Traffic crashes constitute a major, worldwide public-health problem that cause disabilities, life-long suffering, and huge economic losses. When a person is injured in a traffic crash, actions taken by bystanders often are of crucial importance. To perform first-aid actions in a correct manner, bystanders, often laypersons, need both the courage and the knowledge to do so. For preventive purposes, society spends large resources to inform and educate the public in order to enhance people's ability to take correct actions. However, there only is little information on the rate in a population of persons who have had first-aid training, have been bystanders at a traffic crash, on the actions taken by such persons, and on effects of first-aid training on patient care. The aim of this study was to acquire knowledge about: (1) the prevalence of first-aid training; (2) the incidence of being a bystander and of the first aid provided at traffic crashes and other emergencies; and (3) the impact of first-aid training on the risks people take in road traffic. A questionnaire was administered to 2,800 randomly selected persons aged 18-74 years. The response rate was 67.5%. During the previous five years, 39% of the population had received first-aid training, with a higher rate among younger individuals and those with a higher education. After training, 30% of the respondents had used their skills, and 41% took fewer risks in traffic, particularly those who were older or had a lower level of education. Fourteen percent of those with training (significantly more men) had been bystanders at a traffic crash. At 20% of the crashes, a bystander had administered first aid, and one-third of those who provided such assistance had had use of their training. Intensified first-aid training of the general public could lead to citizens who are more cautious in traffic and to bystanders who provide more immediate and adequate first aid at traffic crashes and other emergencies.