As children, it seems that everyone was told while learning to ride a bike that wearing a helmet was a must. For adults, though, these safety devices are often ignored. There are many reasons for this, from a general sense that helmets are uncomfortable and "uncool" to the fact that adults are more confident in their skills as riders.
However, not all accidents can be avoided. When looking at bicycle accidents involving cars, for example, you'll find that the motorists cause many of the crashes. These are often the most dangerous crashes of all, and they are not incidents that even the most skilled riders can completely rule out.