ITE Journal - August 2020 - 29

Source: FHWA

The spiral alignment of a turbo roundabout is generated from
the "turbo block," a series of circular arcs with centers located at
various points along a reference line known as a "translation axis."
The arcs of the turbo block represent the inner and outer edges of
each lane. The inner radius of the turbo block, which represents
the radius of the central island, is selected based on the anticipated
size of the turbo roundabout. The turbo block and position of
the translation axis differs for each turbo roundabout type. The
following figure illustrates an example turbo block for a basic turbo
roundabout at a four-leg intersection with the major road oriented
along the x-axis and the following notation:
TR1: radius of the inside edge of the inside roadway.
TR2: radius of the outside edge of the inside roadway.
TR3: radius of the inside edge of the outside roadway; the
difference between TR2 and TR3 is the width of the lane divider.
TR4: radius of the outside edge of the outside roadway.
Δυ: shift for the TR1 centers, equal to the difference between the
values used for TR3 and TR1.
Δu: shift for the TR2/3/4 centers equal to the difference between
the values used for TR4 and TR2.

Source: FHWA

Figure 2. Turbo roundabout characteristics.

Figure 3. Example turbo block.
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Augu st 2020

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ITE Journal - August 2020

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