Functions, Capacities, and Traffic Safety Characteristics of Some Types of Two-Level Roundabouts

Over recent decades, roundabouts have become increasingly used when building new at-grade intersections or up-grade junctions all over the world. Consequently, control of traffic flows at at-grade intersections and up-grade junctions using roundabouts creates unique design problems. Nowadays, ‘alternative’ types of roundabouts have started to become very popular, especially because of their advantages compared with ‘standard’ roundabouts and standard types of up-grade junctions. Some of these alternative types of roundabouts are two-level roundabouts, which are still currently in the development phase. It is for this reason that they can be called ‘theoretical roundabouts’. Two-level roundabouts are particularly useful in urban and suburban areas with space limitations due to their relatively small footprint. This paper illustrates three new alternative types of two-level roundabouts—‘target’, ‘four flyover’, and ‘roundabout with left and right bypasses’—as well as their functions, capacities, and traffic safety characteristics.