ITE Journal - August 2020 - 28

The turbo roundabout is a roundabout alternative that eliminates
conflict points that are associated with lower-severity crashes that
sometimes occur in modern 2x2 multilane roundabouts, roundabouts
characterized by two entry lanes approaching two circulating lanes.
The turbo roundabout was first designed and implemented in the
Netherlands in the 1990s. Now there are more than 500 turbo
roundabouts worldwide, with more than 370 in the Netherlands.
The turbo roundabout can achieve similar capacities as modern
2x2 multilane roundabouts with potential to enhance relative safety
performance. With a limited dataset, an international crash-based
safety evaluation concluded that conversion of an intersection from
yield-control, signalized, or old-style rotary to a turbo roundabout
was associated with a 76-percent reduction in the number of injury
crashes.4 In addition, the geometric characteristics of the turbo
roundabout result in operational outcomes that should help address
lane selection, lane changing, and entering and exiting behaviors
that can lead to the lower severity, multiple-vehicle crashes in 2x2
multilane roundabouts. This paper provides an overview of turbo
roundabout features, design considerations, and performance
characteristics, along with opportunities for U.S. implementation.

Key Features
The most notable differentiators of the turbo roundabout are
its perpendicular entry, spiral road geometry, and lane dividers
that require drivers to choose the proper lane prior to entering
the roundabout in order to leave the roundabout in the desired
direction. Figure 2 illustrates nine key turbo roundabout features,
several of which are discussed in more detail in this article.

Conflict point frequency for multilane roundabout.

Design Considerations
Turbo roundabouts can be considered at any intersection where
a roundabout is a potential alternative, particularly where
traffic demand indicates the need for a multilane roundabout.
Operational performance models for turbo roundabouts have
not yet been developed for, or adapted to, the context of a U.S.
driving population. However, international research suggests
turbo roundabouts have similar capacities as 2x2 multilane
roundabouts, and that the roundabout capacity models of
the Highway Capacity Manual (HCM) are likely to represent
reasonable capacity estimates for turbo roundabout approaches
with up to two lanes.4, 5 At modern multilane roundabouts in the
United States, the capacity of one entry lane ranges from 300 to
1,100 passenger cars per hour (pc/h), depending on conflicting
flow in the circulatory roadway, implying a total approach
capacity ranging from approximately 600 to 2,200 pc/h for a
two-lane approach.
There are five different types of turbo roundabouts-basic,
egg, knee, spiral, and rotor turbo roundabouts-that differ with
respect to central island design, number of circulating lanes, and
number of approach lanes. The Federal Highway Administration's
(FHWA's) Turbo Roundabouts: Informational Primer contains
illustrations and capacity estimates for each of these turbo
roundabout types.4, 6 The selected type is driven by the desired
capacity and the number of lanes on the approach roadways.
Regardless of the type, there are four unique design elements of
turbo roundabouts-turbo block, central island cutout, perpendicular approach geometry, and lane dividers.

Conflict point frequency for turbo roundabout.

Figure 1. Conflict point frequency for multilane roundabout and turbo roundabout.3
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ITE Journal - August 2020

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