This guide will help long-term pavement performance (LTPP) database users understand the differences between (and, thus, appropriate uses of) the layer thickness data found in different database tables. In addition, as a result of LTPP data analysis completed in 1999, Temperature Predictions and Adjustment Factors for Asphalt Pavements, (FHWA-RD-98-085), the American Association of State Highway and Transportation Officials (AASHTO) adopted standard T317-02, "Prediction of Asphalt-Bound Pavement Layer Temperatures," and balloted a second standard, "Recommended Practice for Application of Temperature Adjustment Factors to Backcalculated Asphalt Modulus, Deflections, and Deflection Basin Characteristics."