ITE Journal - August 2020 - 32

Freight/Large Vehicles

Safety Analysis Methods and Results

The design of some turbo roundabout features is influenced by
the physical dimensions and turning characteristics of the larger
vehicles that will use the intersection. The lane widths of turbo
roundabouts are determined with consideration of the design
vehicle, typically the largest vehicle anticipated to regularly
navigate the intersection. The inside lane is often wider than the
outside lane to compensate for the design vehicle maneuvering
a smaller radius. As with other types of modern, multilane
roundabouts, the decision to allow large vehicles to track across
more than one lane while entering, circulating, and exiting or to
stay within their lane considers other lane width-related considerations, such as right-of-way and safety performance for all vehicle
types and users.
Turbo roundabouts include central truck aprons to better
accommodate larger vehicles that need to navigate the intersection.
Aprons can also be provided on the perimeter of the roundabout to
provide additional turning space for large vehicles.

Given the brief history of turbo roundabouts, international safety
studies based on an analysis of crash data are limited and not yet
available in the context of a U.S. driving population. Dutch research
analyzed crash data at seven intersections-including signalized,
yield-control, and old-style rotary types-that were converted to a
turbo roundabout and found a 76-percent reduction in the number
of injury crashes.4 Polish research found that turbo roundabouts
with a raised lane divider experience a lower crash frequency than
those with paint stripes only. However, the research observed lower
severity crash outcomes in both cases. Only 7 percent of crashes
on turbo roundabouts without a raised lane divider resulted in an
injury, compared to 4 percent of crashes with a raised lane divider.9
Safety surrogate measures resulting from microscopic traffic
simulations or field observations (i.e., time-to-collision, vehicle
speeds, vehicle conflicts, incorrect movements, and incorrect paths)
have also indicated that turbo roundabouts are likely to experience
less frequent and less severe crashes than multilane roundabouts,
due to the reduction of conflict points within the roundabout and
the lower speeds required to navigate the smaller radii.*

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32

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OKL AHOMA CIT Y

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Based on international experience, turbo roundabouts are feasible
options for U.S. intersections where a roundabout is a potential
alternative, particularly where traffic demand indicates the need
for a multilane roundabout. International experience indicates that
the more direct entry geometry and enhanced delineation of lanes
can make it easier for motorists to successfully navigate the turbo
roundabout. Though accepted and present throughout Europe,
the concept is new to the United States and requires education
and outreach for both practitioners and the public. FHWA has
released two new publications focused on turbo roundabouts:
Turbo Roundabouts: Informational Primer and Advancing Turbo
Roundabouts in the United States: Synthesis Report.6, 11
Given the unique geometry and limited knowledge of turbo
roundabouts in the United States, traditional public outreach
methods for roundabouts will need to be modified for educating the
public about turbo roundabouts. In general, an emphasis should be
placed on the fundamental characteristics of turbo roundabouts,
including the reduction of conflict points and the intuitive lane
selection and channelization. Agencies can also emphasize the key
differences between multilane roundabouts and turbo roundabouts,
including the lane divider and the spiral lane markings. Demonstrated safety performance benefits in the United States will be key
to the sustained viability of the strategy. As with other new concepts,
obtaining a set of reliable U.S. turbo roundabout crash modification
factors will require broader implementation by multiple agencies.
* The ability to reliably link safety surrogates to crash frequency and severity remains a topic of ongoing research
and debate.


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ITE Journal - August 2020

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