Conditions and Performance
of the Interstate System
-- After 40
Years
by Clifford M. Comeau
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.
Pavement Conditions
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:
- It uses a standard procedure and can be replicated.
- It provides a consistent measure across jurisdictional lines and diverse functional
systems.
- It is an objective measurement.
- It is consistent with accepted worldwide pavement roughness measurement procedures.
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:
|
Rural |
Urban |
Very good |
11.0% |
9.3% |
Good |
32.7% |
26.3% |
Fair |
23.7% |
23.8% |
Mediocre |
26.2% |
28.3% |
Poor |
6.4% |
12.3% |
Definitions
of pavement condition categories:
Very good - New or almost new pavement; will not require improvement for some time.
Good - In decent condition; will not require improvement in the near future.
Fair - Will likely need improvement in the near future, but depends on traffic use.
Mediocre - Needs near-term improvement to preserve usability.
Poor - Needs immediate improvement to restore serviceability.
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.
Bridge Conditions
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.
Congestion
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.
Conclusion
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.
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