Road accidents are one of the leading causes of death and injuries among adolescents and young adults. Road safety education programs aim to promote safe traffic behavior through information, skills training or fear appeals. During the last decade, an intervention type using victim testimonials has been developed. These types of programs aim to promote road safety by connecting a personal narrative of choices and consequences to the beliefs and behaviors of the audience. Studies on the effectiveness of this type of road safety programs among youngsters who are not yet drivers are still limited. This study used a cluster-randomized and controlled trial design to examine the effect of a Danish school-based road safety educational program using testimonials for students in lower secondary schools in Denmark on a) knowledge of risks factors in traffic and b) two types of behavior specifically relevant for the pre-driver target group: seatbelt use and safe bicycle behavior. Our analysis sample comprised 1007 students from 57 schools with a matched baseline and four-month follow-up response. Linear regression analyses suggest a small positive effect on the students’ knowledge of the three most important risk factors in traffic but not on the age group most at risk. Concerning seatbelt use or overall cycling behavior no effect was found, although a small positive effect was found on helmet use. Neither was the effectiveness modified by gender, parental educational background, or risky cycling behavior at baseline. Although classroom-based interventions using testimonials may have the potential to increase knowledge of risk factors in traffic among adolescents, translating knowledge into safe traffic behavior is challenging. More studies are needed on how to further adapt the intervention to an age group who are not yet drivers. Such research may focus on mechanisms of change including the age-specific relevance of the ambassador's testimonial and on employing strong elements of action guidance.