New AASHTO design guide only one among current issues and trends to follow as highway, road, and bridge industry picks up steam next year.

A new pavement design manual by the American Association of State Highway & Transportation Officials is near implementation, and will change profoundly the way roads and highways are planned and executed in coming years.

It’s just one of a number of exciting trends in asphalt and concrete, innovative materials, robotics, transportation and the environment, bridges, and motorist and worker safety reviewed by Better Roads in this special edition of Road Science. Please join us for this overview of developments for 2005 in soft-wheel transportation in the following pages.

New Design Guide:
Mechanistic-empirical philosophy underlies new pavement guide.

A new pavement design manual — years in the making — is very near implementation by AASHTO and will profoundly impact the way pavements are designed for decades to come.

The design guide — the 2002 Guide for the Design of New and Rehabilitated Pavement Structures — represents a major change in the way pavement design will be performed, and in a way, is an extension of the Superpave system of performance-based mix design specs to all other pavements.

Now available for evaluation is a near-final version that includes the design guide itself, and user-oriented computational software and documentation based on the design guide procedure.

The existing 1993 edition of the AASHTO Guide for Design of Pavement Structures is based on empirical equations derived from the famous, but outdated, AASHO Road Test. This test conducted performance testing between 1958 and 1960 of a limited number of structural sections at one location, Ottawa, Illinois, and with much-reduced traffic levels compared those of the 21st century.

Under the new design guide, a designer of any pavement must first consider site conditions such as traffic, climate, subgrade, existing pavement condition for rehabilitation, and construction conditions in proposing a trial design for a new pavement or rehab. Then, using the software, the trial design will be evaluated through prediction of key distresses and smoothness. If the trial does not meet the demanded performance criteria, the pavement design must be revised until it does.

The mechanistic-empirical format of the design guide adapts it to evolution in truck loads, materials, construction techniques, design concepts, and even computerization. It’s a forward-looking methodology that will take the industry away from the cookbook or recipe specifications and design methods of the Ottawa tests, and into a future that molds design to anticipated performance.

In late 2004, the U.S. DOT announced that in June 2004, safety belt use in the United States reached 80%, the highest level yet recorded.

Mechanistic-empirical are fancy words that describe a very simple concept. Mechanistic refers to the interaction between the materials and structure of a pavement, and how it stresses and strains under load deflection. The principle relates these pavement mechanics to empirical or experimental performance data obtained in field or lab.

The guide uses mathematical models to describe this relationship, and the primary basis for all mechanistic-based pavement performance predictions methods is cumulative axle load applications.

“The benefit of a mechanistic-empirical approach is its ability to accurately characterize in situ material (including subgrade and existing pavement structures),” says the Washington State Department of Transportation in its online tutorial. “This is typically done by using a portable device to make actual field deflection measurements on a pavement structure to be overlaid. These measurements can then be input into equations to determine existing pavement structural support (often called backcalculation) and the approximate remaining pavement life. This allows for a more realistic design for the given conditions.”

The official download page for the design guide, software, and climatic data is www.trb.org/mepdg/home.htm. The near-final review guide is available for inspection and review only in an online .pdf version, but in a configuration that is read-only, non-save, non-printable, and non-editable. See it at www.trb.org/mepdg/guide.htm. A review copy of the software may be downloaded at www.trb.org/mepdg/software.htm.

The current products available for review are described in a four-page flyer, NCHRP Research Results Digest 290: Recommended Mechanistic-Empirical Pavement Design Guide and Software (Phase II) Available for Evaluation. Review it at http://gulliver.trb.org/publications/nchrp/nchrp_rrd_290.pdf.

AASHTO’s June 23, 2004 memo on the same subject is available at www.trb.org/mepdg/AASHTO_memorandum.pdf. Washington State DOT’s tutorial is available at http://hotmix.ce.washington.edu/wsdot_web/.

Petroleum Through Roof:
Higher oil prices will impact the price of asphalt overlays.

Unprecedented world demand for petroleum — combined with international turmoil and natural disasters — is driving the cost of petroleum higher, and that will impact the price of liquid asphalt used in road construction and maintenance, and likely will impact highway trust fund receipts.

Oil prices have risen nearly 70% this year, and briefly topped $55 per barrel in October.

This year’s $55-per-barrel peak is shocking to observers, and is fodder for alarmist broadcast news, but the industry has survived such high levels before. The October 2004 price spike put the price at the same level, in inflation-adjusted August 2004 dollars, as in October 1990, in the run-up to the first Gulf War. And during the Carter administration, during the Iran Hostage Crisis (1979-80), prices shot up to $81 per barrel adjusted to today’s dollars, but plunged to 1960 price levels within four years.

What’s different now is that some of the latest price increase may be caused by new, permanent changes in demand. The economies of China, India, and other Asian nations are growing and moving inexorably to a consumer basis, which prefigures the kind of widespread prosperity enjoyed in the United States and Canada. As their taste for consumer goods rises, so will automobile ownership, and that means all-new customers for oil are coming on the world market, driving and keeping prices higher.

Also putting pressure on world oil prices are economic recovery in the U.S., uneven production from Iraq, threatened strikes in Venezuela, Nigeria, and Norway, and particularly for the United States, the onslaught of hurricanes in the Gulf of Mexico region earlier in 2004.

And even though it’s not at record levels in 2004 dollars, the rising price of oil nonetheless is boosting liquid asphalt prices. “The almost daily records being set for crude oil prices strongly suggest that diesel fuel, asphalt, other materials that have a petroleum component, and freight charges will keep rising,” said Ken Simonson, chief economist, Associated General Contractors, in October.

An average of liquid asphalt prices per ton, recorded by the Asphalt Pavement Association of West Virginia, records this price spike, rising from $157 per ton in January 2004 to $197 per ton in September and October 2004. While many state DOTs have asphalt price escalator clauses in contracts which do not penalize contractors as asphalt prices rise, nearly all governments work with fixed budgets each year. And that potentially means less money to go around for paving and maintenance projects.

Natural Wonder:
Surface transportation directly drives environmental progress.

Roadbuilding and traffic conventionally are decried as a detriment to the environment. But a simple study of today’s road programs shows that, without question, the United States’ surface transportation program is driving improvements to our nation’s environmental quality.

America’s highway construction and reconstruction industry has by far become the No. 1 recycler of waste materials in terms of tonnage, saving tax money and reducing demands on landfills, quarries, and gravel pits, prolonging those resources.

The United States has seen tremendous improvements in air quality, much of it the result of transportation-related improvements. Since 1970, total national emissions of the six most common air pollutants have plunged 25%, reports the Environmental Protection Agency. Significantly, this improvement in national air quality has occurred even while — during the same three decades — the U.S. Gross Domestic Product increased 161%, energy consumption increased 42%, and vehicle miles traveled increased 149%.

Contractor Blythe Construction, Inc., Charlotte, is ramping up its promotion of warm asphalt mix technology in the mid-Atlantic states; here it paves Old Statesville Road in Charlotte.

The problem is, nearly three-quarters of the American public believe air quality has either deteriorated or stayed the same, according to an August public opinion poll and a new study of government data.

“There’s a clear disconnect between the nation’s significant emission reduction progress and public perception,” said Bill Fay, president of the Foundation for Clean Air Progress, which commissioned the study from Meszler Engineering Services, and survey from Wirthlin Worldwide, respectively.

The Clean Air Act sets health-based National Ambient Air Quality Standards for the six most dangerous pollutants: nitrogen dioxide, sulfur dioxide, lead, carbon monoxide, ozone, and particulates. The EPA’s own monitoring data show Americans are breathing far healthier air than was the case in 1970. According to the EPA’s most recent data, collected in 2001-2002:

• The health-based standards for nitrogen dioxide and sulfur dioxide were attained everywhere in the U.S.

• The standards for lead and carbon monoxide were attained in 3,129 of the nation’s 3,132 counties.

• 70% of the nation’s population now breathes air that meets the original NAAQS for ozone.

• Exposure to particulates has been significantly reduced.

However, according to the Wirthlin Worldwide survey, seven out of 10 Americans believe overall air quality has either diminished or stayed the same. “With so many Americans unaware of the dramatic air quality improvements, what we’ve got is a clean little secret,” Fay said.

Better Roads’ research shows that the roadbuilding industry is directly involved in improving the natural environment. For example, the Federal Highway Administration reports that some 5% to 20% of a highway project cost is invested in environmental elements.

Sometimes the industry far exceeds legal requirements. For example, in 2002, our federal surface transportation Wetlands Mitigation Program was creating nearly 2.7 acres of wetlands for every acre taken for road construction, an astonishing achievement. The requirement is 1 acre created (or mitigated) for every acre taken.

Antifreeze Concrete:
Conventional admixtures can lengthen the PCC season.

Using off-the-shelf admixtures, the portland cement concrete construction season could be expanded well into the winter or year-round all across the continental U.S. and Alaska, reported Charles J. Korhonen of the U.S. Army Corps of Engineers earlier this year.

Korhonen, a research civil engineer in the COE’s U.S. Army Cold Regions Research and Engineering Laboratory, said PCC using commercial admixtures was successfully placed in Wisconsin and New Hampshire during subfreezing temperatures without heated enclosures, producing savings of 20 to 90% in materials and placement costs.

Korhonen’s project required off-the-shelf admixtures, and application at 23 degrees F or lower, with adequate strength, constructability, and economy, he said. “CRREL evaluated combinations of commercially available admixtures to depress the freezing point of water and to accelerate the hydration rate of cement,” Korhonen said. “Previous research showed that no single admixture in recommended amounts could provide enough freeze protection to meet the low-temperature requirement.”

Current practices limit the amount of each admixture to concrete, he said. “No limits apply, however, to the number of admixtures in a single batch, and concrete with more than one admixture is not uncommon,” Korhonen said. “CRREL researchers produced candidate formulations by combining several commercial admixtures that met standards, all within the recommended dosages,” he said. “The formulations were evaluated under controlled laboratory conditions, to identify the admixture combinations that could accelerate curing, ensure workability, provide adequate freezing point depression, and not harm the freeze-thaw durability.”

Laboratory tests indicated an appreciable strength gain for the candidate formulations at 23 degrees F with no adverse effects on durability. The formulations were then evaluated in field tests, with the demonstrations of the antifreeze concrete technology taking place at five locations in Wisconsin and New Hampshire.

In one project in Rhinelander, Wisconsin, in a classic full-depth slab repair, a section of pavement 10-feet wide by 22-feet long by 10-inches deep was removed and replaced with antifreeze concrete in below-freezing temperatures. The freshly placed concrete was covered with a layer of plastic to minimize moisture loss, and an insulation blanket was laid over the plastic to speed up the strength development but not freeze protection. “Two years after construction, the antifreeze demonstration sections show no signs of distress,” he said.

At Rhinelander project, the antifreeze technology reduced the cost by nearly 20% compared with the cost of using a heated enclosure. “The savings on other projects were greater,” he said. “For example, one New Hampshire project realized a 90% reduction in materials and placement costs with antifreeze concrete instead of regular concrete under conventional heated enclosures.”

For further information contact Korhonen at CRREL, 72 Lyme Road, Hanover, New Hampshire, 03755, or e-mail Charles.J.Korhonen@erdc.usace.army.mil.

Robotic Work Zones:
Robot traffic barrels keep workers from harm’s way.

Construction work zones will get a good deal safer if Dr. Shane M. Farritor gets his robots in the road. He’s advancing research in self-propelled orange barrels for use in slow-moving maintenance operations, and later in active work zones.

“We’ve completed Phase I and are working on funding for Phase II,” Farritor told Better Roads. “We are building more robots and one of the more exciting applications will be robots following slow-moving maintenance operations. We are working to maintain a wedge of nine barrels for lane closures or pavement sweeping.”

Farritor, an assistant professor in the Department of Mechanical Engineering, University of Nebraska-Lincoln, worked with research assistant Mark E. Rentschler in the initial work, a project of the National Cooperative Highway Research Program Highway IDEA program.

These self-propelled barrels look like conventional barrels, but have a robotic three-wheeled base. Farritor’s plan is for the barrels to be delivered to the roadside by a specially equipped truck, from which an operator controls their deployment using a laptop computer. Each fleet of robots is made up of a motorized lead robot, or shepherd — which is equipped with a ground-penetrating-sonar receiver — and less-expensive motorized barrels. A camera on the truck would send an image of the pavement to the laptop, and an operator directs the barrels by indicating on the computer screen where the barrels should go. The barrels can also move on their own.

The motorized base of each unit has two electric motors which are powered by a 12-volt lead-acid battery. These drive two 7.9-inch-diameter wheels, which permit the barrels to move in any direction. They can move at a speed of 4 feet per second.

The software then calculates the GPS coordinates for the location where the shepherd ought to be placed, and this location is sent to the shepherd via a radio link. The shepherd unit moves into position, then informs the other barrels by radio where to go. These slave units use dead reckoning, such as counting how many times their wheels turn, for instance, to work out their position. Finally, the smart shepherd confirms that its charges are correctly positioned by using a laser-based radar system to correct errors. This laser system also can evaluate if a barrel keeps moving out of place; it can move it out of the operation and close it down.

Phase I prototypes cost about $700 each, but Farritor feels that can be reduced to $200 by using less expensive motors, making them cost-effective if destroyed by a vehicle hit.

Independent, autonomous barrel motion has several advantages, Farritor said. First, the barrels can self-deploy, eliminating the dangerous task of manually placing barrels in busy traffic. Second, the barrel positions can be quickly and remotely reconfigured as the work zone changes. Barrels could continuously follow work crews to maintain optimal placement for safety.

Farritor says the barrels are only the first component of an overall smart work zone system, made up of signs, cones, and perhaps barricades and arrestors.

Farritor’s initial research may be downloaded off the University of Nebraska Web site at http://robots.unl.edu/Files/Papers/2002/IMECE_2002.pdf. A 10-second video clip of five robotic barrels aligning themselves into a wedge may be viewed at http://robots.unl.edu/projects/current/barrel_robots/animation_11_02/2fps.avi.

Continental Drift:
The U.S. explores materials, tests, and specs from Europe.

The European continent always has fostered a closer working relationship between contractors and government agencies than would be acceptable in the United States, and one result is a more adventurous approach toward new materials, products, and end-result specifications.

Now, a report hot off the press elaborates materials advances that a public/private team from American government transportation agencies and industry found during a materials scanning tour of the United Kingdom, Denmark, Germany, and the Netherlands. Sponsored by the FHWA, AASHTO, and the National Cooperative Highway Research Program, the investigators learned how we might improve our domestic highway infrastructure by optimizing the procedures for introducing, approving, and specifying new and innovative materials and products for highway construction.

The Superior Materials, Advanced Test Methods and Specifications in Europe Scanning Tour project is one of more than 50 international scans conducted since 1990.

This scan defined superior materials as those materials and manufactured products that significantly improve performance, are cost-effective on either initial or life-cycle bases, improve motorist or worker safety, or reduce construction time for new, reconstructed, or rehabilitated facilities, including repair and preservation.

Most of Europe is transitioning from methods-based specs and are adopting more functional specifications or requirements for construction materials, the tour found. “Functional specifications are similar to end-result specifications used in the United States, but tend to incorporate elements of a performance specification.”

Europeans use warranties and performance contracts as part of everyday practice, the tour members reported. The specific elements range from short-term (1- to 3-year) materials and workmanship warranties, to long-term (more than 30-year) design-build-finance-operate contracts.

Accelerated load testing and field testing were used to evaluate superior materials for both properties and performance, the tour found, through national testing laboratories with requisite equipment and facilities, especially for the accelerated testing. Some European testing methods that have been used in the U.S. which deserve a second or closer look include:

• Torque bond test.

• Stripe wear.

• Automated raveling assessment.

• Hot-mix asphalt microscopy.

• Polymer content evaluation.

• Pulse (active) thermography.

The Americans saw a selection of materials that met their definition of superior. Like the test methods, a number of these materials already have been introduced in the United States, they said, but the team feels that some show promise for widespread use on these shores. These include:

• Noise-attenuating pavements, including porous asphalt pavements, twin-layer asphalt, and Helmholtz resonators, which are carefully engineered voids cast into a pavement which serve to dampen sound induced by excessive air pressure created as a tire rolls over the pavement.

• High-friction surfaces.

• Waterproofing orthotropic decks.

• Low-temperature asphalt mixes.

• Semi-flexible asphalt.

• Composite pavements.

• Fiber-reinforced concrete inlays.

• Rapid concrete repairs.

• Dynamic road marking.

You may download their report at http://international.fhwa.dot.gov/superiormaterials/superiormaterials.pdf.

Warm Asphalt Mixes:
A North Carolina contractor promotes new HMA technology.

Blythe Construction introduces warm-mix asphalt modifier into hot-mix plant from blue hopper.

Contractor Blythe Construction, Inc., Charlotte, is ramping up its promotion of warm asphalt mix technology in the mid-Atlantic states, following a successful national demonstration at World of Asphalt ‘04 in Nashville in March. The continuing promotion of warm-mix technology from Blythe and competing providers makes warm mix a trend to watch for 2005.

Both the new warm-mix asphalt processes — and their sibling, conventional cold asphalt mixes — will get closer looks by governments as regulators and citizens clamor for low-emission asphalt mixes as part of regional clean air programs in non-attainment areas.

Also, like the so-called antifreeze concrete described earlier, warm asphalt mixes have the potential to expand the construction season into winter months, because temperature loss in transit is not critical to the mix’s performance. However, they cost more than conventional mixes. That’s because warm mixes use sophisticated, proprietary modifiers to allow them to be produced and worked at significantly lower temperatures than conventional hot-mix asphalt, well-below the 300 degree F-plus temperatures of conventional mixes (see Warm Mixes are a Hot Topic, June 2004, pp 30-41).

Blythe — a unit of The Hubbard Group, which is introducing Aspha-min warm mix modifier to North America — placed warm mix on Old Statesville Road in late summer in a project for the city of Charlotte. “We did the municipal project, and plan to follow up with a state job,” Blythe’s Sandy Whitaker told Better Roads.

Blythe’s Aspha-min is a processed sodium-aluminum-silicate crystal (a zeolite) that has been modified to contain 21% water, which is released during processing. The water lubricates, so to speak, the mix to enhance workability and compaction of the mix at lower temperatures.

Aspha-min is one of four warm-mix technologies that are vying to make state DOT specs. The others are WAM Foam, in which two components comprise a system that introduces a soft and hard foamed binder at different points during production; Sasobit, a synthetic paraffin wax which reduces the viscosity of the binder at mixing and compaction temperatures; and Asphaltan B, a low molecular weight esterified wax. All four processes work by reducing mix viscosity.

A review of warm-mix performance already is underway by the National Center for Asphalt Technology for modifier suppliers and for the National Asphalt Pavement Association. Reports are anticipated early in 2005.

Cold asphalt mixes use asphalt emulsions to distribute liquid asphalt binder among component aggregates, and are popular in rural settings where distances between HMA plants and lower traffic volume may preclude. As emulsion water evaporates, the lift cures and the pavement is opened to traffic. Cold mix plants have a lower initial cost than conventional HMA plants, are more easily transported, and may be situated anywhere without EPA permits due to their lack of emissions. They also are amenable to creation of cold mixes with high percentages of reclaimed asphalt pavement.

In the future, as they become more common, warm mixes may encroach on the niche occupied by these ambient-temperature cold mixes. But both warm and cold asphalt mixes constitute new choices for government agencies as they plan their road programs in the decade ahead.

Precast Pavements
Indiana may join states studying precast slabs.

Precast pavement panel is cast on-site for Texas application.

New technologies such as self-consolidating concrete, gaining interest in 2005, can speed manufacture of precast pavement slabs and prefab bridge components.

Indiana likely will join states studying the applicability of use of precast concrete pavement to speed portland cement concrete pavement rehab, according to a June 2004 report from the FHWA.

The new report, Using Precast Concrete Panels for Pavement Construction in Indiana, describes the concrete repair options for states, and discusses how precast panels may work in Indiana (also see High-Performance Concrete Pavements Come of Age, April 2004, pp 42-55).

The report investigated the Texas PPCP and New York State precast Super-Slab methods in-depth, and compared to conventional cast-in-place concrete pavement.

“Although the PPCP and Super-Slab methods have a higher construction cost and the industry is not familiar with this new method, [the] methods possess the advantages of using precast concrete,” the Purdue University investigators say. “The speediness of the laying precast panels not only results in less traffic congestion and delays, but can lower the user costs significantly. In addition, the precasting is under better controlled environment. It leads to a more durable concrete and requires less maintenance.”

Contrasting Texas’ PPCP method with New York’s Super-Slab method, the precast Super-Slab is approximate half the size of precast concrete panel. With its precision of Supergrader, Super-Slab has better subbase preparation for warped slabs, they write. Nevertheless, due to the application of pre-tension and post-tension on PPCP’s concrete, PPCP can have thinner slabs and a more durable concrete in the long run.

“Moreover, [the] PPCP method is derived from  near 20 years laboratory and roadway experiments on cast-in-place post-tensioned method in Texas,” the investigators found. “The theories are sound and empirically proved. This method holds no patents. The detailed information and experimental data are well documented and accessible. It can be reasonably predicted that the unit cost of precast concrete panels will be lowered if the constructed lane length is getting longer and the method can be repeatedly used.”

The study concludes that it is feasible to use in Indiana DOT’s pavement construction. Among the various PCP methods identified in this study, PPCP methods posses many comparative advantages. They recommend that a demo project be undertaken to set the stage for future full-scale implementation.

Review the entire 90-page document at http://rebar.ecn.purdue.edu/JTRP_Completed_Project_Documents/SPR_2779/FinalReport/spr_2779_final/Form1700.pdf.

Outside the Beltway:
Seat belt use at an all-time high.

Seat belt use is at an all-time high, and it validates the wisdom of those who fought federal seat belt use legislation linked to denial of highway funds to those states not enacting such a law.

Such a federal law, strongly favored in the 1980s, would have denied, in the name of motorist safety, funds to states that could be used to improve highway safety. The success of state programs in the absence of federal push justifies rollback of other safety programs which punishes states which don’t implement federal guidelines, such as age-21 drinking laws.

In late 2004, the U.S. DOT announced that in June 2004, safety belt use in the United States reached 80%, the highest level yet recorded. This result is from the National Occupant Protection Use Survey, which is conducted annually by the National Center for Statistics and Analysis in the National Highway Traffic Safety Administration.

According to the U.S. DOT, the 80% usage rate will translate into 15,200 lives saved and a savings of $50 billion in economic costs associated with traffic-related crashes, injuries, and deaths every year.

A state or territory has a primary enforcement law if motorists can be ticketed simply for not using their belts. Under a secondary enforcement law motorists must be stopped for another violation, such as an expired license tag, before being cited for belt nonuse.

In June 2003, 18 states had primary laws, 32 had secondary laws, and one (New Hampshire) effectively had no belt law. In New Hampshire, it is legal for motorists over age 18 to ride unbelted. Primary enforcement laws took effect in Delaware and Illinois in July 2003 and in Tennessee in July 2004.

At press time, the complete report had yet to be posted, but a preliminary descriptive flyer will be very useful and can be downloaded at www.nhtsa.dot.gov/people/injury/Traffic-Safety-Facts/Research-Notes/SeatBelt-Use-2004/images/

Prefab Bridges:
Scanning tour brings prefab bridge concepts home.

Active research in prefabricated bridge applications in the United States will be augmented by a new scanning tour of European prefab bridge technology, completed in April.

The panel visited Japan, the Netherlands, Belgium, Germany, and France and held meetings and site visits with representatives of government agencies and private sector organizations. The countries were selected because of their known use of prefabricated systems. Visiting Japan was particularly important because of their seismic design requirements.

At completion of the scanning tour, the team had identified 35 bridge technologies that, in one or more aspects, were different from current practices in the United States. The scanning team considered all aspects of design, construction, and maintenance of bridge systems composed of multiple elements that are fabricated and assembled off-site, including foundations, piers or columns, abutments, pier caps, beams or girders, and decks. Bridges with span lengths in the range of 20 to 140 feet were the major focus, although longer spans were of interest if a large amount of innovative prefabrication was used.

Use of prefabricated bridge systems is assumed to minimize traffic disruption, improve work zone safety, minimize environmental impact, improve constructability, increase quality, and lower life-cycle costs.

This concept is not new to the U.S. For example, last year Texas researchers described how a new precast bridge design for off-system bridges incorporating precast concrete deck panels on concrete or steel I-beams can significantly reduce bridge closure times, while maintaining quality and practicality of construction. Elimination of the cast-in-place deck pour was emphasized.

The scanning team scheduled 17 presentations at national technical meetings sponsored by FHWA, AASHTO, and other organizations to disseminate information from the scanning tour. In addition, the team has formed a group to prepare Scanning Technology Implementation Plans for the technologies. An advance summary of the scanning tour’s findings in .pdf format may be downloaded at www.fhwa.dot.gov/ bridge/prefab/pbesscan.pdf.

All FHWA International Scanning Tour reports may be browsed and downloaded at http://international.fhwa.dot.gov/links/pubs.cfm.

Reprinted from Better Roads Magazine
December 2004

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