Cook A, Sheikh A. BMJ 2000 v321 p1055
Hospital admissions data for April 1991 to March 1995 was analysed for injuries concerning cyclists and the subset of these involving skull fractures and intercranial injury. The total number of emergency admissions changed little over the research period, but the number with head injury as primary diagnosis fell from 3,393 to 2,571. Reductions occurred across all age groups. As cycle helmet use had increased during this period, the authors attribute the decline in head injuries largely to this factor.
The first calculation presented by Cook and Sheikh is incorrect by an order of magnitude: 35,056 cycling injuries are 0.28%, not 2.8%, of 12.6 million hospital emergency admissions. Elsewhere in their paper the authors greatly overstate the risk of injury when cycling.
The authors state that the 24.2% decrease in numbers of head injuries that they report from 1991 to 1995 is attributable to the increase in helmet wearing, but no evidence is presented either of the magnitude of this increase in helmet use nor of any change in mileage cycled or cycling demographics. In fact, over this period, the number of cyclists killed and seriously injured per 100 million cycle kilometres increased by 8.6% while the figure for all drivers and riders decreased by 16.7% (DETR, 1998). For fatalities there was no change for cyclists against a reduction of 20% for all drivers and riders. These statistics suggest that any decrease in cyclists' head injuries over this period has been more than offset by increases in other serious and fatal injuries among cyclists.
During the study period the UK was in the deepest recession since World War II. Casualties and the severity of injuries fell for all road users. However from 1994 - 1998 (overlapping the study period), cycling fatalities increased by 25% as helmets became more common. This rise in fatalities did not apply to any other road user, whilst the amount of cycling was either stable or in decline.
Other factors are almost certainly involved in the recorded reduction in head injuries.
Road Accidents Great Britain 1998, table 8. Department for the Environment, Transport & Regions, London.