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A modern roundabout is an intersection with a circulatory roadway at which the vehicle speed is low, and the traffic is continuous and circulating in one direction around the central island towards the exits at the approach legs. Modern roundabout design is an iterative process that is composed of the following steps: (1) the identification of the roundabout as the optimal traffic solution; (2) the definition of the number of lanes at the intersection based on the required capacity and the level of service; (3) the initial design of the roundabout geometry; (4) design vehicle swept path, the fastest path analysis, and visibility performance checks; and (5) detailed roundabout design if the results of the performance checks are in line with the design recommendations. Initial roundabout geometry design elements are not independent of each other; therefore, care must be taken to provide compatibility between them. An overview and a comparative analysis of the initial geometric design elements for suburban single-lane roundabouts defined in roundabout design guidelines and norms used in Croatia, Austria, France, the Netherlands, Germany, Serbia, and Switzerland is given in this entry.

Subject reviewTurbo roundabouts are lately a more common choice in designing multilane roundabouts. First guidelines for geometric design of this particular roundabout layout were developed by a Dutch Information and Technology Platform CROW in 2008. Soon after, a number of European countries began to develop their own regulations on turbo roundabouts, adjusted to their traffic cultures and their local conditions. In order to present the diversity of design approaches for turbo roundabout planning in various European regions, comparative analysis of turbo roundabout design procedures described in Slovenian technical specifications, Serbian design manual, German working document, and Croatian and Dutch guidelines is made. Relevant influential parameters used in these procedures are commented. Study presented in this paper may contribute to the success of turbo roundabout planning procedure. Keywords: comparative analysis; geometric design; guidelines; influential parameters; turbo roundabouts Geometrijsko oblikovanje turbokružnih raskrižjaPregledni rad Turbokružna raskrižja su u posljednje vrijeme sve češći izbor pri projektiranju višetračnih kružnih raskrižja. Prve smjernice za geometrijsko oblikovanje ovog posebnog tipa kružnog raskrižja razvijene su od strane nizozemskog istraživačkog centra CROW u 2008. Ubrzo nakon toga, nekolicina europskih zemlja započela je s razvojem vlastitih smjernica, prilagođenih njihovim prometnim navikama i lokalnim uvjetima. Kako bi se dobio uvid u raznolikost pristupa oblikovanju turbokružnih raskrižja u različitim europskim zemljama, provedena je komparativna analiza postupaka njihova oblikovanja opisanih u slovenskim tehničkim specifikacijama, srpskom priručniku, njemačkom random dokumentu, te hrvatskim i nizozemskim smjernicama. Komentirani su relevantni utjecajni parametri koji se u navedenim postupcima koriste. U radu prikazano istraživanje doprinijelo bi uspješnosti provođenja postupka geometrijskog oblikovanja turbokružnih raskrižja.

Challenges that emerge in roundabout design are mostly related to space constrictions and provision of appropriate deflection around the central island. This can result in speed profiles on roundabouts that might reduce their potential as a noise abatement measure. Because of this, the impact of a roundabout on noise levels and its applicability as a traffic calming device and a noise abatement measure should be investigated in the early design stage, by modeling noise levels. In this paper, the following hypothesis is tested: vehicle movement trajectories, defined during the fastest path performance check in roundabout early design phase, can be used as the road traffic noise sources when modeling noise at roundabouts using static noise model. This procedure (1) simplifies the preparation of the noise model, (2) results in a model that is closer to the real-world in terms of traffic flow conditions, (3) allows the noise calculations in the early stages of roundabout design, and (4) includes the influence of roundabout deflection on calculated noise levels. The abovementioned simplifications of the road traffic noise modeling process should encourage the optimization of roundabout geometry in terms of its noise reduction capabilities in the preliminary design phase of these intersections.

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