Cycle helmets are specified by their manufacturers as meeting one or more of the international standards for this equipment. All of the standards test the helmet's protection of only a decapitated headform, (i.e. one with no body attached); and all tests involve only low speed impacts. Impact speeds are less than 6.6 m/s (24 km/h or 15 mph), and in some cases, barely 5 m/s (18 km/h or 11 mph).
Unlike seatbelt tests, helmet test standards do not realistically replicate serious crashes.
Helmets reduce the force of an impact only while the polystyrene liner is compressing. Once the liner is fully compacted, a helmet offers no further protection and passes residual energy straight on to the skull and brain. There is no evidence to suggest that helmets continue to provide a reduced level of brain protection beyond their design limits.
When helmets fail, they do so catastrophically, rather than gradually, by breaking. The breaking of a helmet is not by itself evidence that it has provided useful protection to the wearer. It is common for cycle helmets to fail prematurely, before the polystyrene liner has been fully crushed. Indeed, very often helmets break without the liner compressing at all, perhaps because they have been subjected to oblique forces, not directed at the head, that they are not designed to withstand. If a helmet breaks without its liner compressing, it is likely that no more than superficial protection would have been afforded.
In cases of high impact, such as most crashes that involve a motor vehicle, the initial forces absorbed by a cycle helmet before breaking are only a small part of the total force and the protection provided by a helmet is likely to be minimal in this context