Modern and alternative types of roundabouts – state of the art

Abstract

Over the recent decades, roundabouts have been increasingly used when building new at-grade intersections and up-grade junctions all over the world, and also when rebuilding existing intersections. Modern roundabouts exist in all European countries and mainly worldwide, so we can say that today modern roundabouts are a world phenomenon. No uniform guidelines exist in Europe for the geometric designing of roundabouts as specific circumstances (local customs, habits, traffic cultures...) differ from country to country. Certain solutions that are safe in one country could be dangerous in another. Consequently, most countries have their own guidelines for the geometric designs of roundabouts that are, as far as possible, adapted to their circumstances in these countries and are therefore more acceptable within their surroundings. Today, after many years of experience, there are different ideas about the "ideal roundabout", with some consensus on the crucial effect of rules how to negotiate intersections. There are several different types of roundabouts worldwide today, called the alternative types of roundabouts. Some of them are already in frequent use all over the world (hamburger, dumb-bell …), and some of them are recent and have only been implemented within certain countries (turbo roundabout, dog-bone…) or are still at the development phases (turbo-square, flower, target, with segregated left-turn slip lanes…). Both groups typically differ from the "standard" one-or two-lane roundabouts in one or more design elements, as their purposes for implementation are also specific. Therefore, it was decided that it would be useful to collate, in one article, some of the alternative types that are already in frequent use today in some countries and some of them that are "still coming".

Geometric design of turbo roundabouts

“…During that, input parameters recommended in this paper should be used. Future studies should include the following: (1) definition of new step-by-step turbo roundabout design procedure, based on the rules of design vehicle movement geometry; (2) definition of more relevant input parameters in swept path analysis; (3) definition of relationships between the turbo roundabout radius, design vehicle swept path and circulatory lane width; (4) definition of the influence of various alignments of roundabout approaches on turbo-block dimensions.…”

“…They claim that the main reason the latter countries have less satisfactory experiences with turbo roundabouts is because "road marking does not prevent the lane changing" [4]. German author [29] claims that application of mountable lane dividers is not acceptable because of motorcycle safety, winter service and maintenance requirements, and that the absence of these elements does not have great influence on intersection traffic safety and capacity.…”

“…Distance between the outer centres is labelled with ∆v, and the distance between the inner centres with ∆u. Outer centres represent the centres of the smallest circular arcs R 1 , while the inner centres represent the centres of larger circular arcs R 2 , R 3 and R 4 . Physical separation of the traffic lanes is achieved by 30 cm wide raised mountable lane dividers, made of special concrete elements or stone granite cubes.…”

“…According to Slovenian authors [4], countries with turbo roundabouts can be divided into these groups: countries in which raised mountable lane dividers are used, and countries in which raised mountable lane dividers are avoided (dismissed). They claim that the main reason the latter countries have less satisfactory experiences with turbo roundabouts is because "road marking does not prevent the lane changing" [4].…”

“…The reasons for this are high driving speeds and large number of potential conflicts at roundabout multilane entrances, exits and circulatory roadway. In the past few years road designers have been trying to solve these problems by introducing new roundabout layouts [4]. One such layout, which is used in the engineering practice (in design of new and reconstruction of existing roundabouts) in growing number of countries, is so-called turbo roundabout.…”