Overwhelmingly, cycling is a very safe and healthy activity, and one that has considerable potential to address illnesses such as obesity and heart disease which are the principal causes of premature death in western countries. In Britain these illnesses are more than 500 times more likely to kill than cycling.
Indeed, research (Andersen, Schnohr, Schroll and Hein, 2000) has shown that by far the most effective way a person can increase their life span is to cycle regularly to work.
If the risk of injury when cycling is very small, so the risk of head injury is much smaller. It takes over 3,000 years of average cycling to suffer a serious head injury, and the risk of death through head injury when cycling is very small indeed. Cyclists are a little less likely to die of head injury than pedestrians or car occupants.
If individual cyclists do feel vulnerable or believe that they are especially prone to injury, then first and foremost they should consider good quality cycle training.
Andersen LB, Schnohr P, Schroll M, Hein HO, 2000. All-cause mortality associated with physical activity during leisure time, work, sports, and cycling to work. Arch Intern Med 2000 Jun 12;160(11):1621-8.