Speeding in traffic is a key factor in road deaths and accidents. A largescale randomized controlled trial of an online intervention against speeding offenses in northern Denmark is ongoing. Preliminary quantitative results were presented at ESC 2023 and the final results will be ready by 2025. This study aims at improving our understanding of the theoretical mechanisms that influence speeding behavior. To improve the effectiveness of the intervention, we interviewed potential future participants about underlying beliefs related to speeding as part of the development of the intervention as well as people who had completed the intervention. Pre-intervention study consists of 24 semi-structured interviews, evenly distributed on sex, severity of speeding violation (over/under 30% of legal limit), and geography (town/country). Post-intervention study consists of 32 semi-structured interviews, selected from 240 participants in the intervention, oversampling the most serious speed offenders, in terms of their assessed likelihood of speeding again. Using directed content analysis, we examined participant’s motivations for speeding, highlighting the mechanisms in the theory of planned behavior, in terms of costs and benefits. The pre-intervention interviews assessed behavioral beliefs on the advantages of speeding concerned saving time, avoiding boredom, while the shame of potential criminal sanction was a cost. Normatively, participants believed almost everyone speeds, and that it is mostly unacceptable in urban areas, particularly in the vicinity of schools.S peed limits were seen as guidelines, necessary to curb “idiots”. Control beliefs pertain to work-related stress, low risk of apprehension, and habits. The perceived costs of speeding increase with age, and when (others) children are in the vehicle. The post-intervention interviews were generally favorable to the intervention design, particularly the surprising information of lack of efficacy of speeding. Interviewees generally expected to reduce their speeding following the intervention.