Increased bicycle helmet use in the absence of mandatory bicycle helmet legislation: Prevalence and trends from longitudinal observational studies on the use of bicycle helmets among cyclists in Denmark 2004–2022

•Bicycle helmet use has increased in Denmark without helmet legislation.•An observational study has monitored helmet use in Denmark since 2004.•From 2004 to 2022, helmet use among all cyclists has increased from 6% to 50%•From 2004 to 2022, helmet use among school children rose from 33% to 79%•Non-legislative means and social processes could have been important factors. Introduction: Using a bicycle helmet reduces the risk of serious head injuries among cyclists substantially. This makes it highly relevant to increase the use of helmets and to measure the prevalence of bicycle helmet use over time and across different groups. Method: Since 2004, the use of bicycle helmets in Denmark has been measured observationally in two nationwide time series: one among cyclists in city traffic across all age groups, and one among cycling school children (aged 6–16) around schools. The observations have been conducted on a regular basis in different parts of the country following the same methodology over the years. Results: Bicycle helmet use among cyclists in city traffic in Denmark has increased from 6% in 2004 to 50% in 2022. Among cycling school children, helmet use has increased from 33% in 2004 to 79% in 2022. Throughout the years, helmet wearing rates have been highest among young children and lowest among young adults. Since 2015, female cyclists in city traffic have had a slightly higher helmet use than male cyclists. Discussion: Several factors might have affected bicycle helmet use in Denmark. One possible factor is a nationwide focus on traffic safety education and behavior change campaigns to encourage helmet wearing. Furthermore, among stakeholders on cycling safety there has been consensus on recommending bicycle helmet use and supporting the promotion of helmets while not recommending or promoting helmet legislation. Finally, more safety-oriented behavior in road traffic in general, and self-reinforcing effects of increased helmet use have plausibly been important factors. Practical Applications: Increasing bicycle helmet use in a country where cycling is popular is possible in the absence of mandatory bicycle helmet legislation. Persistent behavior change campaigning and education, stakeholder consensus, higher levels of road safety-oriented behaviors, and self-reinforcing processes could potentially be important factors.