Abstract
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.
Highlights
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’
A growing number of foreign studies have pointed out the poor traffic safety characteristics of ‘standard’ two-lane roundabouts and a lower capacity than what was expected [1]
Some alternative types of roundabouts are already in frequent use all over the world, other types have only been implemented within certain countries, while some are still in the development phase [4] and, can be called ‘theoretical roundabouts’
Summary
A growing number of foreign studies have pointed out the poor traffic safety characteristics of ‘standard’ two-lane roundabouts and a lower capacity than what was expected [1]. The ‘target’ roundabout [5] is designed as two one-lane roundabouts with different outer diameters located on dual levels (Figure 1), and all vehicles turning right at both roundabouts have their own separate right-hand-turn bypass lanes. This type of roundabout allows one to drive from all directions to all directions, and if someone makes an error, e.g., if a driver mistakenly stays in the left-hand lane at the entrance, it is still possible to turn right at the exit (which is not the case for the turbo roundabout). ‘Four flyover’ roundabouts are designed with one large one-lane roundabout (Douter = 80 m) on the upper level, and all vehicles turning left on major roads have their own separate left-hand-turn bypass lanes (R = 35 m) located on another lower level. By physically separating left-hand turning traffic flow on major roads, a one-lane roundabout is obtained, with no crossing and weaving conflict spots
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