WIT Press

Design Of Modern Roundabouts In Urban Traffic Systems


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A. Pratelli


This paper makes an overview on modern roundabouts design and practice. Modern roundabout have become an issue of great interest over the last decade in Italy. This fact is partially drawn on the great success of roundabout traffic solutions in some European countries, where intersections design practice has changed in consequence of the performances gained with roundabouts and their general good acceptance by drivers. Modern roundabouts can have significant impacts and benefits in terms of circulation safety, delays and capacity. These quoted items form as many sections of the paper. Keywords: modern roundabouts; priority-to-circle or offside priority; roundabout design guidelines; circulation safety; delay and capacity. 1 Introduction Modern roundabouts, also called second-generation roundabouts, are based on quite different design criteria from those of traffic circles, or first generation roundabouts, built in the first half of last century. The first generation roundabouts gave priority to traffic flows entering from branches and were designed considering the weaving movements as basic goal. This way, the circles become very large, with long distances between successive branches and with relatively low speeds and circulating flows. On the contrary, modern roundabouts gave priority-to-circle and are designed for lowering speeds, with dimensions essentially defined by the number of branches, expected capacity and by the turning path of larger vehicles. The second-generation roundabouts are classified by the dimensions of their inscribed circle diameter, D, as follows: a) mini-roundabouts, when D is less than 22 metres; b) compact roundabouts, for D


modern roundabouts; priority-to-circle or offside priority;roundabout design guidelines; circulation safety; delay and capacity.