Background: Because of the effects of traffic loads and pavement material properties on pavement performance, understanding the changes in traffic loadings and material properties and the variations in pavement layer thickness is an essential part of the pavement design process. However, the variability of these factors within projects and the changes that occur over long time periods are not well documented. Without this information, selecting appropriate designs for new pavements and rehabilitated pavement structures is a difficult task. The data available from the Long Term Pavement Performance (LTPP) studies are expected to provide such information.

Research is needed to determine the variability of design inputs within projects as well as the changes that occur over long periods of time. The findings of this research will provide guidance for the design of new and rehabilitated pavement structures.

Objective: The objective of this research is to analyze, based on the data available from the LTPP studies, the changes in traffic loadings and material properties and the variations in pavement layer thickness. The research is concerned with the variability within projects and the changes that occur over long time periods; it will not address daily and seasonal changes. The research shall be limited to using the data available in the LTPP Information Management System (IMS) database classified as "Level E."

Tasks: Accomplishing this objective will require at least the following tasks.

Phase I: (1) Collect and review literature relative to characterization of variations in traffic load data and pavement layer properties commonly used as inputs for pavement design. From the data elements available in the LTPP database, identify the elements needed to conduct this research and determine the extent of availability for each. Acquire the needed data and develop a database that includes the data proposed for use in the project. (2) Based on the information and data obtained in Task 1, refine the data analysis plan presented in the proposal. The plan shall address flexible and rigid pavements and deal with vehicle classification and axle load data; it will not deal with equivalent single axle loads (ESAL). The plan shall clearly describe what the research team can realistically achieve with the available data. (3) Prepare and submit an interim report that documents the research performed in Phase I and includes the details of the work planned for Phase II of the project. Following review of the interim report by the NCHRP, the research team will be required to make a presentation to the project panel. Work on Phase II of the project will not begin until the interim report is approved and the Phase II work plan is authorized by the NCHRP.

Phase II: (4) Execute the plan approved in Task 3. (5) Submit a final report that documents the entire research effort.

Status: The revised final report was submitted after additions and revisions to address panel comments.

Product Availability: The final report is available as NCHRP Web Document 48.