Two-Geometry roundabouts: Design Principles

Although concentric multi-lane roundabouts have a higher capacity than single-lane roundabouts, they have the disadvantage of a higher driving speed through the roundabout. They also reintroduce the possibility of lane changing on the roundabout and hence raise the risk of accidents. The authors developed and studied the two-geometry roundabout in an attempt to deal with these and other drawbacks. This type of roundabout has the following features: no lane changing (or at least as little as possible) on the roundabout; lower driving speed through the roundabout; advantages in the manoeuvres of heavy duty vehicles. This paper discusses the concept of the two-geometry roundabout and the details determining success. In detail: first of all, the characteristics of the single-lane and multi-lane roundabouts will be illustrated, focusing on the problems of the latter; after which the development, the main characteristics and the typologies of the two-geometry roundabout will be illustrated (with some examples attached); finally, we will focus on the positive effects of the two-geometry roundabout on heavy vehicles. In the final part, the methods for calculating the capacity of the two-geometry roundabout will also be introduced, and therefore the conclusions of the paper. The paper discusses the concept of the Two-Geometry Roundabout and outlines its principal advantages. We first development, main characteristics and typologies of Two-Geometry Roundabouts are illustrated (with some examples attached); therefore, we focus on the positive effects of the Two-Geometry Roundabout with particular attention on heavy vehicles. Finally, we present methods for calculating the capacity of the Two-Geometry Roundabout and conclude the paper.