FASTEST PATH VEHICLE SPEED ANALYSIS AT STANDARD TURBOROUNDABOUTS WITH VARIOUS APPROACH LEG POSITIONS

Džambas, Tamara; Dragčević, Vesna · 2022 · DOAJ

DOI: https://doi.org/10.13167/2022.24.1

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Summary

This study investigates the applicability of the Dutch calculation model for fastest-path vehicle speed analysis at standard turboroundabouts with varied approach leg positions. Turboroundabouts are multi-lane intersections with spiral circulatory roadways and raised mountable lane dividers, designed to reduce conflict points and vehicle speeds compared to conventional double-lane roundabouts. Current design regulations in the Netherlands, Slovenia, Serbia, and Croatia mandate speed analyses using the Dutch model, which was originally developed for classic single-lane roundabouts. However, previous research identified shortcomings in existing turboroundabout design procedures, leading to the development of an improved design approach based on vehicle movement geometry. This paper examines whether the Dutch model remains valid when applied to these newly optimized turboroundabout schemes, which account for various design vehicle scenarios, circulatory roadway radii, and non-standard approach leg alignments. The methodology involved analyzing 161 standard turboroundabout schemes that fulfilled swept path requirements for two-axle trucks with semitrailers and three-axle buses. These schemes were derived from an initial set of 759 configurations, varying inner radii from 11 to 21 meters and approach leg positions including radial angles from 75° to 105° and translatory shifts up to ±8 meters. For each scheme, twelve fastest vehicle paths were constructed for through movements and right turns, maintaining a 1-meter distance from potential impact points such as roadway edges and lane dividers. Vehicle speeds were calculated using the Dutch model’s formula, which relates speed to the radius of the circular arcs defining the fastest path. These calculated speeds were then compared against maximum recommended speed limits specified in existing regulations. The results demonstrated that the Dutch calculation model is largely inapplicable to standard turboroundabouts with varied approach leg positions. In 81% of the analyzed cases (31 out of 38 schemes), at least one fastest path for a right turn could not be constructed due to the physical presence of raised mountable lane dividers. Additionally, the model’s equation for determining radii for through movements in the side driving direction failed when approach legs were not aligned radially at 90°, as the necessary geometric parameters could not be defined. For the paths that could be constructed, through-movement speeds ranged from 41 to 45 km/h, unaffected by approach leg positions. However, right-turn speeds were notably higher, particularly when approach leg angles varied, often exceeding regulatory limits. The study concludes that the Dutch calculation model does not accurately reflect real traffic situations at turboroundabouts with non-standard approach leg alignments and should not be used for speed analyses until a new, appropriate model is developed. The authors recommend revising existing design regulations to address the impact of various approach leg positions on vehicle speed. Until such revisions occur, they suggest alternative speed control measures, such as vertical traffic calming devices or speed radars. The findings also question the feasibility of achieving prescribed speed limits at large-diameter turboroundabouts in suburban areas, noting that German guidelines, which do not require fastest-path speed verification, may offer a more practical approach for these complex intersections.

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• intersection design