ITE Journal - August 2020 - 30

Figure 4. Curved inside lane introduction.

Figure 5. Flat inside lane introduction.

The central island consists of a traversable portion (mountable
apron) and a non-traversable portion. The non-traversable portion
is typically used for signage, specifically a roundabout directional
arrow sign, as well as landscaping and other features that increase
conspicuity of the island.
Cutouts in the central island introduce the inside lane of the
turbo roundabout on the applicable approaches. There are two
developed methods for these cutouts: a curved entry and a flat
entry. The curved entry was the original design in the Netherlands.
It provides a smooth turning path for approaching vehicles but may
result in more circulating vehicles entering the inside lane. The flat
entry discourages this movement by circulating vehicles and is the
current practice in the Netherlands.
Turbo roundabouts are built with little or no flare or deflection
and smaller entry radii. The angle between entering traffic and
circulating traffic is closer to a perpendicular entry. These approach
features differ from modern multilane roundabouts in the United
States, which typically include flare to gain some capacity increase
and deflection to align entering vehicles "to the right of" the central
island in the desired direction of travel. The entry geometry of
a turbo roundabout generally does not channelize drivers into
the circulatory roadway to the right of the central island, and the
splitter islands generally do not have enough curvature to block
a direct path of approaching vehicles to the central island. This
approach geometry is based on the premise that it will be clearer
to drivers that they are approaching an intersection that should be
negotiated at lower speeds.4 Potential disadvantages include drivers
errantly hitting the central island, making wrong-way left turn
maneuvers to enter the roundabout, and making wrong-way exit
maneuvers into entrance approach lanes.7 International literature
emphasizes the importance of a roundabout directional arrow
sign, placed in the central island in the line of sight of approaching
drivers, that directs drivers to turn right. It also emphasizes the
need for a forgiving design of the central island and sign in the case
that either is struck.

A final key feature of the turbo roundabout is a lane divider
between each circulating lane. In the Netherlands, this lane
divider is raised but mountable, designed with little vertical
profile and a rather flat slope to provide forgiveness for errant
vehicles. The raised lane divider is introduced with a triangular
demarcating feature that large vehicles can pass over.
Poland, Germany, and Canada have implemented turbo
roundabouts without raised lane dividers, in part due to possible
challenges these dividers present to motorcyclists and snow
plowing operations.8 Alternatives to raised lane dividers for
turbo roundabouts include striping and colored or textured
pavement. While these options do not provide a physical barrier
to lane changing, they still communicate this message to the
driver. International research showed that turbo roundabouts
with raised lane dividers experience lower crash frequencies than
those with paint stripes only.9 However, there were lower severity
crash outcomes in both cases. Only 7 percent of crashes in turbo
roundabouts without a raised lane divider resulted in an injury
compared to 4 percent of crashes with a raised lane divider.9

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Pedestrian and Bicyclist Considerations
Pedestrian and bicyclist accommodations at turbo roundabouts
do not differ from other modern roundabouts. Reference can
be made to the Roundabouts Informational Guide and Crossing
Solutions at Roundabouts and Channelized Turn Lanes for
Pedestrians with Vision Disabilities when considering these
user groups.7, 10 Key design considerations for pedestrians
include placing sidewalks along the perimeter, separated with a
landscaped strip or buffer; convenient crosswalks placed where
drivers can be expected to yield and where they are less likely to
be blocked by queued vehicles; and a splitter island that accommodates an accessible and comfortable crossing.
Bicyclist accommodation at turbo roundabouts is consistent
with guidance in the Roundabouts Informational Guide.
Bicyclists can either mix with traffic or utilize separate facilities

ITE Journal - August 2020

Table of Contents for the Digital Edition of ITE Journal - August 2020