Capacity Limits of Basic Turbo-Roundabouts

Kocianova, Andrea · 2016 · DOAJ

DOI: 10.26552/com.C.2016.4.90-98

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Summary

This paper investigates the capacity limits of basic turbo-roundabouts, addressing the critical question of whether these intersections can provide capacity equal to or greater than standard two-lane roundabouts. While turbo-roundabouts are widely recognized for improving traffic safety by reducing conflict points and driving speeds, their capacity performance relative to conventional designs has been a subject of debate. The study aims to quantify total capacity under various traffic conditions to determine the operational advantages and limitations of turbo-roundabouts compared to standard multi-lane alternatives. The research employs a lane-based capacity model grounded in gap-acceptance theory, utilizing formulas proposed by Brilon for turbo-roundabouts and Wu for standard two-lane roundabouts. Driver behavior parameters, including critical gaps and follow-up times, are derived from Slovak technical guidelines. A key methodological distinction involves the assumption of traffic flow redistribution: the model assumes efficient use of both entry lanes at major turbo-roundabout entries, whereas it accounts for the observed underutilization of inner lanes in standard two-lane roundabouts, modeling a 30:70 split between left and right lanes. The analysis evaluates total capacity across varying ratios of major-to-minor entry traffic loads and different proportions of left- and right-turning traffic. The findings indicate that turbo-roundabouts consistently offer significantly higher capacity than single-lane roundabouts, typically 1.2 to 2.0 times greater. When compared to standard two-lane roundabouts, turbo-roundabouts demonstrate a 5% to 25% capacity advantage when major entries carry a dominant share of traffic (e.g., 90:10 major-to-minor ratio). This superiority stems from the efficient utilization of both lanes at major entries. However, this advantage reverses under balanced traffic loads or when minor entries carry significant volume. In scenarios with high left-turning traffic (>70%) or low right-turning traffic (<30%) at minor entries, standard two-lane roundabouts can exceed turbo-roundabout capacity by 5% to 20%. This occurs because turbo-roundabouts restrict right-turning traffic at minor entries to a single lane, leading to underutilization and subsequent capacity failure of the left lane when right-turn demand is low. The study concludes that turbo-roundabouts effectively combine high safety standards with substantial capacity, particularly in networks where traffic flow is concentrated at major entries. The specific geometry forces drivers into defined paths, preventing the weaving conflicts and lane-changing behaviors that reduce efficiency in standard roundabouts. However, the capacity benefit is conditional; turbo-roundabouts are less efficient than standard two-lane designs in environments with balanced entry loads or high left-turning volumes. The results suggest that while turbo-roundabouts are a robust solution for safety and capacity in dominant-flow scenarios, their implementation should consider local traffic composition to maximize performance.

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• intersection design