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|Publication Number: Date: Summer 1996|
Issue No: Vol. 60 No. 1
Date: Summer 1996
The Dwight D. Eisenhower System of Interstate and Defense Highways, from its inception to its fulfillment as the foundation for the National Highway System, has more than achieved its founders' expectations. It has provided a rapid and efficient means of travel to the American public, allowed the growth of a highly efficient trucking industry, and formed a transport infrastructure foundation for the nation's economic growth and development.
It has been 40 years since the establishment of the Highway Trust Fund for financing of the nation's highways, in particular the interstate system. What better time to look at the condition and performance of this system, the core of the newly enacted National Highway System.
The interstate system has served its purposes well. In many instances, anticipated usage levels of the system were reached as much as a decade earlier than expected by the interstate planners. America's reliance on the interstate system creates major challenges for transportation agencies. The system has provided a reliable basis for long distance surface movement and has been fully integrated into the freight logistics of major producers and suppliers. Consequently, the reliability of the system and the preservation of its physical assets are key policy and programmatic concerns for the entire transportation community.
For long- and medium-distance travel by automobile and for freight movement by truck, the system is aiding the mobility and productivity of the nation. In spite of congestion in the larger metropolitan areas, travel on the interstate system is usually faster than on the alternative street systems.
Much of the pavement on the interstate system was constructed 20 to 40 years ago. However, some highways with even older pavements - mostly in the Northeast - were incorporated into the system to provide logical connectivity without increasing the cost of the system for highway users. Some of the pavements have been completely reconstructed over the years. Some are still fairly new. Some have been resurfaced one or more times. Most have undergone some form of rehabilitation, restoration, resurfacing, or reconstruction since the original construction.
Interstate pavement condition and congestion data are taken from the Highway Performance Monitoring System (HPMS), a database that has been in place since about 1978. The states furnish data annually for all of the arterial systems and most of the collector roads, including the interstate. This is a sample section database that provides a statistically valid sample for each of the categories of highway in the data system. More than half of all interstate mileage is included in the sample sections. Thus, the interstate is well represented in the HPMS database.
HPMS is used to track the conditions and performance of our highway systems. This information is reported to Congress on a biennial basis. The most recent such report is for 1995, using 1993 data. Also, Highway Statistics is produced annually; the latest was published using 1994 data.
Historically, the pavement condition data were based on Present Serviceability Rating (PSR). More recently, the International Roughness Index (IRI) has been adopted for this purpose. The reasons for its adoption are:
More than 95 percent of the interstate highway system's pavement has been rated using the IRI within the past two years. Because of the change in rating procedure from PSR over the past few years, it is impossible to show a valid trend of pavement condition over the recent past.
Based on 1994 data, the pavement roughness ratings are:
Definitions of pavement condition categories:
More than half of the interstate pavement mileage is in good or fair condition. This means that this mileage does not require immediate attention, but the fair portion may need resurfacing or other rehabilitation in the near future. A small percentage of the pavements are poor, needing immediate attention, possibly reconstruction. The mileage that is considered mediocre - more than a fourth of the total - will need some type of improvement in the near future to preserve the usability of the surface and to avoid preventable deterioration which may result in reconstruction being required.
All states currently use pavement management systems (PMS) to monitor interstate pavements and to develop cost-effective pavement rehabilitation strategies. The application of PMS enables states to identify and correct deteriorating pavements using relatively less costly measures rather than delaying improvements, when more expensive remedial actions are often required.
The National Bridge Inventory (NBI) contains data for each public road bridge in the nation. This database in updated on a continuing basis by the states. Most bridges are inspected every two years, and the data from these inspections are reported to the Federal Highway Administration and incorporated into the NBI. The number of deficient bridges on the interstate system has declined since 1990.
The number of structurally deficient bridges was reduced from 7.2 to 6 percent, and functionally deficient bridges went from 21.4 to 18.2 percent. A structurally deficient bridge is not necessarily unsafe. It is a bridge that is designated as needing significant maintenance, rehabilitation, or sometimes replacement. Some of these bridges are load-posted for safety, sometimes requiring heavier trucks to take a longer, more circuitous route. Functionally, deficient bridges are those that do not have the lane widths, shoulder widths, or vertical clearances adequate to serve the traffic demand, or the bridges or approaches may be subject to occasional flooding.
The National Bridge Program, with the bridge management programs that the states have implemented, has contributed greatly to the emphasis on bridges and to reducing the number of deficient bridges.
For many years, congestion has been a growing problem on urban interstates and on interstates approaching and connecting major metropolitan areas. Measured by the ratio of the volume of daily vehicle miles of travel (DVMT) to the capacity of the highway to accommodate traffic (V/C ratio), peak-hour congestion increased steadily until about 1988 and since then has stabilized.
This does not mean that total congestion is not continuing to increase; it simply means that, in many cases, peak-hour conditions cannot get much worse. Since 1988, between 67 and 70 percent of urban peak-hour DVMT occurred under congested conditions (V/C of 0.80 or greater). Typically, two-thirds of this congested DVMT occurred under severely congested conditions (V/C greater than 0.95).
DVMT per lane-mile provides a broader measure of congestion, since it relates to the number of hours daily that a roadway is under performance stress. This measure of congestion extent and duration continues to increase. This shows a continuing increase in DVMT compared with capacity and a spreading of congestion over longer periods of the day and on more routes. From 1984 to 1994, the average daily vehicles per lane of urban interstate increased from 9,990 to 12,808.
Typically, when a freeway becomes congested, the average travel speed will drop to 50 kilometers per hour (km/h) or less. If we use a speed of 50 km/h or less as another threshold of congestion, the routes on which 75 percent of urban interstate DVMT occurs are congested for at least two hours in each direction on a typical weekday. About 60 percent of urban interstate DVMT occurs on interstates that are congested up to eight hours a day in each direction.
In summary, pavement conditions have stabilized over the past 10 years; congestion is increasing in extent and duration, if not in severity; and the number of deficient bridges is decreasing.
The interstate system has proven to be invaluable, even essential, to the competitiveness of this nation in the global marketplace. The system of interstate highways is the core of the 260,000-km National Highway System that, with major intermodal connectors and direct connections to the Canadian and Mexican highway systems, will form the basis for a fully integrated 21st century intermodal system to support America's social and economic needs.
Clifford M. Comeau is a highway engineer in the Highway Needs and Assessment Branch of the Office of Policy Development for the Federal Highway Administration.