In 1963, former President Dwight D. Eisenhower predicted that the Interstate System "would change the face of America." Forty-five years and 42,800 miles after its launch, that prediction is a living reality.
In looking at the many innovations in road and bridge building the 20th century brought to this country, the Interstate System continues to stand at the forefront. In connecting the country, it cut travel time from coast to coast from more than 60 days in 1919 to a mere five days today. And in what has been described as the "greatest public works project in history," the program inspired such civil engineering achievements as the I-275 Sunshine Skyway Bridge in Florida and the I-70 Mountain Corridor in Colorado. The Skyway Bridge, with its concrete segmental cable-stayed span, has received accolades for not only its engineering expertise but its stunning visual design. Likewise, the I-70 corridor is at the same time an engineering marvel and a spectacular scenic byway in a highly sensitive mountain environment. For these and many other innovative answers to engineering challenges, the American Society of Civil Engineers cited the system as one of the "Top 10 Civil Engineering Achievements of the 20th century."
The Interstate System also has brought lasting safety benefits. It is estimated that the first 40 years of the system’s existence resulted in approximately 187,000 lives saved and nearly 12 million injuries avoided. Its many accomplishments in safety, efficiency and design have made it a model of infrastructure development the world over.
The building of the Interstate System and the attendant emphasis that was placed on road construction and maintenance in this country spurred highway agencies to try the many innovations that emerged from the Interstate construction. The use of cable-stayed bridge technology on the Sunshine Skyway Bridge and the I-310 Hale Boggs Memorial Bridge in Louisiana, for example, prompted other states to start adopting this new technology, where a continuous girder-type structure typically has one or two towers erected above piers in the middle of the span. From these piers, cables are attached diagonally to the girder to provide additional support. Cable-stayed bridges offer greater stiffness and lateral rigidity than do suspension bridges, making them more stable against wind.
States also were spurred to look at other emerging technologies that held the promise of building better, safer roads and bridges, resulting in benefits that have been dispersed well beyond the Interstate System. In the structures area, for example, another groundbreaking technology that has been more widely adopted is segmental bridge construction. Under this method, box girder spans, column supports and other bridge components are cast prior to assembly. Because this work occurs in a separate casting yard, construction can proceed concurrently, resulting in more rapid project completion with less disruption to the traveling public and the environment.
Additional achievements in bridge technology during the last century include the development of new high-performance materials, which are being used to build more durable structures that require less maintenance and fewer repairs. Bridge beams made of high-performance concrete (HPC) can be stronger and stiffer than those made with conventional concrete, allowing bridges to have longer spans and smaller or fewer structural components. And with fewer components, the bridge can often be built faster and with less concrete, thereby saving money on labor and materials. HPC bridge components also are less permeable than conventional concrete bridge components, making them more resistant to damage caused by freeze-thaw cycles, salts in seawater and deicing chemicals, and other environmental conditions. It is estimated that HPC bridges could have a useful service life of 75 to 100 years.
High-performance steel (HPS) also provides increased strength, as well as greater ease in welding. HPS has only half the carbon and one-tenth the sulfur content of conventional steel. The low-carbon content means that little or no preheating is needed to weld components together, which reduces both construction time and costs. HPS is tougher than conventional steel, resulting in a bridge that can better absorb the impact of traffic loads and has reduced susceptibility to fractures.
Other high-performance bridge materials increasing in popularity include fiber-reinforced polymer composites. These nonmetallic composites, which are typically made up of such fibers as glass, aramid and carbon, as well as polymer resin matrixes, are more corrosion resistant than conventional steel. They are lightweight and easier to handle and install, resulting in more rapid construction. The superstructure for the first all-composite vehicular bridge in the U.S., which opened to traffic in 1996 in Russell, Kan., was installed in one day. This compares to a typical installation that can take months.
Bridge maintenance also has benefitted from innovations. For example, the use of epoxy-coated rebar to protect concrete bridges from corrosion has enabled highway agencies to extend the service life of these structures and reduce life-cycle costs.
Pavement design breakthroughs
The advances in bridge technology have been accompanied by equally significant breakthroughs in pavement design. The end of the 20th century brought the debut of a completely new approach to asphalt mix design: the Superpave system.
The Superpave system’s binder and mix specifications have provided pavement designers with the tools to custom-design an asphalt pavement for the specific weather and traffic conditions at a particular jobsite. By custom designing mixes, pavement engineers can better prevent such pavement distresses as permanent deformation and low-temperature cracking. With asphalt pavements accounting for more than 90% of all paved highways in the U.S. and annual expenditures for asphalt pavements topping $10 billion, the nation stands to reap substantial benefits from the increased durability that the Superpave system can provide. Last year, it was estimated that Superpave pavements accounted for 47% of states’ total hot-mix asphalt road projects, up from a mere 1% in 1996.
Asphalt pavement construction also has benefitted from the shift over the past 25 years from batch asphalt plants to drum-mixer plants. This shift has resulted in increased quality, efficiency and cost-effectiveness, as well as higher production rates and more recycling of reclaimed asphalt pavement.
Efficiency and quality have been improved by the advent of more rapid testing methods, such as use of the nuclear density device, which measures the in-place density of hot-mix asphalt; the binder ignition oven, which can quickly and accurately determine the asphalt content and aggregate gradation of a Superpave mix sample; and lightweight, noncontact profilers, which provide enhanced smoothness control.
On the concrete side, one of the most significant advances in paving equipment was the development of the slipform paver, which allows highway agencies to lay continuous strips of concrete pavement.
Equipment innovations have increased safety. For example, concrete median barriers first began to be used in the 1940s and ’50s to prevent out-of-control vehicles from crossing over highway medians into oncoming traffic. While there are several different concrete median barrier designs, the most commonly used now is the New Jersey barrier. The barrier’s tapered concrete shape is designed to minimize damage to a vehicle by allowing vehicle tires to ride up on the lower sloped face of the device, thereby slowing down the vehicle and redirecting it back onto the highway.
A newer innovation that is saving lives is the use of shoulder rumble strips. These continuous bands of raised material or indentations are grooved into road shoulders to alert drivers who are starting to drift off the road.
Better quality, better environment
Advances in equipment and engineering and design practices have been matched by an increased emphasis on quality. In recent years, more and more states have started using a process known as constructibility reviews to improve their highway projects. This process means that when a state is developing a highway project, the transportation agency and the contractor review the project’s features and look for ways to improve quality, foster innovation and control costs. These reviews save both time and money, but most importantly they result in less disruption to motorists. Value engineering is a similar process where agencies and contractors can review proposed project designs and make value-added changes that do not alter the function of the original design or operation.
Environmental analysis and protection also have taken a leap forward. More environmentally friendly equipment now in use includes pavers with pollution controls. Recycling has gained a foothold, with highway agencies and contractors now using such recycled materials as crushed concrete, reclaimed asphalt pavement and glass and brick as substitute aggregates in pavements. In addition, states are finding success using crumb rubber as an asphalt modifier and fly ash for concrete and soil stabilization.
Concern for the environment is changing the way roads are designed, with more states adopting a context-sensitive design approach that emphasizes the preservation of environmental, community, scenic and historic resources. Highway agencies have increased their efforts to mitigate wetland losses caused by highway construction and maintenance activities. And concern over soil erosion and sedimentation resulting from highway construction projects has led to the development and use of improved erosion control devices, including such innovations as geotextile filter bags, which trap silt, and triangular silt barriers, which can be used to remove suspended soil particles from drainage water.
A new century
The Interstate System is essentially complete now. But with $1 trillion invested in all highway and bridge infrastructure nationwide, our biggest challenge in the 21st century is preserving and maintaining the quality of our national investment. To help meet this challenge, the Federal Highway Administration (FHWA) and state departments of transportation developed the concept of a national highway system (NHS) in the early 1990s as a way of focusing federal resources on the country’s most important roads. While the more than 160,000 miles of the NHS comprise only 4% of the nation’s roads, these routes carry more than 40% of all highway traffic, 75% of heavy truck traffic and 90% of tourist traffic. FHWA is committed to working with our many partners in the transportation community, including state DOTs, industry, city and local highway agencies, academia and the Transportation Research Board, to ensure that we continue to build upon the successes of the last century and invest in the technology innovations that hold the promise for our future.