Front-seat safety belt use in the United States (US) was 80 percent in June, 2004. This rate represents the highest ever for the US, but indicates that there is still a sizable minority of people who do not always use safety belts despite mandatory seat belt laws in all but one state. Changing the behavior of these people will require new and innovative countermeasures. Little research has systematically investigated the effectiveness, feasibility, and acceptance of vehiclebased countermeasures for promoting safety belt use. The purpose of this project was to promote safety belt use in the US by conducting research to develop an effective in-vehicle safety belt reminder system. Project activities included a nationwide survey of part-time safety belt users, development of potential safety belt reminder system ideas, and a series of focus groups with part-time safety belt users. The results indicated that the most effective and acceptable safety-belt reminder system concept was one that was adaptive; that is, one that changes its signal type and presentation modality depending on belt use behavior over some metric (time, distance, or speed). The study also assessed and developed an potential reminder system ideas for informing drivers about back-seat belt use.