This paper presents a naturalistic study of motorcyclists’ behaviours during their commutes in the Paris region. The study focuses on lane splitting, which consists of riding between two streams of slow or stopped vehicles that are proceeding in the same direction. This practice is frequent in dense traffic on French urban expressways and particularly in the Paris region but remains without detailed scientific analysis. In an ergonomic approach, 11 motorcyclists drove for a month with an instrumented vehicle with cameras. The video recordings enabled the description of the driving contexts and the conduct of self-confrontation interviews. The results concern firstly the description of the practice of lane splitting and its significance in their daily journeys. Secondly, the findings indicate the importance of perceptual activities in the participants’ riding behaviour. These perceptual activities are organized around two major processes: (1) an intensive search for information in the environment in order to foresee risky situations and (2) the detection of situations that systematically impair the visual conspicuity of the riders and taking steps to improve it. The findings are discussed with a view to gaining a better understanding of lane-splitting practices. At the time of writing, the results are also being used by the French authorities to improve the training curriculum and modify the legislation that deals with lane splitting.