Fact

Roundabouts - What types of roundabout are there?

There are different types of roundabout. The most important types are the single-lane roundabout and the two-lane roundabout; in addition, a special type of two-lane roundabout is the so-called turbo-roundabout. As the name indicates, single-lane roundabouts only have one driving lane. If there is much vehicle traffic a second lane is needed, thus resulting in a two-lane roundabout. At two-lane roundabouts there are sometimes conflicts due to weaving. This problem is solved by the turbo-roundabout which makes it possible to exit the roundabout without conflicts with other vehicles.

The great advantage of the two-lane roundabout is its larger capacity (see Table 1), but there are also disadvantages compared with single-lane roundabouts: the higher speeds because of the wider traffic lanes, the more complex traffic situation caused by lane changing and weaving on the roundabout, cutting-in conflicts on exit roads, and longer crossing distances for mopeds, cyclists, and pedestrians. This last problem can be solved by using single instead of double access ramps, this, however, does indeed diminish the roundabout's capacity. In addition, a single exit ramp ensures a lower speed and, therefore, a lower collision speed. It also prevents the danger of vehicles being obscured by other vehicles.[i]

Table 1. Rules of thumb for roundabout capacity (CROW, 1998).

 

With the problems of a two-lane roundabout in mind, the turbo-roundabout was developed (see Figure 1). This roundabout has non-concentric markings, which makes it possible to exit a two-lane roundabout without the risk of lateral conflicts i.e. situations in which vehicle sides can touch one another. In essence, a turbo-roundabout has the following features:

  • no weaving takes place on the roundabout;
  • traffic entering the roundabout has to give way to traffic on two lanes;
  • each segment has one single lane on which traffic can choose whether or not to leave the roundabout.

Figure 1. Turbo-roundabout.

 

For long vehicles such as lorries and buses, roundabouts can be an awkward obstacle. This must be allowed for in the design of the roundabout, particularly on roads with a lot of freight traffic. For geometric design characteristics of the various roundabout types we refer to the Dutch guidelines (CROW, 1998; 2002a and 2002b; 2008).


[i] Danger of one vehicle being obscured by another: if traffic on the one traffic lane hinders the crossing traffic's view on the other traffic lane, thus increasing the risk of a crash when crossing over.

Pictures

Fact sheets(s)