The VSS standard "Roundabouts” (SN 640 024) is one of seven subsections of the VSS group "Capacity, Level of Service, Performance". This standard indicates the dimensioning and evaluation of the traffic flow on roundabouts. The dimensioning correlation for one-lane roundabouts with one-lane entry is based on research done in Switzerland in the middle 1980's. The dimensioning correlation for two-lane roundabouts with two-lane entry is based on research done in Germany, although the latest research results have shown that this correlation is no longer valid.
The most recent research results confirm the dimensioning correlation of one-lane roundabouts with one-lane entry. Additionally, a new dimensioning correlation for one-lane roundabouts with extra-wide circulatory roadway and two-lane entry, which are used frequently in Switzerland, has been found. In Switzerland, it has not yet been possible to derive a dimensioning correlation for two-lane roundabouts with two-lane entry.
The main research need is to complete the standard "Roundabouts" (SN 640 024) and provide the corresponding basic principles. Based on empirical measurements, a new dimensioning correlation for evaluating the capacity of two-lane roundabouts with two-lane entry needs to be derived, as well as examining critical gaps and follow-up times.
The objective of this research project is to establish basic scientific principles to amend the standard SN 640 024 "Roundabouts“. Based on empirical measurements, a specific dimensioning correlation for two-lane roundabouts in Swiss traffic conditions should be found. Aside from deriving the capacity of the entry, dependent on the traffic volume of the circulatory roadway, the critical gaps and follow-up times need to be examined. The influence of crossing pedestrians on the capacity of the entry and of the exit also needs to be investigated.
In the first stage the appropriate investigation objects have to be evaluated with high traffic loads in a first step. Since in Switzerland so far only a small number two streaky roundabouts were implemented, a survey by the competent authorities of the cantons is provided at the beginning of the research. In addition to providing the relevant planning documents thereby shall include information to be procured for accidents. Based on the results of this survey appropriate investigation objects can be selected subsequently.
In a second step, the detection of the traffic-related variables using video recording during peak hours, supplemented by the measuring device of the IVT (measuring post), with which can be recorded (location of vehicles in cross-section), the speeds and the track behavior occurs.
Based on the video recordings, the most important traffic related variable are detected. The performance of two streaky roundabouts will be evaluated. On the one hand the conventional regression method will be applied, which already came into its own and otherwise conducted earlier studies on the application. On the other hand second method will be used to derive the performance. This method is based on the border and subsequently gaps, which are determined by the maximum likelihood method.
The evaluation of the accident involves the determination of average values and deviations of the aforementioned absolute and relative accident figures (accident rates, accident cost rate).
Apart from the findings of capacity and traffic safety the present research work permits the following recommendations of the scope of application and design of two-lane roundabouts. To choice the type of a roundabout following should be considered:
• The use of a two-lane roundabout should be evaluated when it’s necessary in terms of performance.
• When the place is limited, the roundabout type 2/1+, that means instead of two marked lanes one, extra wide lane, should also be evaluated. The appropriate use of the type 2/1+ is when the external diameter is between 30 m and 40 m.
• The use of two-lane roundabouts is not reasonable until a diameter of 35 m. Two-lane roundabouts with an external diameter more largely than 50 m does not increase the entry capacity in case of high traffic volumes on the circulatory roadway.
The operational and geometric design of two-lane roundabouts should be planes carefully. Following principles are to be considered:
• Two-lane entries at two-lane roundabouts are to be arranged only when this is necessary in terms of performances and service level (jam). Otherwise a one lane entry is to prefer because of traffic safety.
• Signposts and direction-indicating pavement markings in two-lane entries are to be arranged with caution because the traffic volumes in the left entry cannot be increased in every case.
• To reach the entry capacity mentioned in this research it is necessary that the traffic volumes in both entry lanes are approximately equal (50 % ± 10 %). When there are differences more clear between the traffic volumes in both entry lanes the mentioned capacity will not be achieved.
• For reasons of traffic safety, in the exits of two-lane roundabout must be only one lane. Finally the thanks are addressed to the road authorities and the police stations of the cantons Basle-Land, Fribourg, Berne, Vaud, Lucerne, St. Gallen, Schwyz, Ticino und Zug for the support of the research project with the basic information about the situation of the roundabouts, the traffic volumes and the accident occurrence.