1. Introduction. This paper is concerned with collision risk reduction through the identification, measurement and analysis of contributing factors and the evaluation of risk control measures appropriate to the approaches to the Strait of Juan de Fuca. Risk factors were identified through a traffic survey and collision probabilities were estimated and modified by a simulated modification to traffic routeing. Marine collision risk analyses have tended to describe the circumstances of collisions without comparison to the conditions encountered by marine traffic. In the absence of any test of significance, such an analysis is revealing yet provides an incomplete basis for traffic management decisions. This study examines the importance of contributing factors through such a comparison.