Alternative Types of Roundabouts

In a comprehensive review of roundabouts it could firstly be beneficial to look at their origins. The conventions or rules of usage are as important as the layout. The history of roundabouts shows that these conventions must be conveyed by clear signs and warnings, and by the indications implicit within the layout. If this is done well then drivers will respond logically including moderating their speed.

In the years from 1913 to 1914, Hellier suggested circular traffic systems at places, where several main roads would meet and the main connection of the circular system would prevent overload. At the conference of the local governmental committee on the subject of main roads in 1914, this idea was accepted as positive under the condition that the traffic requirements were met (every intersection should have sufficient empty space, and lawns alongside the intersections would be desirable). The initial phase of development in Europe was interrupted by the First World War. When the British Road Transport Board was set up in 1918, it was suggested that the roads of France should be the model for Europe. Gyratory systems were also used in the USA but there was great difficulty in regulating traffic, local ordinances were unenforceable and flouted, and there was no uniform rule of the road throughout the country. In 1924, at a US national conference, rights of way at intersections, and warning and stop signs were proposed. The “circus” idea continued to spread in the United Kingdom and was frequently recommended for busy junctions of more than four roads. During 1925-26 a lot of gyratory systems were introduced in London. These were simply one-way systems around existing squares with fairly sharp corners. Unfortunately some of the important principles implied in Henard’s concept, e.g. the entries into gaps during circling, operating over a short distance, were being lost. The transfer of these movements to a straight road caused differences in speeds at the conflict points but this may at first have been unimportant when all traffic speeds were quite low. The design was based solely on commonsense and experience [1].

Analysis of literature shows that “modern roundabouts” nowadays exist in all European countries, as well as in more than 60 countries elsewhere in the world, so we could say that they are a world phenomenon.

Today, after many years of experience regarding roundabouts, there are still different ideas about the “ideal roundabout” with little consensus on the crucial effects of rules on how to negotiate intersections. The development of design rules and advice from an extensive body of research should allow civil and traffic engineers to produce the most effective forms of this junction type, even if for a variety of reasons this is not always carried out in practice.

As pointed-out previously, today after many years of experience regarding roundabouts, there are different ideas about the ‘‘ideal roundabout’’. Therefore the development of design rules and advice from an extensive body of research should allow civil and traffic engineers to produce more effective forms of this junction type. It also needs to be stressed that the roundabout has been ‘‘at the development phase’’ since 1902, this development is still in progress, and one of the results of this progress is that several types of roundabouts are in worldwide usage today, called the “alternative types of roundabouts”. Some of them are already in frequent use all over the world, some of them are recent and have only been implemented within certain countries, and some of them are still at development phases. It is because of that we can call them “theoretical roundabouts” .

Roundabouts are an increasingly common form of junction worldwide and their effective design requires detailed analysis of maximum vehicle throughput capacities. Since the 1970s, a series of models have been developed worldwide for an estimation of the capacity of roundabouts, almost all of which have relied upon extensive empirical data due to the complexity of the physical and behavioral processes affecting roundabout entry capacities. However, given the different fundamental principles (and particularly the geographical origins) of models, it is important to have a clear understanding of their limitations and their applicability within new contexts. These models are based on three main methodologies: empirical, gap acceptance and simulation. Due to their limitations, each of these methodologies on their own cannot completely explain the complex behavioral and physical processes involved at roundabout entries, hence all models require strong semi-empirical or fully-empirical bases using data obtained from their countries of origin.

In order to prevent accidents between motorized and non-motorized participants, the following different strategies are known in general: eliminate the risk; separate the non-motorized participants from the risk situation; if that is not possible—insulate the risk; if that is not possible—modify the risk; if that does not work—equip the road infrastructure (with additional road furniture), than control the risk behavior (watch, supervise); when it is not enough—inform and instruct non-motorized (also motorized) participants (through brochures, leaflets, newspapers, television…); when that cannot be done—restrict to approach the risk zone (by legislation or prohibition); the last action to be taken is to start the emergency (reconstruction and new—more safe solution).

Today, modern roundabouts exist in all European countries, and there are also several countries elsewhere in the world where they are numerous (the US, Australia, New Zealand, Israel, and Mexico).