The main motivation of the authors of this article is to establish a rigorous definition of the potential capacity that a motor vehicle driver has to avoid a collision against a pedestrian. Henceforth we will call this capacity avoidability. To calculate the avoidability, it is necessary to analyze time, distance and itinerary, initial position of the pedestrian when exposed to the risk, initial speed; theoretical maximum speed developed by the vehicle and road limit speed; the driver’s reaction time and the influence of the environment; and the interrelation of the initial positions of vehicle and pedestrian with respect to the transversal axis of the road. The definition, categorized by variables, of a driver’s ability to avoid run over a pedestrian in an urban area has an evident usefulness: it allows knowing the influence of the initial speed of a vehicle as an isolated variable and the importance of the road limit speed in the ability to prevent an accident.