Conditions and Performance Chapter Listing

Conditions and Performance Home Page

Introduction

Background

Current Conditions and Performance

Projected Conditions and Performance in 2007

Resources Needed to Maintain and Improve the Interstate System

Addressing Interstate System Needs

Background

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 more than 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 more recently enacted National Highway System. It is also a good time to look at the investment requirements to maintain and improve this 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 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 used in this study are taken from the Highway Performance Monitoring System (HPMS), a database that has been in place since 1978. The States furnish data annually for all of the Interstate and other arterial systems and most of the collector roads. This is a sample section database that provides a statistically valid sample of each of the categories of highway in the data system. More that half of all Interstate mileage is included in the sample sections. Thus, the Interstate is well represented in the HPMS database.

The National Bridge Inventory (NBI) contains data for each public road bridge in the nation. This database is 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. Deficient bridges are classified as structurally deficient or functionally obsolete. A structurally deficient bridge is one that has been restricted to light vehicles (no heavy trucks), one that requires immediate rehabilitation to remain open, or is closed. A functionally obsolete bridge has deck geometry, load carrying capacity, clearance, or approach roadway alignment that no longer meets the criteria for the system of which the bridge is a part, in this case the Interstate system.

For many years congestion has been a growing problem on urban Interstates and on Interstate routes approaching and connecting major metropolitan areas. However, congestion is difficult to measure. Historically the ratio of the volume of traffic to the capacity of the roadway to accommodate that volume has been used as a measure of the severity of congestion. This measure addresses only the peak hour of travel. Delay to the user of the system is now being used in an effort to measure the effects of congestion throughout the day. However, it is difficult to measure delay. The current procedures are based on modeling of speed and delay, and are subject to revisions in the future. Other measures, such as reliability, have been proposed. Reliability is the consistency of the travel time between any two points. This is also difficult to measure, and is not included in this report. The volume of travel per lane, such as VMT per lane-mile, is a measure of the density of travel and is information that is readily available. While it is not directly a measure of congestion, it does provide a valuable indication of travel density on the system.

This study evaluates current conditions and performance of the Interstate system roadways and bridges, and analyzes these data to project the investment requirements for the next 10 years to maintain and improve the system.