1.0 Project Background
Over the past several years, the importance of preserving the existing transportation infrastructure has received increased focus. A fundamental element of the performance of a transportation system is the physical condition of the assets that comprise it. Consequently, the preservation of existing assets is a critical element of the nation's transportation programs and requires the identification of performance measures designed to capture and communicate the physical condition of pavement and bridges. The measures should also capture temporal changes in order to provide early indications of the efficacy of previous work and a basis for assessing options for future work.
The primary goals of this project are to define a consistent and reliable method of assessing infrastructure health with a focus on pavements and bridges on the Interstate Highway System, and to develop tools to provide FHWA and State DOTs ready access to key information that will allow for a better and more complete view of infrastructure health nationally. While initially focusing on the Interstate Highway System, it is the intent of this project to develop methodologies that can be expanded to the National Highway System or any other defined system of pavements or bridges, subject to data availability.
To meet these goals, the scope of this project includes two main tracks:
- Develop an approach for categorizing pavement and bridges as Good/Fair/Poor, which can be used consistently across the country. Performance in this context is based on condition information.
- Develop a methodology for determining the health of a corridor with respect to pavement and bridges. Health in this context is based on factors that go beyond condition.
These tracks are being coordinated with other Federal Highway Administration (FHWA), American Association of State Highway and Transportation Officials (AASHTO) and National Cooperative Highway Research Program (NCHRP) projects focused on performance-based transportation programs.
1.2 SUMMARY OF EXISTING PRACTICES AND DATA
This section summarizes the findings from a literature review conducted as part of Task 2 of this project. For more details, refer to the complete literature review report, dated December 13, 2010.
Although State DOTs collect and track a variety of measures of pavement and bridge condition today, the degree of coverage, consistency of measures, and method and frequency of data collection varies widely. Differences between State DOTs are typically more pronounced for pavements than bridges. Regardless, in order to develop a performance and health assessment approach that can be applied consistently across the U.S., the project team has focused on two national data sets - Highway Performance Monitoring System (HPMS) data and National Bridge Inventory (NBI) data.
HPMS is a national program that includes a sampling of inventory information for all of the nation's public road mileage as certified by the States' Governors on an annual basis. Each year, State DOTs are required to furnish data per the reporting requirements of the HPMS Field Manual. The HPMS requirements and field manual have recently been updated. The results are referred to as "HPMS 2010+".
All State DOTs are required to submit NBI data to the FHWA for all highway bridges on or over public roads, as well as on culverts greater than 20 feet in length. State DOTs and Federal agencies conduct periodic inspections of these structures, prepare and maintain a current inventory of these structures, and report the data to the FHWA using the procedures and format outlined in the
Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation's Bridges.
State DOTs have expressed a desire through direct feedback from AASHTO members to limit any additional data collection requirements.
Pavement Condition Data
Available systems of pavement condition evaluation and monitoring range from State-specific pavement management systems (PMS) to HPMS. Prior to the recent HPMS update, the main measure of pavement condition used in HPMS was International Roughness Index (IRI), which is an indicator of pavement roughness. In addition to IRI, HPMS 2010+ includes data on rutting, faulting, and cracking. Given that IRI is one component of pavement condition, but not condition in its entirety, HPMS 2010+ may allow for development of a combined distress and IRI scoring method.
Outside of the HPMS program, State DOTs collect a wide variety of additional pavement data elements, including longitudinal cracking, transverse cracking, fatigue cracking, rutting, and others. However comparisons between State DOTs are challenging because of differences in the specific items collected, data collection protocols, and methods/equipment used to collect data (e.g. manual versus automated methods, for example). A better understanding of the impacts of these differences will be a key output from the pilot study.
IRI, rutting, and cracking are primarily functional condition indicators. A key missing ingredient in determining pavement condition is to examine the structural response of the layered system. However, most State DOTs do not collect structural response information for PMS purposes.
Bridge Condition Data
The NBI dataset contains condition data by bridge component - deck, superstructure, substructure, channel/channel protection, and culvert. It also contains data on a bridge's functionality, such as underclearances and posting information. Data standards, collection procedures, quality control processes, and calculation methods related to the NBI data set are well established and have been used by State DOTs and the FHWA for several years. The measures currently under consideration by FHWA and AASHTO for national deployment focus on NBI data. Under consideration are good/fair/poor measures based on NBI component ratings, and a measure based on Structural Deficiency status.
Looking beyond the NBI, data required for AASHTO's Pontis bridge management system provide the most potential in terms of national bridge performance measurement. Most State DOTs (over 40) license Pontis. The Pontis database contains all NBI data items, as well as more detailed element-level inspection details. For example, the NBI file contains a single condition rating for a bridge's superstructure. The Pontis database contains additional data on the distribution of conditions by condition state for each structural element of the superstructure, including elements such as girders, stringers, floor beams, etc. Despite the number of DOTs using Pontis, there are still differences in the underlying data. For example, some DOTs do not collect element-level data. Rather, they use Pontis for NBI reporting. Others have modified the bridge element definitions. In addition, The AASHTO Subcommittee on Bridges and Structures has recently updated the definitions of the AASHTO Commonly Recognized (CoRe) Bridge Elements. These new definitions are not currently reflected in Pontis.
Despite the differences highlighted above, bridge condition data collection protocols are considered to be less variable than those for pavement condition data, providing greater opportunities for a national comparison of State datasets.
In many cases, the term "health" is used synonymously with "condition." For this research effort, it is anticipated that "health" will incorporate condition data, but also include additional asset characteristics.
The literature review found two examples of this broader type of health index - one developed by the U.S. Chamber of Commerce1, and one developed by a group of European Nations through the European Cooperation in Science and Technology (COST) initiative.2The Chamber of Commerce approach considers aspects of supply (e.g., highway and transit density), quality of service (e.g., travel time index and fatalities), and utilization (e.g., percent of uncongested lane miles). The COST approach considers safety, comfort, structural adequacy, and environmental factors. In addition, the California DOT has developed a bridge health index that combines condition data from several individual bridge elements into a single, overall index. These efforts help to illustrate the potential and application of the health index approach. They also help to define a general model for developing a health index that can be summarized as follows:
- Determine which asset characteristics (e.g., condition, traffic, safety, etc.) to include in the health index;
- Determine which indicator(s) (e.g., IRI, structural adequacy, vehicle miles travelled, accidents, etc) to use for each characteristic;
- Determine the relative weight of each indicator;
- Normalize the indicators (e.g., on a 1 to 100 scale); and
- Calculate the health index as a weighted average of the indicators.
1Michael Gallis & Associates, et. al., Transportation Performance Index: Complete Technical Report - Measuring and Benchmarking Infrastructure Performance. Prepared for the U.S. Department of Commerce. September 19, 2010.
2European Cooperation in Science and Technology, Transport and Urban Development, Cost Action (354), Performance Indicators for Road Pavement, Final Evaluation Report.