The issue of cycle helmets has long been debated because multiple issues are involved, as examples, safety, health and enforcement of mandatory helmet laws (MHLs).
Some protection by wearing a helmet may occur but it is not so simple because research has reported that when accidents occur helmet wearers report hitting their helmet more often than would occur for non-wearers hitting a bare head. Research in 2019 has reported helmet wearers to have a higher accident rate per hour cycled and the reasons could be due to several factors, see;https://www.researchgate.net/publicatio ... jury_rates
So, assuming some protection may occur in some accidents, it is also evident that there would be more impacts to the helmet and more accidents, especially falls. Therefore, the safety case for wearing a helmet is clearly questionable. Taking Australia as an example, roughly 40 cyclist deaths per year occur in Australia from a population of 25 million, about 8 million ride bikes and includes children. One death per 200,000 cyclists. Even if helmets could help prevent some of these deaths, at a maximize less than one in 200,000 people could benefit in saving lives per year. see
Evaluating Cycling Fatality Risk with a Focus on Cycle Helmet Use Dec. 2018 http://worldtransportjournal.com/wp-con ... 4.4opt.pdf
On the other hand, if 20% fewer people cycle due to MHLs, 1.6 million people would most likely be disadvantaged with increased risk of death due to less exercise.
In 2015 a cost-benefit analysis was provided based on published data and reported;
'The actual risk of serious head injury when cycling is low. The societal health cost factor against the laws is calculated at 109, indicating they cause considerable harm.'
The actual risk of being killed while cycling or suffering a severe head injury is low but the risk of someone being discouraged by having a MHL is high, as examples; Surveys from Melbourne, 1990 to 1991 show 39 more teenagers wearing helmets v 623 fewer cycling and for NSW surveys, 1991 and 1993, show 568 more children wearing helmets v 2658 fewer cycling, see:
Evaluation of Australia's bicycle helmet laws, http://www.cycle-helmets.com/au-assessment-2015.pdf
There is unfortunately no guarantee that the issue will be resolved with various claims and counter claims and the difficulty understanding how extra helmet impacts, a higher accident rate and discouraging cycling has resulted in a negative outcome for Australia. The research claiming a strong result for helmet use is also much weaker than it first appears, see;
Weaknesses with a meta-analysis approach to assessing cycle helmets. Feb 2017 http://worldtransportjournal.com/wp-con ... eb-opt.pdf
Given this information should the helmet law in Spain be tested in the European Court of Justice?