The results of an investigation into reliability of traffic data are presented. Accurate and reliable traffic data are essential to a wide variety of transportation applications, including pavement and bridge design and performance evaluation, programming, planning, and budgeting activities, as well as legislative and administrative policy development. Millions of dollars are spent each year to collect, process, and disseminate these data to users. The development of the AASHTO reliability concept for pavement design was investigated and expressions for traffic data variability were developed. Extensive actual traffic data files were analyzed. The resulting statistics were used to determine the importance of traffic data variability on the reliability of traffic projections for pavement design.