Historical Context The idea to install one-way circulatory traffic management on intersections was struck around 1900, both in the USA and Europe. The devisers regarded the principle of merging and weaving essential for the rotary system and, in practice, right of way had not yet been regulated unambiguously. This resulted in traffic circles performing worse than the devisers had in mind. In the early sixties (20th century) in the United Kingdom, the rule of giving priority to circulatory traffic was discovered to have significant advantages since the change in the traffic operation it enabled roundabouts to be kept small enough to handle a lot of traffic safely. Instead of facilitating merging and weaving at high speed, drivers had no option but to accept a break in the circulatory traffic flow. Systematic research, initiated by Franck Blackmore (1916 - 2008) in the sixties, got this insight worldwide attention. The modern Dutch roundabout is a variation on the British roundabout. In the UK tangential approaches and exits are normally applied, with radial approaches the exceptional when there is a lack of space in built-up areas. On the other hand, in the Netherlands both the approaches and exits are radial, to such an extent that these have become a distinctive feature in the Dutch definition of a roundabout. In the Netherlands the building of the first modern roundabout was initiated by Kars Erné (1932-2010). In addition to the two features mentioned (priority to roundabout traffic and radial connecting legs) the single-lane circulatory roadway has made this modern roundabout a success. The concept for ‘sustainable safety’ offered the road owners a new grip on improving road safety. At many spots, the modern single-lane roundabout is the safest type of intersection but where larger volumes of traffic have to be handled, the compact two-lane roundabout was developed, based on the same concentric design principle as the single-lane roundabout, but handling two lanes of traffic. However, this configuration again introduces the necessity to weave. To avoid serious cutting-in conflicts in the Netherlands and Germany it was recommended to provide the departure leg with a single lane. However, this does not eliminate weave conflicts. Turbo Roundabout based on Sustainable Safety Principle In 1996 the challenge to the author was to develop a two-lane roundabout without weaving conflicts. This study shows that the consistent application of the principles of sustainable safety leads to a safe design of the Turbo Roundabout. Striving for recognisable points of conflict determines the choice of the main shape. But also the design of a detail such as the roundabout shield is largely determined by this principle. Another principle of sustainable safety is realising homogeneity by reducing speed, which leads to the introduction of lane-dividers. The principle of forgivingness is the basis of the mountable shape of the lane-dividers. Design Methodology The spiral shape of turbo roundabouts can be designed using circles with staggered central points. This is made possible by the low driving speed, and it offers a route with fewer steering movements for the driver. Safety is the factor in limiting the number of yielding roundabout lanes to no more than two. This leads to the need to tailor different types of turbo roundabouts to different traffic volume patterns. The number of circle segments on which a couple of lanes have to be continued determines the shape of the turboblock: the way the circle centres are staggered depends on the question the circulating lanes have to shift up one lane outwards over one, two, three or four segments. Requirements Signposting and Lane Marking The principle that drivers choose the correct lane on the approach leg is fundamental to the turbo roundabout. To this end a coherent system of lane arrows (Dutch roundabout arrows analogue to fishhook arrows) on both the road surface and on signposts has been developed. Because the approach lanes to turbo roundabouts tend to be short, it is useful to inform the drivers about the lane division before they reach the point at which they must make a choice. Hence the development of a new type of switch signpost option to the approach lane. Capacity Research Turbo Roundabout To choose the right type of turbo roundabout in a pre-evaluation, in 2000 the MULTI-LANE ROUNDABOUT EXPLORER was developed. Initially this model was of a rather synthetic nature but later it became possible to carry out a simple calibration. Only in 2007 it was possible to do more extended capacity measurements on a turbo roundabout (in Nieuwerkerk a/d IJssel) In the literature two types of model are mentioned to determine the capacity of roundabouts: linear models and exponential models based on gap acceptance theory. The parameters of the first type of model can only be estimated at traffic flow level. The gap acceptance theory is based on the assumption that the behavioural parameters (the critical gap, the follow-on time and the minimum headway) can be estimated at vehicle level, by which the capacity is to be determined. Because the pseudo-conflict (caused by vehicles leaving the roundabout via the opposite leg direction) plays a part on Dutch roundabouts, in the latter method it is necessary to have access to measurements in a saturation situation. In analysing the different data sources (at vehicle level and traffic flow level) it was found that the estimation of vehicle level based parameters corresponds with the results based on the traffic flow level whereas in more complex situations this is not longer the case. In spite of this deficiency in the present gap acceptance theory, it is possible to gain better results with a model based on this theory than with a linear model, provided that the parameters are adjusted for traffic flow measurements under saturation conditions. Further research will be necessary to determine which of the underlying assumptions of the gap acceptance models are accountable for these deficiencies. The findings of this study reveal that for the time being it is necessary, also for this reason, to employ a method of estimation that incorporates both data at vehicle level (gap observations) and observations at circulation level (five-minute observations) in a saturation situation. The functions found make it possible to compare the capacity of turbo roundabouts with the results of measurements on compact two-lane roundabouts in Germany. These data confirm the findings of the MULTI-LANE ROUNDABOUT EXPLORER: the fact that traffic on the turbo roundabout makes better use of the inner roundabout lane, the capacity will be higher than that of a compact two-lane roundabout. Possibilities of a Roundabout Metering Signal The turbo roundabout is designed for operating without traffic lights, just as single-lane roundabouts. Nevertheless, the possibility to expand the capacity of turbo roundabouts by applying signalisation was examined but concluded that a fully signalised turbo roundabout with four legs will not offer any higher capacity than without signals. However, both analysis and simulation found that a roundabout metering signal (RMS) not only results in a more fair distribution of waiting time during peak periods but will also increase the capacity: the capacity of the left lane of the next connecting leg will increase from 15% to 50% without reducing the capacity of the leg on which the RMS is placed. Signalised Turbo Circle It is necessary to enlarge and change the shape of a roundabout to use full signalised equipment effectively. Starting from the radial connecting approach legs, the departure legs must exit the roundabout tangentially. The resulting shape is called a Turbo Circle. And for tailoring a traffic circle expressly to the application of traffic lights, it will be necessary for each lane to be reserved for one direction. For a turbo circle it is more important than for a turbo roundabout that a driver is able to choose the right lane in time. As for handling traffic volumes, this will mostly require short selection lanes – shorter than the approach legs of a signalised intersection with the same number of lanes. But to gain a good overview of the many lanes (mostly more than four), it is necessary all the same for the selection lanes are sufficiently long that two signpost portals can be situated behind each other. In addition to this pre-lane switch, signposts are needed in advance of the actual selection lanes to inform the drivers how to switch lane to the selected destination. Also, roundabout arrows over a 180m stretch on a two-lane approach leg will significantly improve the process of lane switching later on. An analytical model has been derived to compare the cycle times of a turbo circle and a four-leg intersection (with the same number of lanes: in every direction two lanes). From that it may be concluded that a turbo circle proves its usefulness in situations with high traffic volumes. Above 6,000 pcu/h the cycle time of a turbo circle tends to be lower than half of that of a four-leg intersection. Then the total capacity will also be higher under the proviso that two opposite left-turn volumes are not higher than 130% to 160% of the straight-on volumes of these legs. Safety of the Turbo Roundabout Finally, the safety and perception of turbo roundabouts as well as turbo circles have been investigated. The safety of the turbo roundabouts was researched in 2005 by a pre and post study also involving single-lane roundabouts, which necessitates the results of these comparisons to be corrected with certain factors. From literature it is known that without calculating regression-to-the-mean and trends in the overall safety effects, the general safety effects of measurements will be overestimated. In addition, the 2005 study proved that not only the incidental but also the structural differences in risk setting will influence the priority of safety measurements, so measures will have been taken for the most dangerous spots. It has been possible to determine that for the single-lane roundabouts a correction of the accident victims must be reduced from 82.9% to 68.3% to get a good figure of the effectiveness of replacing four-legged (mostly two-way-stop) crossings by single-lane roundabouts. The replacement of intersections by turbo roundabouts has shown a surprisingly high reduction of 82%. The observation time was too short to define the structural component of the effect of the prioritising process by officials, but it is not expected to be less than that for the single-lane roundabouts. Applying the corrections for regression-to-the-mean and the trends in the overall safety effects in the number of injury crashes by turbo roundabout replacements, the reduction percentage of 82% has to be reduced to 76.1%. This is comparable with the figure of 71.8% in the United States for replacement by concentric two-lane roundabouts. Although this is a smaller reduction, this is not a statistically significant difference. But what is striking is the difference in reduction of the total number of accidents by concentric two-lane roundabouts in the US (18 %) and by turbo roundabouts (49 %, in this case not corrected). Investigation of the perception of the turbo roundabout and the turbo circle In 1999 a survey of users of the turbo roundabout in the shape of a bone-roundabout was carried out. The score for clarity of the Coldenhove turbo bone-roundabout was 73% for private car drivers and 87% for truck drivers, a result that can be regarded as positive. In terms of the possibility or otherwise for lane changing, this roundabout is a model for turbo roundabouts built later on, so a similar survey was not repeated for other turbo roundabouts. In unrecorded observations ‘left-turn, short-cut’ manoeuvres were ascertained, which led to a change at the start of the inner circulatory lane of later turbo roundabouts. Private car users of the Doenkadeplein turbo circle judged the route clarity on the circle itself as 52.5 % while truck drivers score it 76.9%, a less positive result than the turbo roundabout. The private car user score in particular prompted improvements to be proposed to both the signing and marking of the turbo circle, and these have been incorporated in the recommendations of chapter 5. New Pre-Signpost In many discussions with users of the turbo roundabout as well as the turbo circle, the aspect of choosing the right lane in time has been mentioned as the main point of critique. Combined with the conspicuous low rate of the surveyability of the lane selection in the approach of the Doenkadeplein, this prompted a change from the general roundabout pre-signpost into a pre-lane switch signpost for the turbo roundabout and the turbo circle. In Conclusion Summarising, it can be stated that the application of sustainable safety principles really leads to a more safe design of a multi-lane roundabout combined with the possibility to handle greater traffic volumes. Meanwhile, it has to be realised that the turbo principle demands more attention to lane selection on distributor roads than road users are used to. An alternative for the pre-signpost has been developed to stimulate better lane selection and this will be incorporated in the national guideline for signposting. Further Research Some questions could not be answered in the context of this study. Therefore recommendations for further research include: In the surveys not all traffic flows – in the form of an origin-destination matrix – on a roundabout have been recorded. Because of the strong indications that the influence of the pseudo conflicts depends on the question of whether the vehicles are coming from the leg ahead or the segment ahead, further roundabout surveys also need to record not only the lane use but also the legs of origin and destination. The traffic flow data of the roundabouts surveyed did not allow to determine whether and how strongly a dominant flow from a single leg influences the capacity of the next entry. Further research that compares roundabouts data with a dominant flow from a single leg is recommended. In 2008 the right-angled start of the inner circulatory lane was introduced (par 3.5.5). It is to be expected that not only the view from the right entry lane will be improved but also the overview on the left entry lane. An item for further research is the question of whether this will also expand the capacity of that left entry lane. The positive effects of roundabout metering signals on the two-lane approach legs have been determined by analysis and simulation. The detector configuration, and especially the fine tuning/adjusting in practice, will be a matter of practical evidence so a corresponding pre and post survey is recommended. The turbo circle was originally designed in a compact shape (with lane dividers 30 cm wide), based on the premise to apply LEDS in the road surface to guide the drivers in their lanes in the conflict areas. But at the moment of building the first turbo circles robust road surface LEDS were not available. Therefore, the turbo circles have been provided with wide splitter islands. But this diminishes their clarity and surveyability. Given the opinion of the car drivers the road surface LEDS are still welcome. On turbo circles with splitter islands, it will be possible to test the robustness of the road surface LEDS, without risking diminished functionality of the traffic circle. Once the robustness of the road surface LEDS has been established, they will also be applied on compact turbo circles. Regarding the short period available for research on the safety of turbo roundabouts, a more extended survey is recommended, with a focus on the safety of bicycles and moped riders related to right-of-way. To reach the level of accuracy comparable with the results of the research on the safety of single-lane roundabouts in this study, the registration of traffic accidents in the Netherlands has to be improved.