Speed Analyses of Intersections Reconstructed into Roundabouts: A Case Study from Rijeka, Croatia

The reconstruction of a standard at-grade intersection into a roundabout is very often motivated by the need to improve traffic safety, and the effects are usually monitored through before–after analyses based on traffic accidents. As in some countries, like Croatia, data on traffic accidents are not available, in this study, a methodology for the estimation of roundabout implementation effects based on measured operating speeds was developed and applied. This methodology enabled the analysis of the development of roundabouts designed according to the Croatian guidelines through the analysis of four roundabout case studies. Statistical analyses of before–after operating speed were performed, and speed time distribution variations, traffic volumes, and design elements were analyzed, compared, and correlated. Statistically significant differences were found between operating speeds before and after the reconstruction, although the expected effect of speed reduction of roundabouts was not observed. The weak correlations observed between the individual design elements of the roundabouts and operating speed highlight the need to analyze the combination of design elements with operational speed. The application of the developed methodology enabled systematic safety analyses of the selected roundabouts. The results of the analyses suggest a need to revise the guidelines.