Speed reduction effects of urban roundabouts

Antov, D.; Abel, K.; Sürje, P.; Rõuk, H.; Rõivas, T. · 2009 · OpenAlex-citations

DOI: 10.3846/1822-427x.2009.4.22-26

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Summary

This study investigates the speed reduction effects of urban roundabouts in Estonia, aiming to identify design factors that influence driver speed choices. The research is motivated by the growing popularity of roundabouts as a road safety measure in Europe, despite local skepticism regarding their capacity and safety for vulnerable users like pedestrians and cyclists. While international evidence strongly supports the safety benefits of roundabouts, Estonia lacked comprehensive statistics on roundabout design and performance. The primary objective was to determine how specific geometric parameters, particularly the inscribed circle diameter, affect vehicle speeds, thereby linking design to safety outcomes. The study employed a two-stage approach: a literature review of international roundabout safety records and an empirical analysis of speed data from 11 urban roundabouts in Tallinn, Tartu, and smaller Estonian settlements. The selected roundabouts varied in size, with inscribed circle diameters ranging from 15 meters to over 85 meters. Researchers classified these roundabouts into five size categories based on diameter. Speed data were collected by video-recording traffic and calculating driving speeds based on the time taken to traverse the roundabout and its measured dimensions. The analysis covered 3,268 vehicles, categorized into cars, heavy goods vehicles (HGVs), and buses. Additionally, GPS-based speed measurements were conducted on a specific route in Tallinn to compare speeds at roundabouts versus the connecting road links. The results demonstrate a clear correlation between roundabout size and vehicle speed. Smaller roundabouts (inscribed diameter < 20 m) exhibited the lowest speed distributions, with the majority of vehicles traveling between 15 and 25 km/h. In contrast, larger roundabouts (diameter > 70 m) showed significantly higher speeds, with a substantial portion of cars traveling between 30 and 40 km/h. Mean speeds increased consistently with roundabout size across all vehicle types, although cars accelerated more rapidly than HGVs and buses as the diameter increased. The GPS data further confirmed that roundabouts effectively reduce speeds to approximately half of the speeds observed on the connecting road segments. The study concludes that the inscribed circle diameter is the primary factor influencing driver speed at roundabouts. This finding supports the broader understanding that reduced speed is the key mechanism behind the improved safety records of roundabouts, as lower speeds significantly decrease the severity and frequency of accidents. The authors suggest that these speed trends can be used to estimate the safety effects of roundabouts in Estonia. However, they note that further investigation is needed to assess the specific safety impacts on pedestrians and cyclists, as current evidence in this area remains inconclusive. The research underscores the importance of proper geometric design in achieving the intended traffic calming and safety benefits of roundabouts.

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