The Long Term Pavement Performance (LTPP) program is a 20-year research study of in-service pavements. The program’s results provide the basis for pavement design, maintenance, rehabilitation, and construction procedures and methodologies for years to come. As with any research endeavor, however, LTPP’s results are strongly influenced by the quality, quantity, and completeness of the input data. LTPP, therefore, has very stringent data requirements. The purpose of this TechBrief is to discuss one of these requirements—site-specific measurements for estimating pavement loadings—and to illustrate the effects of traffic loading data error on LTPP’s ability to develop accurate and reliable design equations. This TechBrief is based on two studies—“Vehicle Volume Distributions by Classification” and “Results of an Empirical Analysis of Alternative Data Collection Sampling Plans for Estimating Annual Vehicle Loads at LTPP Test Sites.”