Cite or link this publication as: doi:10.4233/uuid:e01364ce-78de-465b-a8c8-39e28a4585dd
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.
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.
Signalised Turbo Circle
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.
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).
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.
• 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.