In real accidents bike helmets don’t seem to crush as designed, they break instead. The senior engineer of Bell Sports, the market leader in cycle helmets, has written: “Another source of field experience is our experience with damaged helmets returned to customer service… I collected damaged infant/toddler helmets for several months in 1995. Not only did I not see bottomed out helmets, I didn’t see any helmet showing signs of crushing on the inside.” In 1987, the Australian Federal Office of Road Safety found that in real accidents “very little crushing of the liner foam was usually evident… What in fact happens in a real crash impact is that the human head deforms elastically on impact. The standard impact attenuation test making use of a solid headform does not consider the effect of human head deformation with the result that all acceleration attenuation occurs in compression of the liner. Since the solid headform is more capable of crushing helmet padding, manufacturers have had to provide relatively stiff foam in the helmet so that it would pass the impact attenuation test…”

It appears that helmets break easily, but don’t absorb the impact, see the engineers quoted at http://en.wikipedia.org/wiki/Bicycle_helmet#Criticism_of_current_standards.3B_new_designs. A broken helmet has simply failed, and the widespread anecdotes on the theme of “a helmet saved my life” seem to owe more to wishful thinking than to science. As for the occasional anecdotes about “a car ran over my head” (http://www.kptv.com/news/21541052/detail.html), see the pro-helmet site http://www.helmets.org/smush.htm; if a car goes over your head, I’m sorry to say you won’t be sitting up and praising your helmet. The only known connection between helmets and death is that helmets have strangled a few young children who were wearing helmets while playing off their bicycles, see http://en.wikipedia.org/wiki/Bicycle_helmet for an incomplete list.

To answer your question – fearmongering is the best way to make people strap a lid on their heads. Helmet propaganda may tend to overemphasize the very small dangers of cycling and seldom seems to emphasize its large benefits. At my moderately advanced age it’s far too dangerous not to cycle – regular cycling, Danish style, not too far, not too fast, nearly halves the death rate, see http://archinte.ama-assn.org/cgi/content/abstract/160/11/1621 All-Cause Mortality Associated With Physical Activity During Leisure Time, Work, Sports, and Cycling to Work. Andersen et al, Arch Intern Med. 2000;160:1621-1628. Taking up moderate exercise is about as beneficial as giving up smoking. Bicycling is good for health, but bike helmets don’t seem to be. If you want to boost safety, following the helmet trend doesn’t seem to be a good way to go.