Pavement Preservation Compendium II
Research Frontiers in Pavement Preservation
by Roger M. Larson, Larry Scofield, and James B. Sorenson
In the past, the major emphasis in the area of pavement was on structural design - project specifications addressed the issues of material quality. Today, material properties are being tied directly to structural design and distresses. The surface characteristics that contribute to good functional performance, however, often are ignored until problems develop.
Surface characteristics have gained significance with the shift of focus from new construction and major rehabilitation to pavement preservation. But despite the increasing use of preservation treatments on pavement sections in good structural condition, many state highway agencies still have no specifications for the improved functional performance of the pavements.
Functional performance is determined by how well the pavement serves the user. Until now, riding comfort - a concept developed in 1957 - had been the dominant concern. Today the greater need is to improve other important functional surface characteristics of pavements.
Highway User Surveys
In May 1996, a national survey identified highway user concerns. Safety was first, followed by pavement condition, and then traffic flow. Highway users wanted an increased focus on the quality of roadway surfaces.
A follow-up infrastructure survey in 2000 found that highway users rated improvements to traffic flow, safety, and pavement condition as the highest priorities. The survey also discovered overall increases in dissatisfaction with safety and with pavement condition. These are findings that professional engineers can address.
The results again supported greater consideration of the functional characteristics of pavements. In terms of safety, concerns include pavement markings, friction in wet weather, and clearing accidents more quickly. Pavements need more durable surfaces, a smoother and quieter ride, and better surface appearance.
Good highway drainage is fundamental to increase surface durability by eliminating or minimizing potholes and extending service life. A good cross slope is important for surface drainage, improving ride quality, improving wet weather friction, and reducing splash and spray. Cross-slope deficiencies should be corrected as part of any pavement preservation project.
Because durability affects all other pavement characteristics, higher-quality materials and better workmanship are necessary for cost-effective construction and preservation. Greater attention to materials and workmanship would reduce deterioration and minimize rutting. With current staff reductions at many highway agencies, increased use of warranties, guarantees, or performance-related specifications can help ensure more durable pavement surfaces for highway users. Improved guidelines and incentives for obtaining desirable - not minimal - levels of critical surface characteristics are necessary.
Periodic distress surveys are a means of evaluating surface durability - lack of distress indicates durability. Established warning levels of texture and friction can identify potentially hazardous locations before significant numbers of crashes occur. Cost-effective corrective actions can be undertaken as appropriate.
Most highway users can relate to ride comfort as a criterion for pavements. Several recent publications have addressed the research under way to improve guidance for pavement smoothness. Improvements are needed in the measurement and evaluation of overall smoothness, the detection of bumps, and the identification of roughness that would increase dynamic loading impacts from trucks.
Texture, Safety, and Noise
Pavement texture is often overlooked in project specifications. Many state highway agencies have no requirements for texture or friction on paved asphalt surfaces.
Specifying friction above minimum levels can raise liability concerns. However, considerable evidence shows that higher levels of texture and friction significantly reduce fatalities and injuries - and the resulting traffic delays - and can be cost-effective for congested routes and for work zones. Improved guidance on the desirable macrotexture to reduce splash and spray and hydroplaning and on the microtexture to increase friction at low and high speeds is needed. National Cooperative Highway Research Program Project 1-43, Guide for Pavement Friction, is under way to address this concern.
Safety in work zones - reducing deaths, injuries, and traffic delays - also was a concern for highway users. In 2002, 1,083 highway workers and users were killed in highway work zones. This critical area has few guidelines on texture or friction characteristics, particularly in work zone transitions, which involve lane changing, slowing, or stopping. The demand for friction, therefore, is greater than it is in typical roadway operations. Increasing the texture or friction would have a significant effect in reducing the stopping distance, which would be expected to reduce crashes in highway work zones.
The American Association of State Highway and Transportation Officials (AASHTO) has developed a comprehensive Strategic Highway Safety Plan to reduce highway fatalities by 5,000 to 7,000 annually. Eight or more states are piloting an Integrated Safety Management Process to help implement the plan. Most of the emphasis in safety-related pavement research has been on wet-weather crashes - however, up to 86 percent of all crashes occur on dry roadways. The assumption has been that friction on dry roadways was adequate; however, friction has a significant effect on stopping distance, which can be expected to reduce crashes from roadway departures and intersections.
The Federal Highway Administration (FHWA) also has set safety goals for the next 10 years, including the following performance measures:
- 20 percent reduction in fatalities,
- 20 percent reduction in injuries,
- 50 percent reduction in truck-related fatalities, and
- 10 percent reduction in fatalities at intersections and in roadway departures by 2007.
The prevention of all wet-weather crashes would not achieve these goals. Therefore, a comprehensive program is necessary. Research indicates that up to 70 percent of wet-weather crashes could be prevented with improved texture and friction. A recent study in New York reported that at 40 intersections with high crash rates and low friction values, accidents were reduced an average of 61 percent after the approaches were given a more skid-resistant surface.
More than 3 million crashes occurred at intersections in 2002, causing nearly 9,000 deaths and 1.5 million injuries. Since wet-weather crashes represent about 14 percent of all crashes, improved skid resistance could result in a 10 percent reduction in fatal and serious injuries from crashes and also could reduce travel delays.
The effect of increased texture and friction on reducing crashes on dry roadways also must be considered, however. None of the AASHTO or FHWA goals specifically target the expected overall benefit of increased texture and friction on reducing fatalities, injuries, and the resulting traffic delays; pavement skid resistance, however, is among the topics under roadway departure.
Corresponding performance measures are needed - for example, average macrotexture depths that can be measured continuously at highway speeds - to help monitor whether texture and friction levels on the network are increasing as a result of construction or preservation activities. An analysis of friction and texture versus average crash rate by major roadway classifications would demonstrate more clearly the benefit of increased texture and friction on reducing fatalities and serious injuries. The lack of an accident reduction goal linked to increased texture and friction and a corresponding performance measure to monitor progress is a deficiency to be addressed.
Texture also affects noise. Reduced tire-pavement noise levels will benefit highway users, as well as adjacent property owners. Specifying desirable noise levels has received little emphasis even in noise-sensitive projects in urban areas. Therefore, completed projects have had large variations in noise levels, and the monitoring of noise levels on constructed projects has been limited.
The Arizona Department of Transportation (DOT) has one of the most comprehensive studies under way to evaluate pavement texture characteristics - both friction and noise - on representative surface types in approximately 200 pavement preservation test sections. A goal is to develop ranges of texture-and the resulting friction and noise levels-for a variety of preservation treatments. In addition, both Arizona and California DOTs are pursuing quieter pavement surfaces to reduce noise at the source.
Specifications for a uniform, pleasing surface appearance have received little attention. The FHWA Federal Lands Division, however, has made this a major issue on the projects it administers for the National Park Service. Spot grinding to remove bumps can produce differential friction-differences in textures changing the skid resistance-and also can cause a nonuniform appearance. Surface repairs such as partial lane patches also affect both friction and appearance. Specifications should not reward corrective measures that result in poor appearance or that contribute to differential friction, which may increase skidding crashes.
Traffic markings are particularly important for visibility at night and in poor weather. Sixty percent of roadway departure crashes occur during dark or reduced-light conditions. Excluding alcohol-related crashes, the nighttime crash rate is about twice the daytime rate. Improved durability in traffic markings is required-also important is that the markings do not increase the risk of skidding, particularly for motorcycles.
Rumble strips are being used successfully to warn drivers that the vehicle is departing from the roadway or crossing into an area with a potential for a head-on crash. These low-cost treatments have been effective in reducing crashes.
Technological advances have facilitated data collection and data analysis. In many cases, results are available in real time and presentable in either graphical or statistical formats for pavement management, maintenance management, or safety management systems.
These powerful tools can guide engineering decisions that extend the service life of highways and increase highway user satisfaction. Obtaining the greatest benefit for the highway agency, however, requires increased integration of all management systems.
New high-speed, nondestructive evaluation techniques are available or are in development that will help differentiate structural and functional pavement problems. A rolling-wheel deflectometer is in development that will allow continuous high-speed evaluation of the structural strength of asphalt pavements by monitoring pavement deflections. The instrument also would help distinguish top-down environmental cracking versus bottom-up structural fatigue cracking. In Texas and other states, ground-penetrating radar is being used to locate structural problems in pavements.
Advances in laser technology allow the measurement of a pavement surface macrotexture at highway speeds. This could minimize the need and expense for network-level friction testing. Laser sensors and the newly developed scanning lasers can improve evaluation of rutting, aggregate polishing, bleeding, surface raveling, and aggregate segregation of mixes at relatively low cost.
These tools can improve decision-making for pavement preservation. The techniques will help to improve the surface durability and will reduce the need for frequent, routine, or reactive pavement maintenance.
Portable devices, such as the circular track (or texture) meter and the dynamic friction tester, are available to evaluate pavement texture and friction values and to develop an international friction index. These stationary devices require lane closures for testing but allow a relatively quick comparison of surfaces. Arizona DOT, the National Center for Asphalt Technology, and others are using the equipment in studies.
Performance and Ride
The FHWA Office of Asset Management has initiated a project that uses pavement management systems to evaluate the performance of Superpave mixes. Many states have adopted the Superpave system and need to verify that the forecast benefits-including improved safety, durability, and longer service lives-are being achieved.
These evaluations should substantiate improvements to safety and to surface durability-two of the major concerns of highway users. FHWA's emphasis on pavement preservation also should lead to improved surface durability and should minimize the amount of routine or reactive maintenance of pavement surfaces.
The FHWA Pavement Smoothness Initiative has made significant changes in evaluating ride comfort-for example, adopting the International Roughness Index (IRI) as the standard measurement unit and using the lightweight laser profiler to monitor construction quality and to provide an initial value for monitoring long-term performance. A new effort is under way to develop bump specifications, including grinds or repairs, and to ensure that roughness does not cause dynamic loading by trucks that would increase the rate of structural damage to the pavement.
Few specifications address texture or friction. Texture is important to the friction and noise properties of the pavement surface. The few states that have guidelines typically address the minimum, not the desirable, values. No state has requirements that address the maximum or desirable noise levels for the various surface types.
The FAA guidelines for airport runways are a best-practice example that could be modified to address various highway pavement classes. The FAA guidelines address friction and texture for new construction and for maintenance activities, including desirable friction and texture for new surfaces, maintenance threshold levels, and minimum acceptable levels.
Texture affects both noise and friction and should not be considered independently. Texture and friction should be addressed specifically to reduce the current, unacceptable levels of 43,000 fatalities and 2.9 million injuries annually in highway crashes and to minimize the resulting traffic delays.
Refining the Tools
Technological advances are providing the tools to assist practitioners in developing more cost-effective pavement preservation strategies. The new technologies should enable researchers to develop cost-effective pavement guidelines that contribute to reducing fatalities and serious injuries and that also reduce noise impacts for highway users and adjoining property owners. Additional research must refine these tools further and introduce the advances into widespread use.
Larson is Senior Engineer, Applied Pavement Technology, Inc., Springfield, Virginia; Scofield is Research Engineer, Arizona Department of Transportation, Phoenix; and Sorenson is Construction and System Preservation Team Leader, Federal Highway Administration, Washington, D.C.
From TR News, September-October 2003, pp. 22-25. Copyright, Transportation Research Board (TRB), National Research Council, Washington, D.C. Reprinted with permission of TRB.