This paper presents descriptions of a number of heavy truck inspection and observation studies recently performed in several regions of the United States. These studies were done to establish the relative prevalence of various aspects of heavy truck operation and in-use condition.Results of these “prevalence” studies are presented. They involve heavy truck front brake operability, speed control practices on downgrades and in curves and the prevalence of various safety-related equipment features. The relationship of the findings to accident data and implications for design and enforcement initiatives are discussed. Methodological issues which arise when performing prevalence studies and “lessons learned” are reviewed based on the experience of the five research organizations which conducted the studies.