Thursday, August 19, 2010

Helmet law research hits the headlines

Helmet laws hit the headlines with a new Australian study proclaiming their ineffectiveness at providing safety to cyclists, while in Canada the debate is heading the other way (due to this study - sorry I can't get the full text to review the methods).

The Australian study neatly controls for the number of cyclists and distance cycled by comparing the ratio of head to arm and hand injuries resulting from cycling activities from hospital records. A change in this ratio (lower head injuries per arm and hand injury) would be a clear indicator of the success of helmet wearing in preventing head injury.
The figure above shows the ratio (ICD9) from 1988 to 2000. Helmet laws were introduced in 1991, and self-reported compliance for two age groups (<16years and >16years) are plotted from 1991 to 1995.

The essential argument is that the large decline in the ratio of head to arm injuries occurred before the helmet law, and much before compliance with the law. In the two year period where helmet wearing took off following the legislation (1991 to 1993), the ratio dropped from 0.8 to 0.75 – hardly a success. The drop in the two years preceding the helmet law was from 1.15 to 0.8.

The author suggests that other road safety measures contributed to the decline, while the law itself would have contributed to a decline in the number of cyclists (some evidence for the decline is here) which itself made cycling more dangerous and lead to a flattening of the trend -

The reduction in numbers of people cycling may have actually increased the risk to the remaining cyclists because of Smeed’s Law and the safety in numbers hypothesis.


  1. Is it possible that the decreased ratio in the two years preceding the introduction of the helmet law reflects increased media attention on helmet safety pre-legislation? One could find many examples of issues that are debated by officials in the media, using expert testimony, well before any legislation is introduced. Perhaps many cyclists began using helmets on the back of publicised safety research, prior to the law being introduced, and this could partly explain the early decrease?

  2. The self-reported helmet wearing is plotted on the graph for > and <16yrs age groups. Helmet wearing went from 25% to 85% in the year the legislation was introduced, while the was minimal change in the head to limb injury ratio.

    It would be odd if a massive decline in head injuries resulted from say, 10 to 25% usage in the preceding years (with massive drops even 4 years prior to the law), while a very minor change when helmet wearing went from 25 to 85%.

  3. Yes, fair enough. I didn't notice that data.