How effective are bicycle helmets?

Bicycle helmets are very effective in reducing the risk and severity of head and brain trauma in a crash or fall (see SWOV fact sheet Bicycle helmets). In contrast with many other countries, bicycle helmets are not mandatory in the Netherlands, not even for children. They are not mandatory for ordinary cyclists, pedelec riders and racing cyclists, although the latter usually wear helmets voluntarily [48]. For the use of speed pedelecs, however, that have now been categorised as mopeds, helmets are mandatory (see SWOV fact sheet Pedelecs and speed pedelecs).