Some studies have shown that bicycle helmets reduced the risk of cyclists' head and brain injuries by up to 90%. In countries whose laws require cyclists to wear helmets, such legislation increases helmet-wearing rates rapidly. However, helmets must be fitted and worn properly if they are to be fully effective. Even if used properly, a helmet certified to a recognised standard has limited protective capabilities. The design of current bicycle helmets reflects the minimum performance requirements of helmet standards, so that it is often inadequate in several respects. For example, modern helmets tend to protect too small an area of the head. They are tested for their ability to reduce linear acceleration, but not tested for ability to reduce rotational acceleration, which can also damage the brain. They sometimes fail to provide optimal protection against the more common lesser impacts, because they are tested only against severe impacts. Their straps and buckles are often not strong enough. Their shapes are designed for the heads of adults, not children. Bicycle helmets can and should be improved to protect all cyclists better. One effective way of doing this is to upgrade the performance requirements of helmet standards, and apply mandatory helmet legislation only with reference to adequate helmet standards.
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