This report attempts to document the evidence pertaining to the nature and characteristics of the relationship between motorcycle engine capacity and collision involvement. Information was derived from two sources: international practices and experiences related to licensing restrictions on motorcycle engine size; and scientific studies that have examined the association between engine size and collision involvement. Despite a considerable amount of research no study has yet clearly established whether there is a relationship and, if so, what its nature, magnitude and characteristics are. Moreover, after years of restrictions being imposed on the operation of large motorcycles by novice riders in several countries, the results of such efforts remain mixed. Consequently, no unambiguous bottom-line emerges from the research reviewed in this report--accordingly, any intentions to introduce regulations or legislation that would restrict engine size must proceed with a clear understanding and acknowledgement both of the ambiguous nature of the scientific evidence that would support such a move, as well as the mixed and often disappointing results achieved in jurisdictions where such restrictions have been introduced.
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