# Speed Reduction Capabilities of Two-Geometry Roundabouts
## 4 Discussion  
The main reasons for the implementation of alternative roundabout types are the disadvantages of standard roundabouts regarding specific location circumstances, resulting in low levels of traffic safety or low capacities as well as negative impacts on the environment and mobility [20; 21; 22]. According to [22], the main advantages of two-geometry roundabouts are: (1) they occupy less space than standard modern roundabouts, (2) they are easily passable by long vehicles due to their variable circulatory roadway width, and (3) they ensure low vehicle speed because the required personal vehicle path deflection is created by their circular central island. The investigation presented in this paper has shown that analyzed two-geometry roundabouts are indeed easily passable by the selected long design vehicle. However, the design vehicles' swept paths used in the roundabout design resulted in extremely large circulatory roadway widths along the major axis (x), ranging from 7.95 to 12.25 m. This could result in personal car drivers trying to use this wide circulatory roadway as multiple lanes, thus reducing traffic safety. Circulatory roadway widths along the minor axis (y), ranging from 5.45 to 7.35 m, were mostly wider than the recommended values of 5.5 m [1] and 6.0 m [22]. These wide circulatory roadways should be avoided, but at the same time, the circulatory roadway must allow the unobstructed passage of long vehicles. The solution is to reduce circulatory roadway width to an appropriate size by introducing truck aprons in the roundabout design. A truck apron is a paved edge of the central island raised relative to the circulatory roadway and constructed with a greater cross slope to deter car drivers from crossing it, thus providing greater safety on roundabouts. In the second part of the investigation, the circulatory roadway width along the minor axis (y) was fixed to 5.5 m to meet the requirements of Condition 1 and Condition 2. This intervention in the initial roundabout schemes reduced the circulatory roadway widths along the major axis (x) and increased deflection around the central island, which is favorable in terms of traffic safety and speed reduction. The position of the right carriageway edge entry and exit radii (R\({}_{\text{entry}}\), R\({}_{\text{exit}}\), R\({}^{\prime}_{\text{entry}}\), and R\({}^{\prime}_{\text{exit}}\), Figure 1c), designed according to the design vehicles' swept paths, resulted in values of the expected driving speed through the roundabout (V) higher than the recommended 35 km/h [14] in every analyzed roundabout scheme, including the standard roundabouts. These differences were especially pronounced for two-geometry roundabout schemes with smaller major axis (a) values (up to 16 km/h on schemes with (a) of 18 m). Here, it should be noted that this recommended speed limit at roundabouts is conservative compared to the ones given in roundabout design guidelines and regulations used worldwide. For example, Swiss and American guidelines limit the speed to 40 km/h, and French and Italian guidelines to 50 km/h [19]. At the same time, the CROW model (Equation (3)), used in this investigation for the estimation of vehicle speed, results in higher speed values compared to the FHWA model given in [13], which includes superelevation and side friction factors [16; 19]. According to [2; 8; 9; 13; 27], consistency between the speeds of various movements within the intersection can help to minimize the crash rate between conflicting traffic streams. In the first part of the investigation, 94% of the analyzed paths constructed for the two-geometry roundabout schemes met the requirement for a maximum relative speed on entry (V\({}_{1}\)-V\({}_{2}\)), while only 1 (2%) of the analyzed paths met the requirement for a maximum relative speed on exit (V\({}_{3}\)-V\({}_{2}\)). For roundabouts that did not meet these requirements, lower speed for the fastest entering and exiting movements should be provided.