevrons] and edge line rumble strips, and we're now planning a dynamic curve warning system."
To help such efforts, FHWA is developing a series of technical information products. One publication, Low-Cost Treatments for Horizontal Curve Safety (FHWA-SA-07-002), provides concise information on safety treatments and design features that can improve safety without resorting to costly roadway reconstruction.
FHWA developed the guide by reviewing print and Web-based literature to identify low-cost safety countermeasures and by consulting with contacts from several State and local transportation agencies. For each of the six types of treatments or features presented, the guide provides a description, guidance on application, design elements or materials to use, notes on effectiveness and cost, and references and contacts for more information. The concluding chapter underscores the importance of maintenance in keeping roads safe.
Virginia Uses FHWA Guide in Road Safety ProgramMirroring nationwide statistics, about 60 percent of Virginia's traffic fatalities are the result of cars running off roadways and crashing. But waiting for funding for higher cost roadway geometric improvements is not an option if Virginia is going to drive down the number of injuries and deaths from these roadway departure crashes, says Stephen W. Read, highway safety improvement program manager at the Virginia Department of Transportation (VDOT). As a result, VDOT is undertaking a statewide comprehensive analysis of lane departure crashes, with plans to deploy short-and intermediate-term engineering countermeasures. Because Virginia is one of the few States that maintains county roads, in addition to its State roads, it needs a systematic approach to allocate limited funding. Under its Road Safety Assessments program, VDOT will identify roadway segments with departure crashes to the left of centerline and right of edge line that deserve more detailed safety assessments. "With FHWA's Low-Cost Treatments for Horizontal Curve Safety in hand, local VDOT traffic engineering and maintenance staff will be able to implement the best mix of multiple measures to immediately improve safety," says Read. "Having the concise compilation of treatments in one document will greatly expand the knowledge base and practical use of treatments for those responsible for operations and maintenance."
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In addition, the treatments in the guide typically meet at least one of two objectives for improving safety on horizontal curves discussed in the NCHRP report: (1) reduce the likelihood of a vehicle leaving its lane and crossing the centerline or leaving the roadway, and (2) minimize damage caused by a vehicle leaving the roadway. What follows are a few of the treatments showcased in the guide. These countermeasures are intended to improve safety at curves or winding sections of roadway identified as an existing or potential safety problem. Agencies should consider site-specific issues before implementation.
Basic Treatments
Basic treatments are traffic control devices commonly found on the Nation's roadways and described in detail in the Manual on Uniform Traffic Control Devices (MUTCD). The devices include centerlines; edge lines; horizontal alignment signs as advance warnings (including turn, curve, reverse turn, reverse curve, winding road, hairpin curve, or 270-degree loop), and advisory speed plaques supplementing the horizontal alignment signs. The devices also include one-direction large arrow signs; combination horizontal alignment/advisory speed signs; curve speed signs; chevron alignment signs; and delineators, which are retroreflective devices along the side of the roadway that provide guidance by indicating the alignment, particularly helpful at night or during adverse weather.
Agencies can install these treatments at curves where evidence (such as scarred trees, nicked rocks, or damaged fences or utility poles) shows incidents have occurred, or where engineering judgment or studies indicate a potential need. In deciding which treatments to implement, agencies should consider factors such as posted and operating speeds, geometry, sight distance, unexpected features (such as an intersection hidden by a curve), and traffic volume. Many curves need nothing more than adding the standard horizontal alignment warning sign or the standard centerline and edge line (if paved).
Enhanced Basic Treatments
A visible step up, enhanced treatments are basic treatments modified to help drivers see them sooner and have more time to react, which often means slowing down. Low-Cost Treatments for Horizontal Curve Safety reviews several types of enhancements.
Sign enhancements. Transportation agencies have several ways to call attention to signs that otherwise might be overlooked by drivers. For example, the MUTCD allows use of oversized and larger-than-standard signs for special applications where increased emphasis, improved recognition, or increased legibility is desirable. Agencies also can double-up on basic signs — place them on both sides of the roadway — and increase the opportunity for drivers to see and respond to them.
Flashing lights attract attention. Flashing yellow beacons can supplement advance horizontal alignment signs, but their use depends on an available power source, such as solar panels. Finally, agencies can use sign sheeting with higher retroreflectivity levels to improve nighttime visibility or fluorescent sheeting to make signs more conspicuous and improve visibility during inclement weather conditions, such as fog or the twilight hours.
Pavement marking enhancements. Raised retroreflective pavement markers (RRPMs) may supplement pavement markings. The vertical profile of the markers enables drivers to see them from a greater distance at night and in wet weather. RRPMs also may provide an auditory warning if vehicles drive across them.
Profiled markings of thermoplastic or other durable pavement marking materials can provide most of the benefits of RRPMs, but to different degrees. For example, because they are continuous, profiled markings can provide better path delineation through curves. On the other hand, if the markings' height above the pavement is less than an RRPM, they might not be as visible from as far away or in wet conditions.
Other Traffic Control Device Treatments
Agencies also can employ other treatments, some of which are not specifically mentioned in the MUTCD but are allowed under MUTCD general principles and provisions and have been tried by State and local highway agencies. Several of these are included in the current notice of proposed rulemaking for the next edition of the MUTCD.
Barrier delineation. Nighttime driving poses additional challenges for drivers navigating horizontal curves, as many features, such as barriers, might not be visible. Adding retroreflective devices to existing longitudinal barriers draws attention to the hardware and delineates roadway geometry. Agencies can use individual reflectors or apply strips of sheeting for concrete or cable barriers and metal guardrails. Either application will alert drivers and help guide them through the curve.
Roadside object delineation. For trees, utility poles, and other fixed objects in the roadside area, it is preferable to remove it, relocate it, or shield it — in that order. When none of these is feasible, another option is to mark the obstruction so drivers can see it and navigate away from it. An object marker, retroreflective tape, or other simple delineation device can be an effective, low-cost alternative.
Dynamic curve warning system. This supplemental system uses beacons and/or messages that activate as motorists approach the curve at high speeds. The activated signal section or word message advises drivers to reduce speed. Agencies can develop these systems using off-the-shelf technology. European studies have shown success with this approach, and evaluation is underway in the United States to determine expected crash reductions. As with any sign, design and placement should be carefully considered, especially when variable message signs are used with the system.
Speed advisory marking in lane. Where traditional warning signs prove ineffective for reducing vehicle speeds, agencies can consider placing a supplementary message directly on the pavement. For example, a speed advisory marking consisting of CURVE and XX MPH (the speed recommended for the specific location) may be more effective for a curve on a higher speed roadway.
Rumble Strips
Audible and physical vibrations near the edges of traffic lanes alert motorists when they drift off roadways. According to the MUTCD, "Longitudinal rumble strips consist of a series of rough-textured or slightly raised or depressed road surfaces located along the shoulder to alert road users that they are leaving the travel lanes." Where installed on centerlines, shoulders, or edge lines, rumble strips have proven effective in reducing run-off-road crashes. Placing the centerline or edge line in the rumble strip (making it a rumble stripe) provides an additional dimension, increasing the marking's visibility, particularly in wet weather.
New profiled marking products that can provide an audible warning also are becoming available. Some agencies are installing centerline rumble strips on two-lane roads to alert drivers when they drift into opposing lanes. Although rumble strips typically appear on corridors or segments of roadways, placement on approaches and through individual curves could prove effective in reducing roadway departures. (See also "Rumbling Toward Safety" in the September/October 2003 issue of Public Roads or "The Sound of Safety" in the January/February 2009 issue.)
Minor Roadway Improvements
Although reconstruction projects to flatten curves or correct pavement deficiencies such as superelevation are beyond the scope of the guide, there are a variety of low-cost physical changes discussed that can improve roadway departure safety.
Skid-resistant pavement surface. A wide range of materials are available to increase friction on pavements. These materials range from thin coatings that can be applied with hand-held tools to machine-laid friction courses. Grooving an existing pavement surface also can improve skid resistance. This practice is effective because it increases the friction between tires and road surfaces. According to NCHRP Report 500, improving the skid resistance at locations with high frequencies of wet-road crashes results in 50 percent fewer wet-road crashes and 20 percent fewer total crashes. The treatment also is effective at reducing skidding-related crashes that occur when pavements are dry.
Eliminate edge drop-off. This treatment is different in that it is not an independent project (it must be accomplished during the paving process) and there is virtually no cost. Edge drop-offs, the height difference between the paved and unpaved surface, usually result from erosion or wear from tires. A drop-off greater than 6.3 centimeters (2.5 inches) substantially increases the severity of a crash if the angle of the drop-off is near vertical. Once a vehicle drops a tire off the pavement, the driver might overcorrect, causing the vehicle to swing back onto the roadway and into oncoming traffic, or lose control and overturn.
Research conducted through the Transportation Research Board shows that a pavement edge with a 30- to 35-degree angle greatly improves the chances of a safe recovery, especially for larger drop-offs. Because the cost of this treatment is simply the cost of the device to make the edge shape (around $2,000) and a minor amount of additional asphalt or concrete, DOTs typically choose to apply the treatment to the entire corridor being paved. They also can apply this procedure when adding or paving shoulders at horizontal curves. (For more information, see "The Low-Cost Dropoff Solution" in the September/October 2007 issue of Public Roads.)
Widen shoulders on curves. Shoulders vary in width and surface type. Although engineers design them to accommodate stopped vehicles and to provide side support for the pavement, shoulders also are a safety feature. They provide drivers with safe, flat spaces to recover from roadway departures or to get out of the travel lane if they need to stop. The crash reductions expected from widening shoulders depends greatly on the change in width and on traffic volumes. Although the cost can be prohibitive on an entire corridor, widening at the curves, where vehicles typically need more space, can provide excellent benefits at relatively low cost.
Pave existing shoulders on curves. Agencies can pave turf, gravel, or other unpaved shoulders to increase the friction and smoothness of existing shoulders to provide drivers the opportunity to correct and navigate back to the roadway. Variations in surface color or texture also can provide visual and audible warnings to drivers straying from travel lanes.
Experimental Treatments
Using devices that have not been adopted in the MUTCD requires permission to experiment from FHWA. For example, FHWA has approved optical speed bars and curve advance markings for experimental use by certain agencies. Other agencies that wish to use these treatments must apply individually to FHWA for permission to experiment.
Optical speed bars. Transverse stripes spaced at gradually decreasing distances give drivers the illusion that they are traveling faster than they are, which ideally causes them to slow down. Optical speed bars are best used on road segments where vehicles traveling at high speeds need to slow to navigate curves or other situations. Although the treatment is still considered experimental, a pooled fund study has demonstrated enough success that transportation officials are proposing adding optical speed bars to the MUTCD.
Curve advance marking. The Pennsylvania Department of Transportation (PennDOT) is testing a pavement treatment that consists of two transverse bars, a SLOW legend, and the arrow marking designated for turn lanes to indicate the direction of upcoming curves. The intent is to minimize run-off-road crashes by reducing the upper percentile speed where two-lane roads have high numbers of curve-related crashes. "Preliminary findings with this treatment applied at more than 400 curves indicates a decrease in crashes of 33 percent," says Gary Modi, chief of PennDOT's Safety Management Division.
Maintenance Treatments
Maintaining roadways and their traffic control devices is key to ensuring that drivers can recognize and respond to information in the roadway environment. Regardless of whether treatments discussed here are used, road agencies should conduct routine maintenance to re-stripe pavement markings and replace signs to ensure they remain visible both day and night and meet current standards. Crews should cut foliage to ensure adequate sight distance through curves, especially during the growing season; perform regular shoulder maintenance; and improve drainage around curves to keep pavement dry.
Safety First at FHWA
Maintaining U.S. roadways, their environment, and traffic control devices is an ongoing challenge, but foremost is a commitment to improve safety. Low-Cost Treatments for Horizontal Curve Safety provides practical information on where, when, and how to apply low-cost improvements or design features. Although the guide targets local and tribal highway agencies, State DOTs as well will find it useful to identify alternative treatments for safety problems at horizontal curves.
Cathy Satterfield, P.E., is a safety specialist on the Roadway Departure Team in FHWA's Office of Safety Design. She earned her B.S.C.E. at the University of Minnesota Institute of Technology, is registered as a professional engineer in Idaho, and served in a variety of field positions in her 18 years with FHWA before bringing that perspective to headquarters.
Hugh W. McGee, Ph.D., P.E., is a principal engineer with Vanasse Hangen Brustlin, Inc. A three-degree graduate of The Pennsylvania State University, he has conducted numerous research studies of highway and traffic safety issues for FHWA and the National Cooperative Highway Research Program over the course of his 40-year professional career. He currently serves as the principal investigator of a multiyear technical support contract to FHWA's Office of Safety.
Fred R. Hanscom, P.E., is director of the Transportation Research Corporation in Markham, VA. He has an M.S.C.E. degree and more than 35 years of experience in highway safety research. He has directed or provided significant consulting on 26 FHWA research projects.
Low-Cost Treatments for Horizontal Curve Safety (FHWA-SA-07-002) is available from the FHWA Report Center. To order, e-mail report.center@dot.gov. To download an HTML version, visit the FHWA Safety Web site at http://safety.fhwa.dot.gov/roadway_dept/horicurves/fhwasa07002/ or download a PDF version of the document at http://safety.fhwa.dot.gov/roadway_dept/horicurves/fhwasa07002/fhwasa07002.pdf.
For more information, contact Cathy Satterfield at 708-283-3552 or cathy.satterfield@fhwa.dot.gov.