Using micro-simulation methods for optimisation of proposed elements of roundabouts with regard to their safety and performance, movement of pedestrians and cyclists

Most locations limiting fluency and safety of traffic are those, where traffic flows cross, connect or turn. With the exception of leveled interchanges, conflict of traffic flows occurs on relatively confined areas. Directions of traffic flows intersect at different number of conflict points, which are on intersections the most common settings of traffic accidents. Uncontrolled intersections have certain capacity and safety limits, exceeding of which leads to significant delay of certain flows or to acceptance of dangerously short distances between vehicles, which increases potential risk of collisions. The main advantage of roundabouts is the fact that they eliminate numerous shortcomings of at-grade intersections, such as the number of conflict points by excluding certain movements (i.e. dangerous left turns), reducing the waiting period for an appropriate space-time gap between vehicles when crossing a road with completely different traffic intensity, etc. The major advantages of roundabouts include safer connection of minor roads, simplification of movements of vehicles into a single direction around the central island, avoiding rapid transit of the intersection and enforcing harmonisation of speed, increasing fluency of traffic flow, enabling small angle of crossing vehicles in weaving areas of the intersection. All of these properties significantly reduce risk of traffic accidents and severity of their consequences. The disadvantage of roundabouts compared to regular intersections remains potentially greater space demands for their implementation.
Traffic safety attributes or roundabouts
Speed and its characteristics are the basic safety features of roundabouts. Modern roundabouts are designed in order to constitute physical barriers for excessive speeds, in particular by an adequate direction curvature in sequence mode (progressively reducing the direction radius on the arrival shoulder). The major goal of designers must be to ensure speed consistency of the roundabout, which guarantees minimal speed diversity of colliding traffic flows and thus provides their high stability and improved traffic safety. The compiled manual illustrates procedures for achieving these objectives.
Obviously, it is necessary to comply with general principles of safety, which, however, are often not properly understood:
• simplicity – a roundabout optimally fulfills requirement of simplicity, as vehicles of superior traffic flow approach only from a single direction (left),
• clarity – this includes not only provision of good view conditions, but also minimal information required to be processed by drivers, i.e. avoiding duplicate or unnecessary signs, advertising and commercial information in the view, etc.
• understandability – consistency of horizontal and vertical traffic marks, clarity of information and navigation, allowing correct selection of driving lanes, fluent ride and concentration of drivers on traffic.
Synthesis of foreign and local experience with safety of roundabouts reveals that roundabouts bring significant benefits in terms of traffic safety. Their implementation reduces accident rate and severity of consequences. For cyclists involved in accidents, current statistics are contradictory and ambiguous. This is given also by the fact, that statistics include also roundabouts, which do not meet modern criteria, i.e. they present design and construction defects leading to accidents involving also cyclists.
Optimisation process
The following figure illustrates the principle of simulating optimization procedure. After testing functionality of the simulation model created for the project proposal, statistical evaluation of traffic flow parameters is realised on individual segments of the transport system and the system as a whole. In the case of a comparative study, evaluation is carried out for all variants. Prior to this, selection of appropriate parameters for monitoring and evaluation is performed. This continues with identification of critical spots and local extremes, while it is necessary to define the critical limits. They constitute standard requirements, target quality level or specific requirements of the project. If critical spots are found, it is necessary to adjust or change the project proposal by changing designed elements of the transport system or changing traffic management organization. This is followed by a test of the optimized model in the simulation model and return to the point of statistical evaluation and comparison of variants. This process is repeated until the project proposal fulfills all requirements and, therefore, doesn’t produce critical spots. The last step is the final evaluation, formulation of recommendation and visualization of traffic flow.
In certain cases, it may occur that no optimization change leads to elimination of critical spots. Then, the project proposal is unsatisfactory and it is necessary to search for completely different solution (another type intersection, reduction of transport demand, etc.).
Safety level of the project proposal cannot be quantified by simulation directly, i.e. it is not possible to quantify number of accidents directly, only through simulation of known context of identified parameter values of traffic flow and accident rate. A guarantee of safe design is strict monitoring of correct proposed principles during creation of the project proposal and its optimization changes.
Manual for designing roundabouts
The manual is the main output of research task and its goal is to contribute to proper designing of roundabouts, to understand philosophy of such proposals, to acquaint the public with design principles, latest trends and results of performed micro-simulations.
Accessibility of knowledge and its promotion lead to suppression of myths and misunderstandings, which include mainly glorification of speed and size. Large roundabouts (with diameter exceeding 80m) revealed to be inconvenient in terms of safety in the Czech Republic and abroad. Countries such as the USA, Germany or the Netherlands abolished their implementation in behalf of modern roundabout (diameter within 70m). The same approach is recommended for the Czech Republic. The manual categorizes roundabouts according to the American model, with addition of a turbo-roundabout. Speed and its characteristics and effects are described in the chapter 2.3. Speed Reduction for the Purpose of Higher Safety. The manual recommends establishing physical barriers (such as direction curvature, geometric dimensions) that enforce suitable traffic behavior in terms of driving speed. Procedures of adequate geometric designs are described in the manual. Psychological barriers in terms of traffic signs limiting speed do not guarantee reaching the target state and require supervision and restrictions (punishment for failure), therefore they are less efficient. Non-motorised road users (pedestrians and cyclists) are an integral part of the manual. For pedestrians, it is recommended to establish low-speed environment (the lower the speed, the higher the chances of surviving collision team in exponential relation) with dividing islands (except for roundabouts). For cyclists, it is recommended lead cycle routes in low intensities lanes (up to 8000 veh/24h), or on separate paths in affiliated space. Leading cycle routes on separate lanes on the round-belt proved to be dangerous and inappropriate. Performance of a roundabout is documented by international empirical knowledge and by testing of alternatives through a simulation program. The manual defines capacity of symmetrically loaded roundabouts for each category, capacity of specific load configuration of a turbo-roundabout and capacity of entrance of a roundabout, depending on share of freight vehicles, pedestrians and on distance of conflict points. Optimization process utilising simulation is described, including optimization scheme with description of possible restrictions of such a procedure.
 

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Manual for designing roundabouts