Ian Walker, a researcher at the University of Bath, has uncovered an unfortunate tendency in human psychology. Drivers are more likely to hit cyclists who are wearing helmets. The hypothesis is that they compensate for the sight of a bare head by giving a wider berth when passing. On average, they gave a cyclist with a helmet 8.5cm (3.5″) less space. This was confirmed observationally by Walker himself, who used a sensor to evaluate 2,500 such incidents in Salisbury and Bristol. Motorists were most cautious around him when we wore a female wig and no helmet. Wearing the wig earned another 14cm (5.5″) of clearance, on average.
The study also found that larger vehicles are more likely to cut it close: “The average car passed 1.33 metres (4.4 feet) away from the bicycle, whereas the average truck got 19 centimetres (7.5 inches) closer and the average bus 23 centimetres (9 inches) closer.” That’s especially discouraging, given how the relative masses of a cyclist and a bus affect the dynamics of a possible colission.
A more comprehensive examination of the results (PDF) is available online.
The situation demonstrates the kind of game theory situation so common in safety and security. If you fall by the grace of your own actions, you are better off wearing a helmet. If a car actually does hit you, having a helmet is probably also a good idea. The degree of trade-off between reducing the probability of occurrence and mitigating the probable consequences is rarely easy to set effectively.
The take home lesson: there is a bit more reason to be wary of helmets, but cross-dressing could save your life.