Motorcycle engine size and risk of moderate to fatal injury from a motorcycle crash.

Current New Zealand law requires that motorcyclists with a learner or restricted licence ride a motorcycle with an engine capacity of 250 cc or less. Previous research has reported inconsistent findings regarding the relationship between cubic-capacity and risk of a crash. We sought to determine: (1) compliance with the law; (2) if the risk of an injury crash is increased for learner/restricted licence holders who do not comply with the cubic capacity regulations; and (3) whether the risk of an injury crash increases with increasing capacity of the motorcycle. A population-based case-control study was conducted in the Auckland region over a 3 year period from February 1993. Among the controls, 66% were riding motorcycles with a capacity greater than 250 cc. The percentages for those with: full, learner and restricted, and no licence were 82, 29 and 60%, respectively. There was no evidence that learner and restricted licence holders who did not comply with the cubic capacity requirement were at increased risk. It should be noted however, that 75% of those who were complying were doing so on motorcycles of 250 cc or less. Relative to motorcycles of less than 250 cc the risk of an injury crash was elevated by at least 50% for all cubic capacity categories, with the exception of the 251-499 group. There was, however, no consistent pattern of increasing risk as cubic capacity increased. The findings of this study coupled with the fact that cubic capacity is a poor measure of power suggest that, if cubic capacity was to remain the sole basis for restricting learner and restricted licence holders, consideration should be given to having a substantially lower cubic capacity than 250 cc. An analysis of risk in terms of power to weight ratio and style of motorcycle may provide a more useful insight into the benefits of motorcycle design restrictions for novice riders.