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Assessing and Managing the Ecological Impacts of Paved Roads 2 History and Status of the U.S. Road System INTRODUCTION The road system in the United States has evolved over time to a complex network of physical structures that include roads, bridges, and overpasses, all designed to carry an enormous amount of traffic. The system has been created and continues to be changed and maintained by an equally complex set of human systems, centered on a hierarchy of governmental agencies with their associated financial support. The road system provides unlimited access for millions of Americans. The current system of paved roads handles a volume of traffic on the order of 2.9 × 1012 vehicle miles per year,1 or about 8 billion vehicle miles per day (DOT 2003). The first section of this chapter presents a brief history of the system. The second section defines terms relevant to the system and their spatial patterns at different scales. The third section describes ownership and maintenance responsibilities. The fourth section assesses the current status of the road system, which is still undergoing modest growth and requires many resources to maintain, operate, and repair. A BRIEF HISTORY OF THE U.S. ROAD SYSTEM We provide this brief historical overview for three reasons: (1) to show that the layout of the current road system is unlikely to change 1 One vehicle traveling 1 mile constitutes a vehicle mile.
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Assessing and Managing the Ecological Impacts of Paved Roads dramatically and that most development will be done along the current spatial template; (2) to show that the road system is carrying an increasingly heavy load, thus increasing congestion; and (3) to show that increased maintenance is required because of the aging road system. As we point out later, maintenance provides opportunities for mitigating or reducing the adverse ecological effects of roads, and such opportunities should be taken advantage of. A large and extensive road system was already in place in the United States when cars became a major mode of transportation in the early twentieth century. The pattern of the system mirrored land uses and transportation corridors of the nineteenth century. Roads were narrow, primarily composed of dirt and gravel, and for the most part, followed existing topography. Before 1900, only 4% of the roads were paved, leading to poor and unreliable traveling conditions. Yet this system formed the template for the current system. Indeed, the road system has less than doubled in length since 1900, but the capacity has multiplied to accommodate an ever-increasing demand (Forman et al. 2003). The development of the road system occurred in distinct eras, paced in part by technological transportation developments and resource availability. Each era marked a distinct change in a suite of variables (public values, policy, and fiscal resources) that influence road development. The historical context for roads is an important consideration because history affects the current ecological effects of roads. For example, the designers of a modern interstate highway would be more likely to be sensitive to the hydrological and ecological effects of the project than the designers of a two-lane rural road built with county funds or 50 years ago without federal review. In addition, ecological impacts, environmental mitigation, and simple scale of the road surface area vary widely by road type. For example, depending on the scale of concern, an eight-lane interstate highway connecting major cities would have much greater fragmenting effects than a two-lane rural road. Early Roads and Turnpikes Early colonial routes were mostly natural surfaces intended to allow for the passage of wagons. These roads were built mainly to complement an extensive waterway transportation system. Roads provided local access and allowed the movement of people and goods where canals or
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Assessing and Managing the Ecological Impacts of Paved Roads other water courses were not viable. The roads and waterways rarely competed with one another. The federal government became involved in road construction to develop interior lands and a national postal service and to defend remote territories. Settlers purchasing land from the government generated revenue for the federal government to build roads. This revenue was an important source for road building and maintenance, especially in sparsely populated areas. In 1806, the federal government began its most ambitious project to date: construction of the National Road, also known as the Cumberland Road, from the Potomac River at Cumberland, Maryland, inland to the Ohio River at Wheeling, West Virginia. By 1831, revenue began drying up, completed sections of the Cumberland Road were turned over to the states, and the federal government halted all road funding. Local governments, however, oversaw all road construction and operation from the 1830s to the 1920s because the states had little interest in these activities. A major growth in immigration spurred western migration, and by the late 1880s, there were many established routes within and between cities, as well as established routes for interstate and transcontinental travel. Interest in improving roads began again in the late 1800s as bicycles proliferated and in the early twentieth century as cars became more common (Forman et al. 2003). The first response to improving roads was oiling of the naturally surfaced roads. Oiling was followed by paving, using an asphalt surface. Most of the paving was done on a small scale until World War I. After the war, the federal government greatly increased paving and road building to “get the farmer out of the mud.” The funding mechanism for the improvements was a tax on gasoline sales. By 1923, 33 states had imposed a gas tax, and by 1929, all states had imposed the tax. The federal government enacted the first national gas tax in 1932, later converting the tax on gasoline and other transport-related products, such as tires, into a dedicated trust fund for highways (Forman et al. 2003). Federal Aid Highways In 1893, with the creation of the Office of Road Inquiry in the U.S. Department of Agriculture, the federal government renewed its interest in road development. The first serious effort was in 1916 when the U.S.
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Assessing and Managing the Ecological Impacts of Paved Roads Congress created the Federal-Aid Highway Program, which continues as the basic program for federal support of highways. The Federal-Aid Highway Program specified that states receive federal funds subject to various conditions, one of which required the states to match federal funds dollar for dollar. The federal program also required states to create a state highway department that was technically oriented and managed in accordance with the principles of scientific management and administration. The state-level department was meant to serve as a partner in the federal program and was given sufficient authority to supervise the expenditure of the funds. Federal funds could not be passed to localities. Reliance on the states as the senior partners in the Federal-Aid Highway Program continues to the present, although local officials in counties and municipalities have played a stronger role since the early 1960s (Forman et al. 2003). Interstate Highway System A few intercity highways, such as the Merritt Parkway in Connecticut, and the New Jersey and Pennsylvania turnpikes were built in the 1930s through the 1950s, and a few major cities had some limited-access, divided, multilane highways. There was no national system of freeways, however, until 1956, when the U.S. Congress enacted a plan to build and finance the National System of Interstate and Defense Highways, now known as the interstate highway system, to serve auto, truck, and strategic military needs. The interstate system was to be 42,500 miles of four-lane divided highways with limited access throughout. Standard vertical and horizontal clearances were designed to support military vehicles, such as trucks carrying tanks. The federal government would pay 90% of the cost (Forman et al. 2003). The interstate highway system was considered complete in 1990 and could be enlarged only if a state used its own funds to build a road to interstate standards and then petitioned the federal government to have the route added. The post-interstate era began in 1991 with the passing of the Intermodal Surface Transportation Efficiency Act (ISTEA). This act and the following reauthorization are discussed in Chapter 5. For the past 20 years or so, the funding and management of the interstate highway system and the national highway system (NHS) are distinct, but their environmental approaches are similar.
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Assessing and Managing the Ecological Impacts of Paved Roads Present U.S. Road Network There are approximately 4 million miles of public roads in the United States and 8.3 million lane miles. The majority (76% of lane miles) of paved public roads in the United States are two-lane rural highways, and the remainder are urban and rural multilane roads. The road network is expanding slowly, having only 55,000 lane miles built between 1987 and 1997 (less than 0.2% increase per year). About 80% of the expansion to the system is from road widening. As discussed in Chapters 4 and 6, this mode of growth has implications for the types of methods and the data used for assessment and management of ecological impacts. DEFINITIONS AND CHARACTERISTICS OF ROADS Vehicle Miles of Travel The annual vehicle miles of travel (VMT) is a measure of the demand and use on the road system. Statistics such as annual VMT help transportation agencies plan for the future. The amount of VMT has been steadily increasing (Figure 2-1). By 2000, annual travel on the nation’s highways reached about four times the travel level in 1960, or an estimated 2.7 trillion VMT. FIGURE 2-1 Time course of number of vehicle miles traveled by year from 1960 to 2000, indicating travel on rural and urban portions of the highway system. Source: FHWA 2001b.
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Assessing and Managing the Ecological Impacts of Paved Roads Road use is increasingly made up of urban traffic. At some point in the 1970s, total VMT on urban roads exceeded that on rural roads (Figure 2-1). Urban roads and streets now carry about 1.7 trillion VMT, or about 61% of total VMT. Data from the past decade indicate that the rate of road use increased more rapidly than the rate of system growth in length. From 1990 to 2000, travel increased 28.9%, yet the total miles of roads in the United States increased by only 2.1%. The increased volume is attributed in part to the increasing urbanization of the country. As urban populations increased and urban boundaries expanded, urban travel increased 30.6%, and rural travel increased 24.9% from 1990 to 2000. During the 1990s, the largest increase (41.9%) in road use occurred on the principal arterial urban roads (not urban interstates) (Table 2-1). Road Density Less than half of 1% of the land area in the United States is covered by roads of all kinds, not including rights-of-way, parking lots, and driveways. The United States has 1.2 miles of roads for every square mile of land area, much less than many other developed nations. Japan has a road density approximately 4 times greater than that of the United States. Germany, France, and England have road densities 2.5 times greater than that of the United States, and those densities continue to grow even in Europe’s high-density environments. However, Canada has a density of only about 0.16 miles per square mile of land. Densities also vary greatly across the United States, the highest being in New Jersey and the lowest in Alaska. Road Surfaces Of the 4 million miles of roads in the United States, about 2.3 million miles (59%) are paved. There are two general classes of materials used for construction of pavement: flexible pavements (asphalt) and rigid (hydraulic cement concrete) pavements (AASHTO 1993, http://www.fhwa.dot.gov/infrastructure/materialsgrp/cement.html). Flexible pavements generally consist of a prepared roadbed or subgrade, subbase, base, and surface layer. In some cases, the base is used as a drainage layer, or the drainage layer may be part of or below the subbase. In contrast, rigid pavements have
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Assessing and Managing the Ecological Impacts of Paved Roads TABLE 2-1 Functional System Changes, 1990-2000 Functional System Rural VMT % Change 1990-2000 Urban VMT % Change 1990-2000 Total VMT % Change 1990-2000 % of Total Travel Inter state 270,315 34.5 397,288 41.3 667,603 39.4 24.1 Free ways/expressways — — 178,105 38.6 178,105 38.6 6.4 Principal arterial 249,137 41.9 401,237 18.9 650,374 27.4 23.5 Minor arterial 172,780 10.5 326,855 37.5 499,635 27.5 18.1 Major collector 210,496 9.9 — — 210,496 9.9 7.6 Minor collector 58,571 16.3 — — 58,571 16.3 2.1 Collector — — 137,008 27.5 137,008 27.5 5 Local 128,332 31 237,239 23.5 365,571 26.7 13.2 Total 1,089,631 24.9 1,677,732 30.6 2,767,363 28.9 100 Source: FHWA 2001b.
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Assessing and Managing the Ecological Impacts of Paved Roads only a subbase between the prepared roadbed and pavement slab. Another distinguishing characteristic of flexible versus rigid pavements is the distribution of traffic load over the prepared roadbed. Rigid pavements tend to distribute the traffic load over a wide area, and flexible pavements, made with pliable material, do not spread loads as widely and thus usually require the additional base layer and greater thickness for optimal traffic-load transmittal. Of the paved roads, approximately 95% use flexible pavement, and 5% use rigid pavement. Types of Roads The current road system consists of several types of roads placed into a functional class based on traffic volume and general use. Unless otherwise stated, the following information was taken from the Federal Highway Administration (FHWA 2001b). Roads are classified on the basis of the types of function provided. Functional types of roads that constitute the highway network include (1) interstate highways, (2) arterials, (3) collectors, and (4) local streets or access roads. These four major types of roads can be combined into groupings for administrative and funding purposes. One major grouping is the NHS, which comprises the interstate highways and a large number of the high-volume arterial roads. Although not exclusive, most of the considerations of ecological impacts that occur in subsequent chapters consider the impacts from the first three functional types listed above. As discussed in Chapters 3 and 4, ecological considerations such as environmental mitigation and simple physical scale vary by road type. Recognizing that a road does not serve traffic needs by itself is basic to the development of any logical highway system. Travel involves movement through a network of interrelated roads and streets. The movement channels through an efficient hierarchical system that includes lower-order roads that handle short and local trips to higher-order roads that connect regional and interregional traffic and longer trips. In addition to movement, access is a fundamental function of roads. The principal function of local roads, almost 70% of the total mileage, is access, whereas freeways limit access. Federal law requires functional designations of roads in urban and rural areas for funding purposes. This classification is done by state transportation agencies and is mapped and submitted to Federal Highway Administration (FHWA) to serve as the official record for the federal
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Assessing and Managing the Ecological Impacts of Paved Roads highway system. The distinction between rural and urban areas is made using federal census data to create federal-aid and urban-area boundaries (WSDOT 2003). Urban roads occur in a census area with an urban population of 5,000 to 49,000 or in a designated urban area with a population greater than 50,000. Rural roads are defined as any road not located within the urban-area boundary. High-Speed Limited-Access Highways The best known high-speed limited-access highway system is the current interstate highway system. Because of the need to accommodate heavy freight traffic, these roads are the most expensive to build and maintain. Other types of limited-access highways include some roads within the NHS or some state limited-access roads, such as the New York Thruway. These limited-access state highways are funded and administered under their own sets of legal standards. Despite important administrative differences, the committee determined that the differences in ecological impacts of different types of limited-access highways are minor. The interstate system accounts for only 1.2% of the nation's total miles of roadway (Table 2-2), yet interstate highways convey 24% of the annual VMT in the United States and 41% of total truck VMT, suggesting their importance to commercial transportation. These statistics also indicate the potential for the interstate system to deliver greater levels of contaminants to air and water than the total miles of interstate roadway would suggest. However, other factors suggest that the contaminant load is not simply proportional to VMT. For example, vehicles traveling at interstate speeds may emit some pollutants at a lower rate than vehicles operating on local streets. TABLE 2-2 Interstate Highway System—Key Statistics Interstate System Total Highway System Interstate System Share (%) Road miles 46,677 3,951,098 1.2 Lane miles 209,655 8,328,856 2.5 VMT (billion/yr) 667 2,767 24.1 Source: AASHTO 2002a. Reprinted with permission; copyright 2002, American Association of State Highway and Transportation Officials, Washington, DC.
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Assessing and Managing the Ecological Impacts of Paved Roads Other federal highway investment, in addition to the interstate highway system, is reflected in components of the NHS. The NHS consists of the many routes with such designations as U.S. 1 or U.S. 89. These roads may be high-speed limited-access highways, arterials, or collectors, depending on the location and configuration of the roadway. The NHS includes urban and rural roads that serve a wide variety of transportation functions. The NHS comprises 163,000 miles of roads, 46,477 of which are interstate highways. The NHS serves major population centers, intermodal transportation facilities, international border crossings, and major travel destinations. It includes the interstate systems, other rural and urban principal arterials, highways that provide access to major intermodal transportation facilities, strategic highway network connectors, and the defense strategic highway network (Figure 2-2). The NHS carries over 44% of the nation’s travel, but it makes up only 4% of the nation’s total public roadway miles (Table 2-3). Although more than 70% of NHS miles are in rural areas, almost 60% of VMT on the NHS take place in urban areas (Figure 2-3). FIGURE 2-2 Map of National Highway System in the 50 states, the District of Columbia, and Puerto Rico. Source: FHWA 2001b.
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Assessing and Managing the Ecological Impacts of Paved Roads FIGURE 2-3 Miles of road and use of roads in urban and rural portions of the National Highway System. Source: FHWA 2001b. TABLE 2-3 National Highway System Mileage and Travel in Rural and Urban Areas Rural Urban Total NHS Mileage Interstate 33,150 13,527 46,667 Other NHS 85,882 28,629 114,511 Total NHS 119,032 42,156 161,188 NHS Percent of Total Mileage Interstate 0.8 0.3 1.2 Other NHS 2.2 0.7 2.9 Total NHS 3.0 1.1 4.1 NHS Travel (millions VMT/yr) Interstate 270,315 397,288 667,603 Other NHS 224,340 333,335 557,675 Total NHS 494,655 730,623 1,225,278 NHS Percent of Total Travel Interstate 9.8 14.4 24.1 Other NHS 8.1 12.0 20.2 Total NHS 17.9 26.4 44.3 Source: FHWA 2001b.
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Assessing and Managing the Ecological Impacts of Paved Roads FIGURE 2-6 Example of hub-and-spoke pattern of roads found in Washington, DC. Source: MapQuest 2005. Source: MapQuest 2005. Reprinted with permission; copyright 2005, MapQuest.com, Inc., and GDT, Inc. The MapQuest.com logo is a registered trademark of MapQuest.com, Inc. the automobile in everyday life. Suburban areas grew in number and size to house families. Residential areas are often built with cul de sacs and a small number of entry points to reduce the amount of pass-by traffic in residential neighborhoods (Berkovitz 2001). In the conventional pattern of land use, each type of land use (residential, commercial, retail, and industrial) is separated from the others. Conventional land-use patterns result in more of a hub-and-spoke or circulatory pattern, with businesses in the center of town and residential areas surrounding the city. Some of the longest commutes in metropolitan regions are made by residents who live at the metropolitan edge and who work in downtown areas (FHWA 2002b). Engineering Structures In addition to roads and roadsides, the road system includes many engineering structures. These include concrete barriers, guardrails, noise barriers, bridges, culverts and pipes, and overpasses and underpasses. Each of these structures has a particular ecological effect.
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Assessing and Managing the Ecological Impacts of Paved Roads Concrete Barriers Rigid safety barriers separating lanes on roads are common, especially in urban areas. The most common type of these structures is called a Jersey barrier. Some Jersey barriers are used for traffic separation on freeways and interstate highways, and many others are used only temporarily. Temporary units are used mainly to enhance safety in construction work zones. The height of Jersey barriers averages around 32 in., but some are as high as 57 in. Rigid safety barriers are also commonly used longitudinally on major nonlimited-access highways to safely separate the two directions of traffic and preclude left turns and U-turns. There is current concern with barriers being formidable obstacles to small- and large-animal movement across highways. Barriers can block animals when they attempt to cross the highway, making them vulnerable to traffic mortality. Guardrail and Right-of-Way Fences Many different types of horizontal flexible and semirigid barriers, commonly known as guardrails, line the roadsides and medians of the roadway system. As with concrete barriers, guardrails are intended to constrain errant cars and trucks. Guardrails normally provide space beneath and above the rail that allows movement of wildlife. The principal difference between rigid safety barriers and guardrails is the amount deflection that will occur when they are hit. Right-of-way fences are used to keep people and animals from entering the berm, shoulders, and travel lanes of the highway. Many limited-access highways are lined with right-of-way fences, which enhance safety and reduce traffic mortality of large animals by reducing the crossing of animals. Wide Medians American Association of State Highway and Transportation Officials (AASHTO) safety information reflects a continuum of preference among lane-separation technologies. The safest system is one with wide grassy medians and no hard barriers, where there is sufficient room for errant vehicles to recover before entering an opposing lane of traffic. No rigid barrier is needed if the median is approximately 50 ft or more in
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Assessing and Managing the Ecological Impacts of Paved Roads width. Medians less than 30 ft in width should have a rigid barrier (AASHTO 2002a, TRB 2003, Mak and Sicking 2003). Despite the safety offered by wide medians, there can be offsetting concerns because the median increases the width of the road right-of-way, including additional cost, availability of land in some locations, and compatibility of a wide right-of-way with nearby land uses. Although wide medians in an additional loss of natural habitat, they may have less ecological impact than a narrow, paved median with a barrier or guardrail. Another advantage of a wide median is that the highway can be cost-effectively widened in the future with minimal disruption to traffic and to the adjacent properties, although if that happened, the ecological benefits of a wide median would be minimized or eliminated. Overpasses and Underpasses The decision to construct a new road over or under an existing facility is site-specific. Generally, minor roads should pass over major roads. This configuration takes advantage of off-ramp traffic being able to decelerate on the upgrade and the on-ramp traffic being able to accelerate on the downgrade (Sharpe 2004). Although highways generally are constructed over waterways, the choice to span or pass under requires extensive analysis. Sometimes tunnels are built instead of bridges, as was the case with the 2.75-km Fort McHenry tunnel, the largest underwater highway and widest vehicular tunnel, submerged in a trench at the bottom of the Patapsco River in Baltimore, Maryland. A tunnel was constructed instead of a bridge because of concerns that a bridge would tower over a historic site, Fort McHenry. Often it is desirable to depress the major road to reduce noise impacts and improve aesthetics. Span lengths, angle of skew, soil conditions, drainage, and the maintenance and protection of traffic on the existing route all must be considered. Three-dimensional models of the alternatives are sometimes used to obtain informed public input into the decision. Noise Barriers Noise barriers are designed and built primarily to muffle highway traffic noise in residential areas, schools, playgrounds, and other sensitive
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Assessing and Managing the Ecological Impacts of Paved Roads receptor areas (see Chapter 5 for a discussion on noise standards). Noise barriers may also be retrofitted on highways to enhance the surrounding community. Almost all sound walls in the United States have been constructed since 1986, and about two-thirds have been constructed of precast concrete and block. The other one-third of barriers are constructed of various materials, including wood, earth berm, and metal. The average noise barrier height is 10 to 16 ft, although some are over 23 ft. Bridges, Culverts, and Pipes The road system in the United States includes over 580,000 bridges, of which approximately 481,000 (83%) are over waterways. Ongoing construction efforts to address deficient bridges offer a singular opportunity to address and mitigate important ecological issues. These issues are discussed in Chapter 3; mitigation of adverse effects of bridges and culverts is discussed in Chapter 4. The condition of bridges and other engineering structures is discussed later in this chapter. OWNERSHIP AND MAINTENANCE RESPONSIBILITIES Understanding the ownership and maintenance responsibilities for roadways is important for planning and managing the coordination of environmental and planning issues discussed in later chapters of this report. Local town, city, and county governments own and maintain 77.4% of the nation’s roads. The federal government owns only 3.0% of the roads, including those in national forests, parks, military areas, and Indian reservations. Thus, coordination of transportation and environmental issues (including ecological protection) is often a state or local planning concern. Individual states own the remaining 19.6% of the roads, which includes most of the interstate system (Table 2-4). CURRENT STATUS AND FUTURE TRENDS In 2000, FHWA (2001b) assessed the status of the U.S. road system. It reported that most of the existing road surfaces in the system are in good condition but that an aging set of bridges and other engineering structures are a challenge to the system. The system is carrying higher
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Assessing and Managing the Ecological Impacts of Paved Roads TABLE 2-4 Ownership of U.S. Roads and Streets Jurisdiction Rural Mileage % Urban Mileage % Total Mileage % State 663,755 21.5 111,539 13 775,294 19.6 Local 2,311,269 74.7 746,341 86. 3,057,610 77.4 Federal 116,724 3.8 1,484 0.2 118,208 3.0 Total 3,091,748 100.0 859,364 100 3,951,112 100.0 Source: FHWA 2001b. volumes of traffic than in the past. Many roads are approaching design capacity as congestion increases. These issues are discussed in the following sections. Travel Congestion on Principal Arterial Roads in Urban Areas The use of urban roads continues to outpace the use of rural ones. The use and subsequent congestion are evident on both interstate and principal arterial roads. Although interstates play a key role in intercity connectivity, military support, and efficient long-distance travel, they also support local travel. Commercial uses (moving freight within metropolitan areas), noncommercial uses (providing access to airports and commuter travel), and individual uses are increasing the use of urban interstates. Reflecting the evolving expectations that state and local officials have for the interstates, growth in travel on their urban segments has been greater than on the rural portions, a trend that has continued since 1960 (Figure 2-6; FHWA 2001b). Travel per lane mile in rural areas has more than doubled since 1960. Travel per lane mile in urban areas has increased even more, growing roughly 25% during the 1990s. Urban interstate congestion is high in nearly half the states. Forty-one states in a recent General Accounting Office survey predicted that congestion would be high or very high in a decade (AASHTO 2002a). The traffic volume-to-service flow (capacity) ratio (V/SF) is one measure of congestion. As the ratio approaches the theoretical value of 1.00 (volume of traffic is equal to service flow capacity of the facility), traffic slows down and eventually stops. Values of V/SF greater than or equal to 0.80 indicate congestion. Level of service (LOS) is another concept used in transportation work to describe different levels of congestion. LOS is determined by comparing the volume of traffic on a road section to the road’s capacity to handle the volume. LOS A is free-
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Assessing and Managing the Ecological Impacts of Paved Roads running traffic, and LOS F is gridlock (Table 2-5 and Figure 2-7). Previously, FHWA and state Departments of Transportation would try to attain LOS C or better during the peak hours; now, due to heavy travel demand, they have to accept LOS D or even LOS E as goals during rush hour. Many sources suggest that travel is becoming more congested. The percentage of travel under congested conditions (percentage of daily traffic on freeways and principal arterial streets in urban areas) has slowly increased from just over 26% in 1987 to over 33% in 2000 (Figure 2-8). However, off-peak travel has increased significantly in most major metropolitan highway systems in the past decade. Urban interstates are the most crowded. In 2000, 50% of peak-hour travel occurred under congested conditions (FHWA 2002b). Researchers estimated that the annual costs due to lost productivity from congestion-related delays were $78 billion in 1999 (FHWA 2002b). Pavement Condition Over the past several years, the overall condition of the pavement on the NHS and the interstate system has improved. In 2000, on the basis of the international roughness index (IRI), 93.5% of the NHS and 96.6% of the interstate system had acceptable ride qualities. The IRI is an indicator of pavement performance, which uses an objective instrument-based rating system to measure the ride quality of a road. Pavements with an IRI of less than 95 have good or very good ride quality, those with an IRI of 95-170 have an acceptable ride quality, and those with an IRI of more than 170 have an unacceptable ride quality. The NHS and the interstate system have a 6.5% and 3.4%, respectively, unacceptable ride quality (Figure 2-9). TABLE 2-5 Level-of-Service Chart for Major State Highways in New Hampshire Based on 2002 Traffic Data Level of Service Description Miles No Congestion (LOS A and B) 1,233 Moderate congestion (LOS C and D) 1,191 Congested (LOS E and F) 305 Total 2,729 Source: Modified from NHDOT 2004.
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Assessing and Managing the Ecological Impacts of Paved Roads FIGURE 2-7 Traffic congestion map showing a concentration of congested highways in the southeastern and south-central regions of New Hampshire. Green: no congestion (LOS A and B); yellow: moderate congestion (LOS C and D); and red: congested (LOS E and F). Source: NHDOT 2004.
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Assessing and Managing the Ecological Impacts of Paved Roads FIGURE 2-8 Percentage of vehicle miles traveled on urban freeways and principal arterials occurring under congested conditions from 1987 to 2000. Source: Schrank and Lomax 2001. Reprinted with permission; copyright 2001, Texas Transportation Institute. FIGURE 2-9 Pavement surface condition of the national and interstate highway systems. Source: FHWA 2001b. The condition of interstate pavement has improved in recent years, reflecting greater federal funding and the states’ commitment to maintaining these roads. Because of increasingly heavy use, most of the system requires substantial maintenance. Porous pavement is a newer technology for the primary highway system. Structurally, porous pavement has not proved to be as durable to heavy trucks and high traffic volumes as other road surfaces because of movement of water under the road, resulting in cracking. Porous pavement also is a problem in states having to deal with snow removal because the sand and salts used in snow removal clog the openings. The most promising application for porous pavements is in parking lots, but even most of these fail over time as a result of compaction.
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Assessing and Managing the Ecological Impacts of Paved Roads Bridge Conditions Many of the nation’s bridges and road structures (overpasses and underpasses) were built many years ago and are in need of maintenance and, in many cases, rehabilitation. These features, which represented considerable expense at the time of design and construction, also require ongoing maintenance. In 2000, 29% of the nation’s estimated 590,000 bridges were considered structurally deficient or functionally obsolete (Table 2-6). These categories are defined below. Structurally Deficient Structurally deficient bridges have deteriorated structural components and are usually closed or restricted to use only by light-duty vehicles. These bridges are not necessarily unsafe. In general, strict adherence to signs limiting speeds or quantity of traffic on bridges will provide sufficient safeguards for these bridges. Functionally Obsolete About half of the nation's 590,000 bridges were built before 1965, and a fourth are more than 50 years old. Although bridges that are properly cared for can be considered virtually permanent, their age means that they need substantial maintenance and may be functionally obsolete (AASHTO 2002a). TABLE 2-6 Conditions of Highway Bridges Condition Total Highways No. % Structurally deficient 88,150 15.0 Functionally obsolete 81,900 14.0 Acceptable 415,492 71.0 Total 585,542 100 Source: FHWA 2001b.
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Assessing and Managing the Ecological Impacts of Paved Roads Functionally obsolete bridges cannot safely support the type or volume of traffic using them. These bridges have older design features that limit them from carrying current traffic volumes and modern vehicle weights and sizes. They can also cause more environmental damage, such as alteration of floodplain and riparian habitats, because of water flow restrictions. Investments made possible by ISTEA and the Transportation Equity Act for the Twenty-First Century (TEA-21) have permitted bridge improvements. The investment needed to repair the backlog of bridges has decreased in tandem with the reduction in bridge deficiencies. The bridge investment backlog is now $52 billion, based on an evaluation using the new National Bridge Investment Analysis System. Table 2-7 below shows the breakdown on bridge needs. Progress has been made through increased funding under ISTEA and TEA-21, priorities set at the state and other levels, and reduced investment requirements that meet economic analysis criteria. SUMMARY This chapter provides an overview of the status of the U.S. road system. The perspective has been on the entire road system at the broadest spatial scales. The spatial template for the system is a combination of established use histories (following older, established routes) and planning. The network comprises different functional types of roads (interstate, arterial, connector, and access) as well as rural or urban areas where roads traverse. The functional roadway types are used for planning and funding purposes. The characteristics of the current U.S. road system indicate that most of the growth in the paved system now occurs via widening existing roadways rather than adding new routes to the system. TABLE 2-7 Backlog of Bridge Investment Needs Type of Investment Costs (Billions) Bridge replacement needs $37.2 Bridge improvement needs (widening, raising, strengthening) $3.1 Maintenance, rehabilitation, and reconstruction needs $11.6 Total $51.6 Source: AASHTO 2002a.
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Assessing and Managing the Ecological Impacts of Paved Roads Vehicle use of roads has steadily increased in the past century. Since the 1960s, urban road travel has become greater than rural road travel; yet the later has more than doubled, and that has important ecological impacts, as discussed in Chapter 3. Indicators of congestion in urban areas are increasing. The extant road system requires large amounts of maintenance and replacement to maintain its current function. Although paved sections of the NHS are considered to be in very good condition for driving, 3 of 10 bridges are structurally deficient or obsolete.
Representative terms from entire chapter: