The role of accident studies in roadside safety problem identification, the various levels of accident data available, and the problems associated with them are discussed in this paper. Police level accident data is useful for identifying the extent of safety problem, but is limited in its usefulness for determining accident relationships or evaluating safety countermeasure effectiveness. Problems associated with police level accident data include location identification, definitions and reporting criteria, and accident report data elements. In-depth accident data, from clinical studies to general purpose and special studies, provides the necessary details that are lacking in police level accident data, but are much more expensive and thus limited to only specific studies. Furthermore, both policereported and in-depth levels of accident data are faced with problems associated with exposure, unreported accidents, and time lag.