Driver fatigue is estimated to be a (contributing) factor in 15 to 20% of crashes, but estimates in individual studies vary widely. Drivers who are tired are less attentive and react less quickly and less adequately than drivers who are not tired. They also get irritated and frustrated more easily.
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A bicycle helmet is intended to protect cyclists against head and brain injuries when they are involved in crashes. The helmet does not prevent bicycle crashes (see the SWOV fact sheet Cyclists for general bicycle safety measures). International research shows that in case of a crash helmeted cyclists are 60% less likely to sustain serious head/brain injuries and 70% less likely to sustain fatal head/brain injuries than cyclists not wearing a helmet.