The Analysis of Roundabouts through Visibility

Visibility is one of the most important issues in almost all geometrical norms for road and intersection design. This paper deals with the question of visibility on roundabouts but from a different point of view in respect of the norms that have currently been adopted regarding capacity and safety issues. Attention is focussed on the simple consideration that many trajectories, in well designed roundabouts, are curvilinear and therefore two possible measures of distances between vehicles can be calculated along their trajectories and directly (such as the Euclidean distance) between the vehicles themselves. The first measure is associated with the distance in terms of the conflict point and the second one represents the sight distance between the vehicles. These two distances are generally different on a roundabout. The aim of the paper is to demonstrate both analytically (through a mathematical representation) and experimentally (by using trajectories processed from video images shot on a working roundabout) that in many cases the visual distance is less than the distance to the conflict point. This difference changes in time (and space) according to the speeds of vehicles. Comparisons between the two approaches, applied to a real case, are also provided. Three cases are taken into consideration (1- two vehicles both circulating both in the circulating roadway, 2-one is circulating in the circulating roadway and the other is entering, 3-both vehicles are entering) highlighting the effect that visibility can produce.