An Overview of Converting Kabul’s Traffic Circles and Rotaries to Modern Roundabouts

Circular intersections, in the form of traffic circles and rotaries, were built around the world in the first half of the twentieth century. However, with the rise in traffic flow, regular traffic congestion and accidents forced authorities to look for alternative solutions and, in many cases, signalized intersections became the preferred alternative. In the 1960s, the UK introduced concept of modern roundabouts and a new set of priority rules for the circulating traffic. Since the inception of modern roundabouts, they have enhanced both operational and safety aspects at these locations, even when used to replace large traffic circles/rotaries, proving that circular intersections can still exist in urban transportation networks. Kabul City, the capital of Afghanistan, had more than 30 traffic circles and rotaries where regular congestion is present. Ten of them are recently changed to signalized intersection while the rest remains as either traffic circle or rotary. This report focused on investigating if the option of modern roundabout is a suitable, or even preferred alternative to some of the existing traffic circles and rotaries in Kabul City. The research started with collecting geometric data for existing traffic circles/rotaries, followed by a preliminary analysis where they were scored for modern roundabout conversion suitability, based on initial geometric data of size, location, and shape. An on-line survey and one follow-up interview were conducted with Kabul engineers to characterize existing traffic circles/rotaries and their views on potential conversions to modern roundabouts. Finally, one of the current traffic circles was selected for a more detailed operational analysis via microsimulation, using the tool called PTV VISSIM. Supported by literature, it seems that large traffic circles/rotaries can become a more desired location for modern roundabout conversion and as result as scored higher in terms of suitability in preliminary analysis. In the meantime, the survey conducted with Kabul’s engineer showed either traffic is regulated by a traffic officer or no priority rules are applied in existing traffic circles. It also showed that aggressive driver’s behavior is often noticed due to the lack of strict rules and planning from the authorities. Moreover, the survey showed the skepticism amongst responders in regard to the recent improvements with signalized intersection at traffic circles. Regarding the modern roundabout Kabul engineers were open to the option based on a more detailed data analysis. The Sensitivity analysis with microsimulation tool VISSIM showed that the additional bypass lane further enhances the operation. The results also showed that after certain flow rate thresholds there is dramatic increase in delays and queues. Moreover, when traffic includes higher portion of left turns a modern roundabout becomes more a desirable option. The findings from geometric data, survey, and sensitivity analysis have provided evidence that modern roundabouts should be considered as a potential alternative in Kabul City. However, prior to any implementation, significant data gaps should be closed, so a more detailed analysis can be concluded on their suitability