In case of a fall or crash, the use of a bicycle helmet reduces serious head/brain injury by 60% and fatal head/brain injury by 71% on average. These are so-called ‘best estimates’. With 95% certainty, the risk reduction for severe head and brain injury is between 54% and 65%, and the risk reduction for fatal head and brain injury is between 44% and 85%. This has become apparent from the most recent meta-analysis by Høye [19]. This study compared injuries of helmeted and non-helmeted cyclists by combining the results of 55 studies from different countries. These studies were mostly case-control studies and met strict scientific requirements. Overall, the protective effect of bicycle helmets was the same for both children and adults [19] [20].
Most studies from the aforementioned meta-analyses have been carried out in the United States, Canada and Australia, and a few in Asia and Europe; none of these in the Netherlands however. In those countries, the infrastructure and traffic composition as well as bicycle use are often different from the Netherlands. It is possible, that bicycle crashes in the Netherlands may be of a different nature, which has consequences for the protective effect of bicycle helmets. However, there is no a priori indication of a larger or smaller effect in the Netherlands. The protective effect of a bicycle helmet was also evident, for example, in a Dutch study analysing the injuries of nearly 2,000 casualties admitted to a trauma centre after a bicycle crash (2007-2017 period; [21]). The small group of cyclists who had worn a helmet at the time of the crash (7.5%) had significantly fewer head and neck injuries, subdural or intracerebral haemorrhages and skull (base) fractures than the casualties that had not worn helmets. Of the casualties who had worn helmets, 2% died; of those without helmets, 6% died. However, this difference was not statistically significant.
The bicycle helmet effectiveness found in the meta-analyses is based on case-control studies. This is the most common way to investigate the effectiveness of bicycle helmets (see below for more information). In addition, biomechanical research and research with computer simulations and time series analyses on the effect of bicycle helmets have also been conducted (again explained below). In general, all these types of research find a significant effect on reducing head and brain injuries resulting from a fall or crash; only in time series analyses does the effect appear to be smaller.
Case-control studies
In case-control studies, the effectiveness of bicycle helmets is determined by comparing cyclists involved in crashes resulting in head/brain injuries (case) to injured cyclists without head/brain injuries (control), and then verify whether the cyclist involved was wearing a helmet. The major advantage of case-control studies is that they concern real crashes that have actually happened. Case-control research into the effectiveness of bicycle helmets is done when there are no data about the (differences in) bicycle kilometres travelled by helmet users and non-users (exposure). Such data generally do not exist. This method has, however, also been criticised, since it could result in an overestimation of the effectiveness of bicycle helmets [22]. Yet, other researchers did not find any evidence of this overestimation [23] for example). In general, a well-designed case-control study is considered as a reliable indication of the effect of helmets. An experimental design, for example a randomised control study, in which the researcher randomly selects who is and who is not going to wear a helmet, is inappropriate for ethical reasons. That is why case-control studies are standard in this area of research [20].
Biomechanical research
For biomechanical research, bicycle helmets are tested for shock absorption in a laboratory. During the test, a dummy head with and without a helmet falls down. It is estimated that a bicycle helmet will reduce the risk of serious brain damage in a fall from a height of 1.5 metres from almost 100% to 10%; and in a fall from a height of 2 metres to approximately 30% [24]. In these tests, helmets were used meeting the legal US requirements. The legal requirements for testing bicycle helmets differ by continent (see [25] for an overview of helmet standards).
Computer simulations
In studies using computer simulations, both the impact on the head as well as the possibly protective effect of a helmet are simulated in a model. On the basis of simulations of three types of single bicycle crashes, researchers conclude that helmet use may reduce the risk of concussion by more than 50% and the risk of a skull fracture by more than 90% [26]. Computer simulation of bicycle-car crashes also show that bicycle helmets may reduce the risk of serious injury, on average by 40% [27]. This kind of simulation research shows that not only the impact speed but also the impact location on the head determines the protective effect of a bicycle helmet [27] [28].
Time series analyses
By means of a(n) (interrupted) time series analysis, the effectiveness of an intervention (for example mandatory helmet use) may be determined by the number of casualties before and after the intervention. Several studies have used this method to determine the effectiveness of bicycle helmets, or rather of mandatory helmet use [29] [30] [31]. The studies looked at the share of cycling casualties with head or brain injuries before the intervention and during a series of fixed times after the intervention. In general, this kind of study finds a lower effectiveness than studies with a more experimental design. This will partly be due to the fact that not everyone actually wears a helmet. A disadvantage of this kind of study is that they have a prolonged time scale (several years) and that in this period other factors (such as other road safety measures, bicycle use, change in behaviour) may also have had an effect on the prevalence of head and brain injuries.
