Two-Geometry Roundabouts: Design Principles

Although multi-lane concentric roundabouts have a greater capacity than single-lane roundabouts, they may encourage increased driving speeds and decrease in safety. Furthermore, lane changing is possible in multi-lane roundabouts, thus increasing the risk of crashes. To address these drawbacks, the authors present the “Two-Geometry” Roundabout. This type of roundabout discourages lane changing within the roundabout, lowers driving speed through the roundabout, and facilitates the manoeuvres of larger and longer vehicles. The paper discusses the concept of the Two-Geometry Roundabout and outlines its principal advantages. We first delineate characteristics of single-roundabouts and multi-lane roundabouts, focusing on the problems of the latter. Next, the 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.