Advancing Turbo Roundabouts in 

the United States: Synthesis Report

FHWA Safety Program Cover images by Arcadis

 Turbo Roundabouts, Netherlands

http://safety.fhwa.dot.gov
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ii

Technical Documentation Page 

1. Report No.

FHWA-SA-19-027

2. Government Accession

No.

3. Recipient's Catalog No.

4. Title and Subtitle

Advancing Turbo Roundabouts in the United States: Synthesis Report

5. Report Date

September 2019

6. Performing Organization Code

7.Author(s)

Porter, R., Gooch, J., Peach, K., Chestnutt, C., Moore, B., Broeren, P., &

Tigelaar, J.

8. Performing Organization Report No.

9. Performing Organization Name and Address

Vanasse Hangen Brustlin, Inc (VHB)

940 Main Campus Drive

Raleigh, NC 27606

10. Work Unit No.

11. Contract or Grant No.

DTFH61-16-D-00005L (VHB)

12. Sponsoring Agency Name and Address

Federal Highway Administration Office of Safety

1200 New Jersey Ave., SE

Washington, DC 20590

13. Type of Report and Period

Synthesis Report - September 2019

14. Sponsoring Agency Code

FHWA

15. Supplementary Notes

The contract manager for this report was Jeffrey Shaw.

16. Abstract

A turbo roundabout has the same general operating characteristics as modern roundabouts but utilizes notably different

geometrics to address conflicts associated with common crash types in multilane roundabouts. International experience suggests

turbo roundabouts are adaptable to the U.S. context, providing an effective roundabout solution for higher-volume intersections.

The goal of this document is to synthesize existing published resources (e.g., reports, papers, presentations, videos, and tools) on

the topic of turbo roundabouts from international and domestic sources. The synthesis has three major sections: 1) geometric

design, 2) capacity and operational performance, and 3) safety performance. The synthesis provides specifics regarding the

treatment of various users, including motorists, pedestrians, cyclists, heavy vehicles, and motorcyclists. The synthesis also

includes sections on education and public outreach approaches targeting non-technical audiences. The geometric design of a

turbo roundabout is governed by the horizontal swept path of a design vehicle and a fastest path analysis for a passenger car. Key

geometric designs include selection of turbo roundabout type and determination of functional combinations of the inner radius,

cross section elements, and spirals within the turbo block. Turbo roundabout capacity is dependent on type but is generally

similar in magnitude to modern multilane roundabouts. Methods in the HCM 2010 produced reasonable estimates of turbo

roundabout capacity in Poland. Research available to-date also indicates that, with smaller radii and raised lane dividers, the

reduced vehicle speeds and elimination of weaving in a turbo roundabout is expected to produce fewer and less severe crashes

than multilane roundabouts and significantly fewer and less severe crashes than other traditional control types.

17. Key Words:

roundabout, turbo roundabout

18. Distribution Statement

No restrictions.

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this

page) Unclassified

21. No. of Pages

62

22. Price
iii
iv

Table of Contents

Introduction ................................................................................................................................... 1 

Geometric Design .......................................................................................................................10

Inner Radius ........................................................................................................................11

Turbo Block..........................................................................................................................11

Cross Section Elements ......................................................................................................12

Central Islands.....................................................................................................................15

Lane Divider.........................................................................................................................17

Approach Geometry.............................................................................................................17

Signing and Markings..............................................................................................................21

User Considerations ................................................................................................................23

Motorists ..............................................................................................................................23

Pedestrians..........................................................................................................................24

Cyclists ................................................................................................................................25

Motorcyclists ........................................................................................................................27

Freight..................................................................................................................................27

Capacity and Operational Performance......................................................................................28

Analysis of Turbo Roundabouts...........................................................................................28

Safety Performance ....................................................................................................................34

Evaluation of Crash Data ........................................................................................................34

Safety Surrogates....................................................................................................................36

Summary of Safety Findings ...................................................................................................39

Education and Public Involvement..............................................................................................39

Demonstrated Success ...........................................................................................................40

Signage ...................................................................................................................................40

Key Findings............................................................................................................................41

Summary and Next Steps ...........................................................................................................42

References..................................................................................................................................45
v 

List of Figures 

Figure 1. Graphic. Exit-circulating conflict of roundabout design.................................................. 2 

Figure 2. Photograph. Aerial view of turbo roundabout in Delft. ................................................... 4 

Figure 3. Graphic. Conflict point frequency for multilane roundabouts. Image based on 

Vasconcelos et al., 2014............................................................................................................... 5 

Figure 4. Graphic. Conflict point frequency for turbo roundabout. Image based on Vasconcelos 

et al., 2014. ................................................................................................................................... 6 

Figure 5. Graphic. Various turbo roundabout center island geometries: egg. Image based on 

Dzambas et al., 2017 with capacity value from Fortuijn, 2009b.................................................... 7 

Figure 6. Graphic. Various turbo roundabout center island geometries: basic. Image based on 

Dzambas et al., 2017 with capacity value from Fortuijn, 2009b.................................................... 8 

Figure 7. Graphic. Various turbo roundabout center island geometries: knee. Image based on 

Dzambas et al., 2017 with capacity value from Fortuijn, 2009b.................................................... 8 

Figure 8. Graphic. Various turbo roundabout center island geometries: spiral. Image based on 

Dzambas et al., 2017 with capacity value from Fortuijn, 2009b.................................................... 9 

Figure 9. Graphic. Various turbo roundabout center island geometries: rotor. Image based on 

Dzambas et al., 2017 with capacity value from Fortuijn, 2009b.................................................... 9 

Figure 10. Graphic. Turbo roundabout features..........................................................................10

Figure 11. Graphic. Sample turbo block. Image based on Dzambas et al., 2017.......................11

Figure 12. Graphic. Roundabout shield used in the Netherlands. ..............................................15

Figure 13. Photograph. Old design for starting the inner lane used in the Netherlands. ............16

Figure 14. Photograph. New design for starting the inner lane used in the Netherlands............16

Figure 15. Graphic. Dutch designs of raised lane dividers with no modifications. Image based on 

Overkamp & van der Wijk, 2009. Note: all measurements in m. ................................................18

Figure 16. Graphic. Dutch design of lane dividers with modifications for snow plowing. Image 

based on Overkamp & van der Wijk, 2009. Note: all measurements in m.................................. 18

Figure 17. Photograph. Lane divider for turbo roundabout at Victoria International Airport........ 19 

Figure 18. Photograph. Example “frog”.......................................................................................20

Figure 19. Photograph. Example of guide signage to be placed upstream of a turbo roundabout.

....................................................................................................................................................21

Figure 20. Graphic. Lane use arrows for turbo roundabout entry lane accommodating through 

and right turn movement as used in the Netherlands, Image based on Overkamp & van der 

Wijk, 2009. ..................................................................................................................................22

Figure 21. Graphic. Lane use arrows for turbo roundabout entry lane accommodating left turn 

movement as used in the Netherlands, Image based on Overkamp & van der Wijk, 2009........22

Figure 22. Graphic. “Fishhook” lane use arrows for roundabout approaches.............................23

Figure 23. Photograph. Example of a chicane in a splitter island to encourage cyclists to perform 

a two-stage crossing...................................................................................................................26

Figure 24. Photograph. Example of a grade separated bicycle facility at a turbo roundabout in 

South Holland. ............................................................................................................................26

Figure 25. Equation. Capacity equation. Image based on Bovy, 1991. ......................................28

Figure 26. Equation. Modified conflicting traffic volume including the pseudoconflict of exiting 

vehicles.......................................................................................................................................30

Figure 27. Equation. The Hagring (1998) capacity model for a roundabout entry lane. .............30
vi

Figure 28. Equation. Roundabout capacity model. Image based on Brilon and Wu, 2006.........32

Figure 29. Equation. SPF for all crashes on turbo roundabouts in Poland. Image based on Kiec 

et al., 2018. ................................................................................................................................. 35

Figure 30. Equation. SPF for PDO crashes at turbo roundabouts in Poland. Image based on 

Kiec et al., 2018. .........................................................................................................................35

Figure 31. Equation. Calculation of calibration factor for conflict I. Image based on Mauro et al., 

2015............................................................................................................................................37

Figure 32. Equation. Crash prediction model for roundabout approaches in New Zealand. Image 

based on Turner et al., 2009.......................................................................................................38

Figure 33. Equation. Crash prediction model for roundabout approaches in the U.S. Image 

based on Chen et al., 2013.........................................................................................................39

Figure 34. Equation. Crash prediction model for roundabout approaches in Italy. Image based 

on Chen et al., 2013....................................................................................................................39
vii

List of Tables

Table 1. CMFs for converting intersections to roundabout. .......................................................... 1 

Table 2. Standard design values for basic turbo roundabouts in the Netherlands. Table based 

on Overkamp & van der Wijk, 2009. ...........................................................................................14

Table 3. Approach capacity comparison table. Table based on Overkamp & van der Wijk, 2009.

....................................................................................................................................................29

Table 4. Values used for gap acceptance. Table based on Fortuijn, 2009a; Fortuijn & 

Hoodendoorn, 2015; Saric & Lovric, 2017. .................................................................................32

Table 5. Differences in percent of total intersection crashes by type on turbo roundabouts with 

raised and painted lane dividers. Table based on Macioszek, 2015. .........................................35
viii

List of Abbreviations 

AADT average annual daily traffic 

ADA Americans with Disabilities Act

ARR Australian Road Research

CMF crash modification factor 

HCM Highway Capacity Manual

PAR potential accident rate

PDO property damage only 

pce passenger car equivalents

ROW right-of-way

SPF safety performance function 

SSAM Surrogate Safety Assessment Model 
1 

Advancing Turbo Roundabouts in the United 

States: Synthesis Report

Introduction 

Implementing modern roundabouts saves lives and reduces serious injuries resulting from 

intersection and intersection-related crashes. As planned points of conflict, crashes attributed in 

some way to intersections contribute significantly to traffic fatality and injury numbers in the U.S. 

Approximately half of all crashes and half of fatal and serious injury crashes occur at or near 

intersections. In the single year of 2017, more than 9,000 people were killed in intersection and 

intersection-related crashes (NHTSA, 2017). In stark contrast, there were a total of 46 fatalities 

at roundabouts built in the U.S. over the nine-year period spanning 2005 to 2013, a time period 

in which the total number of roundabouts in the U.S. grew from a few hundred to a few thousand 

(Steyn et al., 2015). Roundabouts reduce the number and severity of intersection conflicts by 

eliminating crossing conflicts and lowering operating speeds through the intersection.

Eliminating or reducing the number of conflict points eliminates or reduces the chances of 

crashes from occurring. In addition, research on energy transfer in multiple-vehicle crashes and 

the human body’s tolerance to resulting forces shows that the smaller conflict angles and lower

speeds at roundabouts will result in lower probabilities of fatalities and serious injuries in 

crashes that do occur. Such results are evident in roundabout safety performance to-date. A 

review of three-star, four-star, and five-star crash modification factors (CMFs) in FHWA’s CMF 

Clearinghouse shows that 90 of 105 roundabout CMFs indicate reductions in expected crash 

frequencies from implementing both single and multilane roundabouts for all, angle, and rearend crashes, as well as fatal and injury crashes of all types (Rodegerdts et al., 2010). As one 

telling example, the CMF Clearinghouse includes “five-star” CMFs of 0.28 for fatal crashes and 

0.56 for injury crashes attributed to converting traditional at-grade intersections to roundabouts 

(see table 1).

Table 1. CMFs for converting intersections to roundabout.

Crash Severity “Five-Star” CMF Prior Condition Proposed 

Condition

Source

Fatal crashes 0.28 Yield-, Stop-, or 

SignalControlled

Intersection

Single or Multilane 

Roundabout

Elvik, 2017

Injury crashes 0.56 Yield-, Stop-, or 

SignalControlled 

Intersection

Single or Multilane 

Roundabout

Elvik, 2017
2 

Implementing roundabouts requires agency and stakeholder buy-in and support, which can 

sometimes be difficult to obtain, even with their proven safety performance record. FHWA has 

invested heavily in advancing roundabout planning, design, and analysis practices; delivering 

training; and developing educational and outreach materials to advance the consideration and 

use of roundabouts (FHWA, 2018). This has led to tremendous progress in roundabout 

implementation throughout the U.S. and resulting safety and operational performance benefits. 

Though single-lane roundabouts make up most of the roundabouts in the U.S., multilane 

roundabouts have become more common. Some constructed multilane roundabouts have 

experienced higher frequency of low severity crashes. This has led to some negative public 

perceptions for multilane roundabouts that may slow the growing momentum of their use. For 

example, a multilane roundabout in Madison, Wisconsin received negative publicity when a 

local news organization identified it as the top crash site in the city. Another multilane 

roundabout in Appleton, Wisconsin received attention from local press in 2017 when it 

experienced 30 reported crashes in the first two months after opening. While these articles 

typically provide caveats noting that most of the crashes are minor, they still shed negative light 

on roundabouts from a public perception perspective. 

The FHWA Roundabout Technical Summary describes conflicts between exiting and circulating 

vehicles at multilane roundabouts within the context of discussing larger separations between 

entries and subsequent exits (see figure 1). The Technical Summary notes that this situation 

arises because entering vehicles have more opportunity to begin traveling next to circulating 

traffic as opposed to crossing the path of exiting vehicles. This is a conflict that is unique to 

multilane roundabouts. Adapting the descriptions of operational configuration and driver 

behavior from Gustafson (2018), the larger separations may make drivers in the outer lane 

incorrectly think that it is up to drivers in the inside lane to select a gap in the outside lane before 

exiting the roundabout. In other words, the geometry (specifically the larger separation) may 

imply a “concentric roundabout,” even though the marking is for a “crossing roundabout.” 

Source: FHWA

Figure 1. Graphic. Exit-circulating conflict of roundabout design. 
3 

An examination of multilane roundabouts in Minnesota (Leuer, 2016) observed two common 

crash types:

1) Crashes resulting from entering traffic not yielding to traffic within the roundabout.

2) Crashes resulting from drivers changing lanes inside the roundabout because they 

selected the incorrect lane on the approach.

Leuer (2016) noted that unfamiliar drivers may tend to select the outside lane of a multilane 

roundabout (even when turning left) since they expect to exit the roundabout from the outside 

lane. Additional evidence for this possible explanation was uncovered in a driving simulator 

study, which showed that (Molino et al., 2007):

• More drivers correctly chose the right lane (94.8 percent) than the left lane (82.3 

percent) when asked to make different movements through a multilane roundabout.

• For the movements through a multilane roundabout where either lane was an option, 

only 44 percent of drivers correctly understood that either lane was an option. 

Some countries have implemented a modified version of a multilane roundabout, known as a 

turbo roundabout (see figure 2). A turbo roundabout has the same operating characteristics as 

modern roundabouts but utilizes notably different geometrics to address the conflicts associated 

with the common crash types in multilane roundabouts. In describing turbo roundabout 

operational principles, Fortuijn (2009b) outlined the following key turbo roundabout features:

• A second lane is inserted opposite of at least one entry lane.

• Traffic approaching the roundabout on at least one leg must yield to traffic in two, and no 

more than two lanes on the roundabout.

• Smooth flow is encouraged by a spiral alignment.

• Mountable raised lane dividers discourage lane changing within the roundabout.

• Each segment of the roundabout includes one lane from which drivers can choose 

whether to exit or continue around the roundabout. 

• At least two exit legs are two-lane.

• The diameter of the roundabout is kept small to encourage lower speeds through the 

roundabout.

• Approach legs are at right angles to the roundabout.

• Roundabout shields cut off views of the horizon for approaching vehicles. 

• Mountable aprons offer sufficient width for longer vehicles.

The spiral road geometry and separated lanes of turbo roundabouts require drivers to choose 

the proper lane prior to entering the roundabout in order to leave the roundabout in the desired 

direction. Figure 3 and figure 4 show that the turbo roundabout eliminates the conflicts 

associated with the common crash types in multilane roundabouts. At the two-lane exits of a 

turbo roundabout, drivers in the inside lane execute a “turn” to exit the roundabout, as in 

concentric roundabouts described by Gustafson (2018). However, the turbo roundabout 

eliminates the requirement in concentric multilane roundabouts of exiting drivers in the inside 

lane having to first cross the outside lane. This is done by physically forcing drivers in the 

outside lane to exit (Gustafson, 2018). A crash-based safety evaluation suggests conversion of 
4 

an intersection from yield-control, signalized, or old-style rotary to a turbo roundabout is 

associated with a 76 percent reduction in injury crash frequency (Fortuijn, 2009b).

 ©2019 Google Earth®.

Figure 2. Photograph. Aerial view of turbo roundabout in Delft. 
5 

Figure 3. Graphic. Conflict point frequency for multilane roundabouts. Image 

based on Vasconcelos et al., 2014.1 

 

1 Gustafson (2018) offered new definitions for conflict point types that he concludes are more universally 

applicable to roundabout conflict areas: 1) crossing conflict: a conflict point where two through 

movements intersect, with two input traffic streams and two output traffic streams, 2) joining conflict: a 

conflict point where a through movement terminates at another through movement, with two input traffic 

streams and one output traffic stream, and 3) separating conflict: a conflict point where a through 

movement originates by departing from another through movement, with one input traffic stream and two 

output traffic streams.
6 

Figure 4. Graphic. Conflict point frequency for turbo roundabout. Image based 

on Vasconcelos et al., 2014. 

Fortuijn (2009b) described five common turbo roundabout geometries (see figure 5 through 

figure 9): the basic, knee, egg, spiral, and rotor. All, through different center island geometry, 

reinforce the key turbo roundabout principles and features, and reduce conflict points. Variants 

shown in the figures include:

• Egg – similar to a basic turbo roundabout, but with only one approach lane on minor 

approaches.

• Basic – inside lane added on major approaches, two lanes on each approach.

• Knee – the inside lane is only added on one approach, two lanes on each approach.

• Spiral – three circulatory lanes, inside lane only added on two approaches, two 

approaches with three lanes and two approaches with two lanes.
7 

• Rotor – three circulatory lanes, inside lane added on each approach, three lanes on 

each approach.

International experience suggests turbo roundabouts are adaptable to the U.S. context. 

Adoption of the turbo roundabout in the U.S. should resolve some of the issues at multilane 

roundabouts, providing an effective roundabout solution for higher-volume intersections. The 

goal of this document is to synthesize existing published resources (e.g., reports, papers, 

presentations, videos, and tools) on the topic of turbo roundabouts from international and 

domestic sources. The synthesis is organized into three major sections: 1) geometric design, 2) 

capacity and operational performance, and 3) safety performance. The synthesis provides 

specifics regarding the treatment of various users, including motorists, pedestrians, cyclists, 

heavy vehicles, and motorcyclists. The synthesis also includes sections on education and public 

outreach approaches targeting non-technical audiences.

Figure 5. Graphic. Various turbo roundabout center island geometries: egg. 

Image based on Dzambas et al., 2017 with capacity value from Fortuijn, 2009b. 
8 

Figure 6. Graphic. Various turbo roundabout center island geometries: basic. 

Image based on Dzambas et al., 2017 with capacity value from Fortuijn, 2009b. 

Figure 7. Graphic. Various turbo roundabout center island geometries: knee. 

Image based on Dzambas et al., 2017 with capacity value from Fortuijn, 2009b. 
9 

Figure 8. Graphic. Various turbo roundabout center island geometries: spiral. 

Image based on Dzambas et al., 2017 with capacity value from Fortuijn, 2009b. 

Figure 9. Graphic. Various turbo roundabout center island geometries: rotor. 

Image based on Dzambas et al., 2017 with capacity value from Fortuijn, 2009b. 
10

Geometric Design

The turbo roundabout was invented in the Netherlands in 1996 (Fortuijn, 2009b) with the initial 

design guidelines developed by the Dutch Information and Technology Platform (CROW, 2008; 

Overkamp & van der Wijk, 2009). These documents define turbo roundabouts by the following 

features:

• A turbo roundabout has at least two lanes for some portions of the circulatory roadway. 

• The driver performs circulatory lane selection on the approach.

• The turbo roundabout entrances are yield control; yielding vehicles yield to no more than 

two lanes of conflicting traffic.

• Raised lane dividers within the circulatory roadway physically discourage weaving and 

lane changing.

• Exiting from the roundabout can only occur from the lane where the vehicle entered the 

roundabout.

Fortuijn (2009b) described these features, along with additional notable elements shown in 

figure 10. 

© Fortuijn 2009b.

Figure 10. Graphic. Turbo roundabout features. 

The geometric design of a turbo roundabout is governed by the horizontal swept path of a 

design vehicle and a fastest path analysis for a passenger car. The geometric design process 

consists of iterating through different combinations of the inner radius, cross section elements, 

and spirals within the turbo block. Designers select the inner radius value, lane widths, edgeline 

offsets, and lane divider widths, and then design the radii that produce the desired spirals. 

Designers perform swept path analyses with the selected design vehicle. If there is a violation in 

the swept path analyses (the wheel track of the design vehicle leaves the designated travel 
11

lane), designers adjust the inner radius and/or cross-sectional elements as needed and the 

turbo block is redesigned.

Inner Radius

The inner radius, R1 in figure 11, serves as the radius of the central island, the radius of the 

inside travel lane, and is the base value for the rest of the geometrics in a turbo roundabout. In 

principle, the smaller the inner radius, the lower the speed with which a vehicle will navigate the 

roundabout. Dzambas et al. (2017) found standard inner radius values for the Netherlands, 

Slovenia, and Serbia, of 10.5 m (34.4 ft) for a “mini” turbo roundabout, 12 m (39.4 ft) for a 

“regular” turbo roundabout, 15 m (49.2 ft) for a “medium” turbo roundabout, and 20 m (65.6 ft) 

for a “large” turbo roundabout. Croatian values were found to be 0.05 m less for the mini, 

medium, and large designs. 

Turbo Block

Turbo roundabouts create a smooth, spiraled vehicle path by shifting the centers of the radii 

defining the locations of the inside and outside lanes along a translation axis. The combination 

of these circles and the translation axis is referred to as the turbo block (Fortuijn, 2009b). Figure 

11 is an example of a turbo block for a basic four-leg, two-lane turbo roundabout. 

Figure 11. Graphic. Sample turbo block. Image based on Dzambas et al., 2017. 

According to Fortuijn (2009b), the orientation of the translation axis is based on the major 

approaches (those with two lanes) and should provide similar curvature for all through vehicle 

movements. Overkamp and van der Wijk (2009) stated the correct position for a translation axis 

is where “the distance between the right edge of each entry leg and the inner curve of the outer 

lane of the roundabout after one quarter turn [are] more or less equal” (p.70). Dzambas et al. 

(2017) recommended an orientation of “five minutes until five” (right side is rotated 57.5-degrees 

around the center below the x-axis) for a four-leg intersection and “ten past eight” (the left side 
12

is rotated 65-degrees around the center below the x-axis) for a three-leg approach in situations 

where the major road is in an East-West orientation (x-axis). The translational axis should be 

rotated based on the orientation of the major road approaches. An additional check from 

Overkamp and van der Wijk (2009) is that the outside edge of the minor entrance should 

intersect the outside edge of the outer lane after the translation axis.

The turbo block in figure 11 contains four radii, where:

• R1 is to the inside edge of the inside lane. 

• R2 is to the outside edge of the inside lane. 

• R3 is to the inside edge of the outside lane. 

• R4 is to the outside edge of the outside lane.

The difference between R1 and R2 is the width of the inside lane plus the widths of the inside 

edgeline and the edgeline delineating the raised lane divider. The difference between R2 and 

R3 is the width of the lane divider. The difference between R3 and R4 is the width of the outside 

lane plus the widths of the edgeline delineating the raised lane divider and the outside edgeline. 

When meeting at the translation axis, R1 should eventually join with R2 and R3 should

eventually join with R4. A close examination shows that R1 has a different arc center along the 

translation axis than R2, R3, and R4; the reason for this is the presumed difference in widths of

the inside and outside lanes (Overkamp & van der Wijk, 2009). The distance between the arc 

centers on the two sides of the translation axis (e.g., distance between the centers for R1) is 

called the shift. The shift can differ for the R1 centers and the R2/3/4 centers. The shift for the 

R1 centers (∆vv in figure 11) is equal to the distance between the inside edge of the inside lane 

and the inside edge of the outside lane. The shift for the R2/3/4 centers (∆uu in figure 11) is the 

distance between the outside edge of the inside lane and the outside edge of the outside lane. If 

the inside and outside lanes are the same width, the shift value for all radii are the same (∆vv =

∆uu). The location of the center points (also called the arc center biases) is half the distance of 

the shift from the center of the circle (point CG in figure 11) along the translation axis 

(Overkamp & van der Wijk, 2009).

Turbo blocks can differ across different types of turbo roundabouts. Overkamp & van der Wijk 

(2009) provide sample turbo blocks and related details for the star, rotor, and spiral turbo 

roundabouts. The basic principle is the same; a translation axis is needed for every spiral.

Cross Section Elements

The widths of the cross-section elements of a turbo roundabout, specifically the lane widths, are 

dependent on the selection of the design vehicle and the inner radius (R1). Guidance from 

Croatia, Slovenia, Serbia, and the Netherlands recommend wide enough lanes to prevent the 

design vehicle from tracking over the traversable apron or raised lane dividers in a swept path 

analysis (Dzambas et al., 2017, Croatian Design Guidance, Slovenian Design Guidance, 

Serbian Design Guidance, Overkamp & van der Wijk, 2009). Typically, the inside lane of a turbo 

roundabout is wider than the outside lane to provide more room for the design vehicle to 

maneuver around the smaller radius path. 
13

Overkamp & van der Wijk (2009) note seven cross section elements that require a defined 

width:

1. Inner edgeline offset – the distance from the inside edge of the inner lane to the central 

island, including the inside edgeline pavement marking. 

2. Inside lane width – width of the inside lane. 

3. Divider inner line offset – the distance from the outside edge of the inner lane to the 

raised lane divider, including the outside edgeline pavement marking. 

4. Raised divider width – the width of the raised lane divider. 

5. Divider outer line offset – the distance from the inside edge of the outer lane to the 

raised lane divider, including the inside edgeline pavement marking. 

6. Outside lane width – width of the outside lane. 

7. Outer edgeline offset – distance from the outside edge of the outer lane to the edge of 

the roundabout, including the outside edgeline pavement marking. 

The combined width of elements 1, 2, and 3 is considered the width of the inside roadway, while 

the combined width of elements 5, 6, and 7 is the width of the outside roadway. These values, 

combined with the inner radius, define R2, R3, and R4 as well as the shift for the turbo block. 

Overkamp & van der Wijk (2009) provide radii and cross section element widths for four 

standard inner radius values of a basic turbo roundabout. The design values, provided in table 

2, are based on a two-axle truck with a three-axle semitrailer design. Dzambas et al. (2017) 

found Slovenia and Serbia guidance have the same values, while Croatia uses roadway widths 

that are 0.05 m wider, which translates to larger R4 values. Turbo roundabouts in the Czech 

Republic are typically much larger due to significantly larger minimum lane widths for the inside 

and outside lanes (inside lanes are 2 to 3 meters wider than other countries and outside lanes 

are 1 to 2 meters wider) (Smely et al., 2015; Skvain et al., 2017).
14

Table 2. Standard design values for basic turbo roundabouts in the Netherlands. 

Table based on Overkamp & van der Wijk, 2009. 

Feature Measurement in 

m [ft] 

Measurement in 

m [ft] 

Measurement in 

m [ft] 

Measurement in 

m [ft] 

Inner radius of 

the inner lane 

(R1)

10.5 [34.5] 12 [39.4] 15 [49.2] 20 [65.6]

Outside radius of 

the inner lane 

(R2)

15.85 [52.00] 17.15 [56.27] 20.00 [65.62] 24.90 [81.70]

Inner radius of 

the outside lane 

(R3)

16.15 [52.99] 17.45 [57.25] 20.30 [66.60] 25.20 [82.68]

Outside radius of 

the outside lane 

(R4)

21.15 [69.39] 22.45 [73.66] 25.20 [82.68] 29.90 [98.10]

Width, inside 

roadway

5.35 [17.55] 5.15 [16.90] 5.00 [16.41] 4.90 [16.08]

Width, outside 

roadway

5.00 [16.41] 5.00 [16.41] 4.90 [16.08] 4.70 [15.42]

Width, inside lane 4.70 [15.42] 4.50 [14.76] 4.35 [14.27] 4.25 [13.94]

Width, outside 

lane

4.35 [14.27] 4.35 [14.27] 4.25 [13.94] 4.05 [13.29]

Lane divider 

between driving

lanes

0.30 [0.98] 0.30 [0.98] 0.30 [0.98] 0.30 [0.98]

Shift of inner arc 

centers along the 

translation axis

5.75 [18.87] 5.35 [17.55] 5.15 [16.90] 5.15 [16.90]

Shift of outer arc 

centers along the 

translation axis

5.05 [16.57] 5.05 [16.57] 4.95 [16.24] 4.75 [15.58]

Overrun area 

(truck apron) 

width

5.00 [16.41] 5.00 [16.41] 5.00 [16.41] 5.00 [16.41]

Fastest path 

speed for a 

passenger car in 

km/h [mph]

37-41 [23.0-25.5] 37-39 [23.0 – 

24.2]

38-39 [23.6 – 

24.2]

40 [24.8]
15

Central Islands 

Central islands in turbo roundabouts are similar to central islands in modern roundabouts. They 

are bordered by a traversable apron. Dzambas et al. (2017) found differences in guidelines for 

the apron: Overkamp & van der Wijk (2009) recommend a width of 5 m and a design that allows 

vehicles longer than 22 m to travel through the inner lane. Croatia, Slovenia, and Serbia indicate 

the purpose of the apron is for emergency vehicles and for emergency stops, recommending 

values between 2.0 and 2.5 m. One specific use of the central island is considered important in 

the Netherlands, where the space is used to mount, what Fortuijn (2009b) calls a “roundabout 

shield” that serves two purposes: blocking the horizon and directing the driver to turn right. A 

sample roundabout shield is shown in figure 12. This sign is especially important as approaches 

to turbo roundabouts have a radial entry with no flare and come into the circle at a right angle. 

These signs convey a sense of obstruction to the driver (Fortuijn, 2009b). Fortuijn (2009b) noted 

that the sign “must be collision-friendly, and the central island of the roundabout must not be 

provided with a raised edge. A study of the literature shows that [these points are] not always 

appreciated by researchers from outside the Netherlands.” 

© Fortuijn 2009b. 

Figure 12. Graphic. Roundabout shield used in the Netherlands. 

The geometry of the central island is affected by the addition of the inside lane at major 

approaches. Originally, this was done in the Netherlands using smooth curvature matching an 

entering vehicle’s path. However, this led to circulating vehicles entering the added lane, an 

undesired movement in turbo roundabouts. The country therefore adopted a flat lane addition 

approach (Overkamp & van der Wijk, 2009). The differences in the original and revised

approaches to the inside lane addition are shown in figure 13 and figure 14.The shape of the 

central island will depend on the turbo roundabout type.
16

 ©2019 Google Earth®. 

Figure 13. Photograph. Old design for starting the inner lane used in the 

Netherlands. 

 ©2019 Google Earth®. 

Figure 14. Photograph. New design for starting the inner lane used in the 

Netherlands. 
17

Lane Divider

The raised lane divider is a key feature of turbo roundabouts and is a significant deviation from 

modern multilane roundabouts in the U.S. Overkamp & van der Wijk (2009) highlight the 

advantages of the raised lane divider, including reductions in weaving, operating speeds, and 

the number of conflicting traffic streams for some entering vehicles. The dividers should be 

sized and constructed to encourage the desired behavior from drivers, while still being 

mountable and forgiving to striking vehicles (Fortuijn, 2009b). Dutch lane dividers vary 

depending on needs. Overkamp & van der Wijk (2009) provided examples of lane dividers with 

no modifications and with modifications for snow plowing and intersections heavily trafficked by 

lowboys (or low-loaders). Figure 15 and figure 16 illustrate these examples. Though most 

countries who have adopted turbo roundabouts use them, some countries, such as Germany 

(Brilon, 2015) and Poland in some cases (Macioszek, 2015), choose not to use these dividers

over concerns that include motorcycle safety, snow plowing, and maintenance. A review of 

materials for this synthesis did not uncover any formal evaluations to verify these concerns. The 

first known application of a turbo roundabout in North America at Victoria International Airport in 

Canada used a flat lane divider with a textured pavement treatment (see figure 17). The width in 

figure 17 is 1.2 meters from white line to white line.

Some countries, such as the Netherlands, Croatia, and Slovenia, include a widened, traversable 

object at the start of the lane divider. Overkamp & van der Wijk (2009) call this feature a “frog” 

and say its purpose is to call attention to the lane divider and prevent circulating vehicles from 

changing lanes. An example frog is pictured in figure 18. There are small differences in the 

specific dimensions across national guidelines (Dzambas et al., 2017).

Approach Geometry

For standard multilane roundabouts, NCHRP 672 defines three types of approach geometry:

1. Radial – the centerline of the approach intersects with the center of the roundabout.

2. Offset Left – the centerline of the approach passes through the roundabout to the left of 

the center.

3. Offset Right – the centerline of the approach passes through the roundabout to the right 

of the center.

Turbo roundabouts are constructed with radial approaches, which have the benefit of reducing 

changes to the alignment, minimizing roadway area, and maintaining exit curvature to keep 

vehicle speeds low through exiting the roundabout. Additionally, turbo roundabouts are built with 

little or no flare and minimal entry radius. Flare is provided in multilane roundabouts to provide 

adequate entry widths for the design vehicles; using a flare allows for lane width upstream to be 

kept at a standard width and minimizes the required right-of-way (ROW). The entry radius is 

typically increased in multilane roundabouts to ease the entry of design vehicles. Minimizing 

flare and entry radius allows turbo roundabout to achieve greater speed reductions than 

traditional multilane roundabouts.
18

Figure 15. Graphic. Dutch designs of raised lane dividers with no modifications. 

Image based on Overkamp & van der Wijk, 2009. Note: all measurements in m. 

Figure 16. Graphic. Dutch design of lane dividers with modifications for snow 

plowing. Image based on Overkamp & van der Wijk, 2009. Note: all 

measurements in m. 
19

©2019 Google Earth®.

Figure 17. Photograph. Lane divider for turbo roundabout at Victoria 

International Airport. 
20

©2019 Google Earth®.

Figure 18. Photograph. Example “frog”. 
21

Signing and Markings

Due to the turbo roundabout having no lane changes within the circulatory roadway, adequate 

and informative signing and pavement markings, especially on the approaches, are vital. Dutch 

guidelines recommend mimicking lane usage arrows on a guide sign 400 m prior to the 

roundabout entry; an example of the signage is provided in figure 19. This is followed by 

overhead lane signs supplemented with pavement markings 40 m from the roundabout 

entrance. Additionally, the Dutch guidelines encourage that the lane usage arrows (examples 

shown in figure 20 and figure 21) on the signs and the pavement markings be identical 

(Overkamp & van der Wijk, 2009). No lane use arrow pavement markings are required within 

the circulatory roadway, as the raised lane divider and spiral directs vehicles to their desired 

exits (Silva et al., 2013).

©2019 Google Earth®. 

Figure 19. Photograph. Example of guide signage to be placed upstream of a 

turbo roundabout. 
22

Figure 20. Graphic. Lane use arrows for turbo roundabout entry lane 

accommodating through and right turn movement as used in the Netherlands, 

Image based on Overkamp & van der Wijk, 2009. 

Figure 21. Graphic. Lane use arrows for turbo roundabout entry lane 

accommodating left turn movement as used in the Netherlands, Image based 

on Overkamp & van der Wijk, 2009.
23

Some countries do not use the Dutch arrow design, rather they use the “fishhook” design shown 

in figure 22, drawn from the U.S. Manual on Uniform Traffic Control Devices (FHWA, 2012). For 

instance, Poland does not have specialized turbo roundabout arrows and uses traditional 

intersection arrows at their roundabouts (Macioszek, 2013b). Wankogere et al. (2017) found 

that U.S. drivers in a simulator preferred the “fishhook” design over the Dutch arrow design, with 

55.6 percent of drivers identifying fishhook pavement markings and 62.2 percent of drivers 

identifying fishhook signs as being “very easy to understand” (the responses for the Dutch arrow 

design were not reported). 

Source: FHWA

Figure 22. Graphic. “Fishhook” lane use arrows for roundabout approaches. 

User Considerations

Motorists 

Motorist considerations have been addressed to some degree in the previous sections of this 

synthesis. A selected number of key points are summarized here. When entering a roundabout, 

drivers are required to 1) select the proper lane for their destination, and 2) yield to vehicles in 

the circulatory roadway. To make step 1 easier for drivers, designers provide signing and 

pavement markings along the approaches. Some research has been performed on driver laneselection process in multilane roundabouts. Molino et al. (2007) found drivers in a simulation 

were more likely to choose the right lane when the right lane was required for the desired 

movement (95 percent) than the left lane (82 percent). A review of driver behavior at multilane 

roundabouts in Minnesota found drivers tended to select the outside lane regardless of the 

desired movement because the drivers expected to exit the roundabout from the outside lane 

(Leuer, 2016). For step 2 (yielding to drivers in the circulatory roadway), designers provide 

adequate sight distance for entering vehicles to identify conflicting vehicles in the circulatory 

roadway. Leuer (2016) found crashes caused by entering traffic failing to yield to circulating 

traffic to be common on multilane roundabouts in Minnesota.
24

Given the key operational characteristic of no lane changing within turbo roundabouts, signing

and pavement markings to assist with lane selection on the approach is very important. 

Information should be provided to drivers far enough from the roundabout entrance to allow 

them to choose the proper lane. Wankogere et al. (2017) performed a simulator study with U.S. 

drivers to compare lane selection in turbo roundabouts and multilane roundabouts, finding that 

drivers chose the correct lane 93 percent of the time approaching turbo roundabouts compared 

to 81 percent approaching multilane roundabouts.

The radial entry also impacts motor vehicle navigation of the roundabout. Combined with the 

reduced approach curvature, vehicles arrive at turbo roundabouts nearly perpendicular to the 

circular roadway, rather than at an angle in modern roundabouts. As a result, a roundabout 

shield (see figure 12) is used to catch the attention of the driver and block the view of the 

horizon, guiding the driver to turn right (Overkamp & van der Wijk, 2009). The radial entry also 

reduces vehicle speeds as it requires both entering and exiting vehicles to navigate curvature, in 

contrast with an offset approach alignment (Rodegerdts et al., 2010). 

Pedestrians 

Accommodations for pedestrians at roundabouts are typically limited to the perimeter with 

crosswalks placed on the approaches with some offset from the circulatory roadway. 

Rodegerdts et al. (2010) recommend designing pedestrian facilities to discourage crossing to 

the central island, to minimize the crossing distance on the approaches, and to provide enough 

distance from the circulatory roadway to the crosswalks to allow drivers to focus separately on 

the vehicle-pedestrian and the vehicle-vehicle conflict. In addition, the authors provide the 

following guidelines:

• Provide a splitter island on each approach in line with the crosswalk with an appropriate 

width for the context, e.g., a minimum width of 6 feet for a two-stage accessible crossing. 

• Place crosswalks at least one vehicle length from the circulatory roadway to prevent 

vehicle queues from spilling into the roundabout.

• Crosswalks can either be placed perpendicular to the curb line or to the centerline of the 

splitter island: 

o Perpendicular to the outside curb line produces the shortest individual lane 

crossing distances and better facilitates construction of sidewalk ramps that are 

compliant with the Americans with Disabilities Act (ADA). 

o Perpendicular to the approach minimizes total crossing distance but creates 

skew between the pedestrian path and vehicle path. 

• The crosswalk path through the splitter island should be cut down to pavement height. 

• ADA ramps should connect crosswalks to the sidewalk. 

• Sidewalks along the perimeter should be separated from the roundabout by planter 

strips. The sidewalk path can either follow the curvature of the roundabout or have a 

direct connection between each approach’s crosswalk. 

• Sidewalks should have a minimum width consistent with the July 2013 NPRM version of 

the Public Right-of-Way Accessibility Guidelines (PROWAG) published by the U.S. 

Access Board.
25

Keeping pedestrian crossings on the approaches is also recommended for turbo roundabouts in 

the Netherlands (Fortuijn, 2003). The crossings consist of a splitter island of at least 3 m in 

width to provide adequate time for pedestrians to identify conflicting vehicles in the second 

stage of their crossing. 

Cyclists

At a roundabout, a cyclist can either mix with motor vehicle traffic or, when available, utilize 

separated facilities. The decision as to which treatment is adopted is based on context, 

weighing factors such as cyclist volume, motor vehicle volume, complexity of the roundabout, 

adjacent infrastructure and land use, and available right-of-way. 

In general, cyclists mixed with vehicular traffic should feel comfortable in roundabouts because 

vehicle speeds are typically slowed to a similar range as cyclists (Rodegerdts et al., 2010). 

NCHRP 672 includes the following guidelines for bicycle accommodation at roundabouts:

• Limit roundabouts to single-lane when possible because cyclists have issues with 

weaving vehicles in multilane roundabouts. 

• Smaller radii can limit vehicle speeds which increases comfort for cyclists. 

• Bicycle lanes on approaches should be terminated 100 ft before the edge of the 

circulatory roadway and before any crosswalks. Taper rates for ending the lane vary 

based on the width of the bike lane.

• On exit lanes, the taper to reintroduce the bike lane should begin after the crosswalk. 

• If cyclists are to use the sidewalk, in most situations a minimum 10-foot-wide sidewalk is 

recommended. Bike ramps should be added where needed. Careful attention should be 

paid to differentiate between the ADA ramps and bike ramps.

In the Netherlands, separate cycle paths outside of the roundabout are recommended 

(Overkamp & van der Wijk, 2009). Modifications to the crosswalk and splitter island opening are

often made by introducing a chicane to force cyclists to treat the crossing as a two-stage 

maneuver (see figure 23). When crossing, cyclists have the right-of-way in urban areas, while 

they must yield to vehicle traffic in rural areas (CROW, 1998). It is common to see gradeseparated bicycle facilities in the Netherlands for higher speed multilane and turbo roundabouts 

with significant pedestrian and/or cyclist volume; an example is provided in figure 24, which 

contrasts the at-grade crossing in figure 23 (Overkamp & van der Wijk, 2009).
26

©2019 Google Earth®.

Figure 23. Photograph. Example of a chicane in a splitter island to encourage 

cyclists to perform a two-stage crossing. 

©2019 Google Earth®.

Figure 24. Photograph. Example of a grade separated bicycle facility at a turbo 

roundabout in South Holland. 
27

Motorcyclists

While roundabouts are beneficial from a motorist safety perspective because they reduce 

vehicle speeds and the frequency of conflicts, motorcyclists require additional attention with 

regards to roundabout design. Even low speed crashes involving a motorcycle can result in a 

serious or fatal injury, especially when riders do not use a helmet or other personal protective 

equipment. Between 2008 and 2012, motorcyclists accounted for 25 percent of all serious and 

fatal injuries at roundabouts in Victoria, Australia (Beer et al., 2014). Roadway features that can 

have a significant impact on motorcycle safety performance at roundabouts include curbing

presence/location, surface friction, pavement markings, drainage, sight distance (especially 

rider conspicuity), radii, the roadside environment, and surface conditions. 

Milling et al. (2016) described some issues pertaining to roundabouts that can be harmful to 

motorcyclists. These included wet pavement from sprinkler systems for landscaping on the 

central island or the perimeter, curbing on the roundabout, adverse crown on the circular 

roadway, insufficient surface friction (motorcyclists need to “lean” to navigate the geometry of a 

roundabout), and a lack of design features on the approaches that are tailored to slowing 

motorcycle speed. Roundabout features that make them more suitable for motorcycles include 

more forgiving roadsides (include flexible and breakaway signage), a mountable curb when curb 

is required, high-friction pavement, and adequate superelevation (Beer et al., 2014).

The most applicable motorcyclist concern with respect to turbo roundabouts is the presence of 

curbing, as curbing is used for the raised lane divider. As mentioned previously, Germany does 

not include the raised lane divider in their turbo roundabouts (Brilon, 2015) because of concerns 

for motorcyclists. Some turbo roundabouts are accompanied by signs warning motorcyclists of 

raised lane dividers within the roadway; the signs are provided 50 meters upstream of the turbo 

roundabout and are repeated in the central island (CROW, 2008; Overkamp and van der Wijk, 

2009).

Freight 

The geometric design of roundabouts is often based on a large design vehicle, such as a singleunit truck or multi-unit tractor-trailer combination. Such designs may involve a combination of a 

larger diameter and/or wider lanes. However, overly generous geometry may encourage higher 

operating speeds, countering the speed management goals of roundabouts. Therefore, 

designers seek to balance speed management with accommodating larger vehicles. To achieve 

a balanced design, modern roundabouts may include a mountable apron along the perimeter of 

the central island, and sometimes along the radius returns between adjacent legs, which allow 

vehicles longer than the design vehicle additional room to maneuver when necessary 

(Rodegerdts et al., 2010).

For the design of turbo roundabouts, most countries seek to design the turbo roundabout so that 

the wheel path of the design vehicle does not leave the designated travel lane in a swept path 

analysis (Dzambas et al., 2017). However, turbo roundabouts still provide traversable aprons, 

along with a “frog” at the beginning of the raised lane divider, to provide forgiveness to vehicles 

larger than the design vehicle.
28

Capacity and Operational Performance

The ability to estimate the capacity of turbo roundabout alternatives under U.S. driving 

conditions will be key to their consideration and adoption. Capacity is “the maximum sustainable 

hourly flow rate at which persons or vehicles can reasonably be expected to traverse a point or 

a uniform section of a lane or roadway during a given time period under prevailing roadway, 

environmental, traffic, and control conditions” (Transportation Research Board, 2016, p.9-4). 

The capacity of a roundabout is defined by the capacity of each entering lane. Currently, there 

are several different methods for determining roundabout capacity. NCHRP Synthesis 488, with 

36 responding States, documents use at the time the research was performed. Familiarity with 

the fundamentals of these various approaches will help with interpreting analyses specific to 

turbo roundabouts that are reported in the literature to-date. 

Analysis of Turbo Roundabouts

Fortuijn and Harte (1997) made initial efforts to estimate turbo roundabout capacity by modifying

a capacity model developed by Bovy (1991) to account for distribution of traffic between the 

circulating lanes in a multilane roundabout (see Figure 25). The Dutch multilane roundabout 

explorer, Meerstrooksrotondeverkenner, incorporated this initial work. Capacity values for 

various roundabouts and other intersection control types, estimated using these approaches 

with adjustment factors from Bovy (1991), are provided in table 3. 

Figure 25. Equation. Capacity equation. Image based on Bovy, 1991.

Where:

γ = adjustment factor for entry capacity, changes with the number of circulating lanes. 

β = adjustment factor for circulating flow, changes with the number of circulating lanes. 

α = adjustment factor for exiting traffic, changes as a function of distance along the circulatory 

roadway between the exiting and entering conflict points. 

vexit = traffic volume exiting at the subject approach, in vehicles per hour.
29

Table 3. Approach capacity comparison table. Table based on Overkamp & 

van der Wijk, 2009. 

Type of 

roundabout/intersection

Practice capacity2 in 

peak hour (+/- 10% of 

AADT), all entries 

combined

Theoretical capacity3

in peak hour (+/- 10% 

of AADT), all entries 

combined

Conflicting Traffic4, 

vvpppppppppppppppppp

Single-lane roundabout 2,000 2,700 1,350 to 1,500

Multilane roundabout with 

single entry and exit lane 

2,200 3,600 1,500 to 1,800 

Multilane roundabout with 

two entry lanes and single 

exit lane

3,000 3,600 1,800 to 2,000 

Multilane roundabout with 

two entry and exit lanes

3,500 4,000 2,100 to 2,400

Turbo roundabout with two 

entry and exit lanes (basic 

design)

3,500 3,800 1,900 to 2,100

Spiral roundabout 4,000 4,300 2,000 to 2,300 

Rotor roundabout (three 

entry lanes and two exit 

lanes)

4,500 5,000 2,500 to 2,800 

Signalized roundabout (3’2 

entry lanes)5

8,500 11,000 4,200 

Minor road stop- or yieldcontrolled intersection with 

left turning lane

1,500 1,800 1,100

Four-leg intersection with 

traffic signals (entries 3’1 

travel lanes)

3,500 4,000 3,800

Four-leg intersection with 

traffic signals (entries 3’2 

travel lanes) 

7,500 8,000 3,800 

 

2 Field observed capacity values under peak hour operations, in passenger cars per hour.

3 Calculated assuming a ratio of 5 to 2 for major road volume to minor road volume (3 to 2 for spiral and 

rotor roundabout) and for every three through-vehicles on an approach, one turns left and one turns right. 

Measured in passenger cars per hour.

4 Measured in passenger cars per hour.

5 3’2 entry lanes implies there are 3 lanes on the major road approaches and 2 lanes on the minor road 

approaches.
30

The capacity model was updated by Fortuijn (2009a), who wanted to account for the nonlinear 

relationship between circulating flow and entry capacity while incorporating the effect of 

pseudoconflicts. He did this by adding a proportion of exiting traffic to the conflicting volume at 

the entry, as shown in figure 26 (Rodegerdts et al. [2015] would later mimic this approach with 

U.S. data). Fortuijn (2009a) asserted that incorporating this effect into the Hagring (1998) model 

results in a good basis for turbo roundabout capacity models. The Hagring model Fortuijn 

referred to is described in figure 27. 

Figure 26. Equation. Modified conflicting traffic volume including the 

pseudoconflict of exiting vehicles.

Where:

vconflict with pseudo = the modified conflicting traffic in the circulatory roadway, incorporating the effect 

of pseudoconflicts. 

d = proportional effect of exiting traffic. 

Figure 27. Equation. The Hagring (1998) capacity model for a roundabout entry 

lane. 

Where:

i = index for the circulatory lane (the sum and product operations should be repeated for each 

circulatory lane). 

tM,i = the minimum headway between free circulating vehicles in circulatory lane i.

Fortuijn (2009a) compared observed capacity under saturated conditions in 5-minute intervals at 

a single-lane roundabout entry with a predicted value at the entry, calculated using the 

Troutbeck (1984) model. The author used least-squares regression to estimate passenger car 

equivalents (pce) for light trucks (1.9 pce), heavy trucks (2.4 pce), and the proportional effect of 

exiting traffic, which was found to range from 0.2 to 0.5. It should be noted that the larger the 

proportional effect of exiting traffic, the smaller the entry capacity. 

Fortuijn (2009a) observed driver behavior on approaches of various orientations at single-lane, 

multilane, and turbo roundabouts and used these observations to calibrate a microsimulation in 

VISSIM. Calibration of the microsimulation was implemented using accepted and rejected gaps, 

follow-up times, and headways in the conflicting traffic stream. After calibrating the simulation, 

the author found four notable results:

1. Circulating traffic volume has a significant effect on the critical gap within the simulation.
31

2. The relationship between circulating traffic and entry capacity was linear for single-lane 

roundabouts and nonlinear for turbo roundabouts.

3. Vehicle distribution between circulating lanes in multilane roundabouts influences 

approach capacity.

4. Capacity models should consider each circulating lane separately.

Fortuijn and Hoogendoorn (2015) revisited gap acceptance-based capacity models for turbo 

roundabouts. The authors applied modifications to the Hagring (1998) model, adjusting for 

variance in driver behavior, the non-exponential distribution of gaps in the circulatory roadway, 

and correlation in gaps between lanes in the circulatory roadway. The authors also pointed out 

that Germany sees poor utilization of the left-entry lane in multilane roundabouts, and that 

drivers are more likely to use this entry lane the longer they will travel within the roundabout, 

meaning they are most likely when making a left turn and least likely when making a right turn

(Brilon and Baumer, 2004). The authors noted Dutch and German turbo roundabouts have 

higher entry capacities than German multilane roundabouts because of higher left-lane usage. 

Other authors would go on to adopt the Hagring model for turbo roundabouts as well (Giuffre et 

al., 2012; Savric and Lovic, 2017). Silva et al. (2014) used the Hagring model, with M3 

distribution parameters calibrated to Portugal by Vasconcelos et al. (2012), to compare turbo 

roundabouts and multilane roundabouts in a Portuguese context, concluding that turbo 

roundabouts only show increases in capacity over multilane roundabouts when the proportion of 

right turns from a minor approach is greater than 60 percent of the approach volume.

Not all turbo roundabouts capacity evaluations employed the Hagring model. Mauro and Branco 

(2010) estimated turbo roundabout capacity by adapting the capacity formula developed by 

Brilon and Wu (2006), which is described in figure 28, to turbo roundabout conditions. For each 

approach, the authors used the capacity model for the relevant number of conflicting lanes. For 

right-lane entries, the number of conflicting lanes is one and the conflicting traffic is the 

circulating traffic in the outside lane. The equation varies for left-lane entries. On approaches 

where the inside lane is created across from the approach (typically a major approach), there is 

one conflicting lane and conflicting traffic is the circulating traffic in that lane. For left-turn lanes 

where the inside lane is already present, the number of conflicting lanes is two and the 

conflicting traffic is the total of circulating traffic in the outside lane and inside lane. Fixed values 

were used for follow-up time and minimum headways; it is unclear where these values came 

from. The authors found that multilane roundabouts provide higher approach capacity than turbo 

roundabouts for major road approaches, while turbo roundabouts provide higher approach 

capacity when most of the circulating traffic is in the inside lane. 

Tollazzi et al. (2016) also adopted this approach to model turbo roundabouts when performing a 

comparative analysis of innovative roundabout types; they determined turbo roundabouts 

perform best when most entering vehicles are turning right. 
32

Figure 28. Equation. Roundabout capacity model. Image based on Brilon and 

Wu, 2006. 

Where:

nc = the number of conflicting lanes at the entry. 

tmin = minimum headway between vehicles in the circulating lane, in seconds.

Macioszek (2016) used the appropriate HCM equation for each scenario on the turbo 

roundabout, and, compared with Polish-calibrated capacity equations (Macioszek, 2013a) that 

are based on Tanner (1962), found the HCM produced reasonable estimates of turbo 

roundabout capacity compared to Polish models.

Driver Behavior on Turbo Roundabouts

Little research has been done outside of the Netherlands to assess driver behavior at turbo 

roundabouts. Guerrieri et al. (2018) observed traffic flows at a turbo roundabout in Maribor, 

Slovenia, and found average critical headways ranging from 4.03 to 5.48 seconds and average 

follow-up headways ranging from 2.52 to 2.71 seconds. As a basis for comparison, Rodegerdts 

et al. (2015) founded average critical headways of 4.9 seconds for single-lane roundabouts and 

ranging from 4.3 to 5.5 seconds for multilane roundabouts; they found average follow-up times 

of 2.6 seconds for single-lane roundabouts and ranging from 2.1 seconds to 2.7 seconds for 

multilane roundabouts. Fortuijn (2009a) and Fortuijn and Hoodendoorn (2015) used differing 

critical headway and follow-up times in their turbo roundabout analyses depending on if it was a 

major or minor approach and if it was the left or right lane. These are shown in table 4. These 

are likely lower because Dutch drivers are generally more experienced and comfortable using 

roundabouts. 

Table 4. Values used for gap acceptance. Table based on Fortuijn, 2009a; 

Fortuijn & Hoodendoorn, 2015; Saric & Lovric, 2017. 

Entry Lane 

Approach

Major 

Direction Left

Major Direction 

Right

Minor 

Direction Left

Minor Direction 

Right

Critical gap—tc (s) 3.55 3.80 3.15 3.70

Follow-up time—tf

(s) 

2.30 2.30 2.25 2.80

Wankogere et al. (2017) used a driving simulator study to compare U.S. driver behaviors in 

multilane roundabouts with turbo roundabouts. The study consisted of 46 participants navigating 

multilane and turbo roundabouts with different signing and pavement marking schemes. The 

study found that, in turbo roundabouts, a minimum of 92.5 percent of drivers selected the 
33

correct lane on the approach compared with 81.4 percent for a multilane roundabout. This is 

important for two reasons. First, with nearly 20 percent of drivers selecting the wrong lane for a 

multilane roundabout, weaving within the circulatory roadway is likely to arise, which can lead to 

crashes and reduced capacity. Second, the increase in correct lane selection for the turbo 

roundabouts shows the effectiveness of good signing and lane marking schemes on the 

approach, as well as limits issues with lane changing in the roundabout, which are restricted 

due to the presence of raised lane dividers.

Summary of Turbo Roundabout Traffic Operations Research 

Overall, it appears the identified capacity research for turbo roundabouts is based on evaluating 

the ability of existing gap acceptance capacity models to estimate turbo roundabout capacity. 

These models can be calibrated to local conditions using observed driver behavior 

characteristics, such as critical gap, follow-up time, and bunching of circulatory vehicles. Several 

of these studies stated that, at least in some conditions, turbo roundabouts have higher capacity 

than multilane roundabouts, although the difference is not substantial (Bulla and Castro, 2011; 

Fortuijn and Hoogendoorn, 2015; Giuffre et al., 2012; Mauro and Branco, 2010; Saric and 

Lovric, 2017; Silva et al., 2014). Most likely, this arises from the lane dividers within the 

circulatory roadway, which limit lane changing within the roundabout and reduce the number of 

conflicting vehicles that a portion of entering vehicles need to concern themselves with. Having 

to only focus on one circulating lane is important, as the HCM 6th edition states “some drivers 

who choose to enter the roundabout via the right entry lane will yield to all traffic in the 

circulatory roadway due to their uncertainty about the path of the circulating vehicles” (p.22-25).

With raised dividers between the inside and outside lane of the circulatory roadway, drivers 

entering from the right entry lane can confidently only search for gaps in the outside circulating 

traffic.

Another potential reason for improved capacity in turbo roundabouts could be the right-angle 

approach of entry lanes. Skewed entries, particularly for older drivers, can present challenges 

when assessing gaps (Staplin et al., 2001). It is not unreasonable to expect that a less skewed 

approach may see higher capacity as it will be easier for drivers to judge gaps. A third reason 

for the higher capacity could be better utilization of the inside lane by movements for which it is 

an option. Turbo roundabouts have tighter entry radii values, which, according to the U.K. 

models, will result in lower capacity because of the associated decrease in entry speeds. With 

these counterbalancing effects in place, HCM approaches for multilane roundabouts, with 

conflicting vehicle counts specific to the turbo roundabout, will likely result in reasonable 

estimates for the first turbo roundabouts in the U.S. as demonstrated in Poland by Macioszek 

(2016). 
34

Safety Performance

Modern roundabouts have a well-documented history of reducing crash frequency and severity 

at intersections upon installation (Rodegerdts et al., 2010). Traditional four-leg intersections 

have 32 vehicle-vehicle conflict points, while modern single-lane roundabouts have 8 vehiclevehicle conflict points. Where a single-lane roundabout does not provide sufficient capacity, 

multilane roundabouts can be considered. As discussed in the introduction to this synthesis, 

some constructed multilane roundabouts have experienced higher frequency of low severity 

crashes. The introduction also provided multiple reasons for this observation centered around 

an analysis of conflict points and driver behavior. The remainder of this section focuses on 

safety performance of turbo roundabouts in general, and relative to multilane roundabouts, 

based on analyses of crash data and safety surrogates.

Evaluation of Crash Data

Because turbo roundabouts are relatively new, observational study designs and statistical 

analyses of crash data on turbo roundabouts are limited both in quantity (i.e., few studies have 

been performed with few sites) and quality (with regards to whether they used more reliable 

methodologies described by Gross et al., 2010). Fortuijn (2009b) performed a before-after 

analysis of seven intersections with varying traffic control (signalized, stop-control, yield-control, 

and old-style rotary) that had been converted to single lane and turbo roundabouts. After 

accounting for priority selection (the improved sites had a history of elevated crash frequency 

compared to the national average), national safety trends, and the effects of statistical 

averaging, Fortuijn (2009b) found that converting from yield-control, signalized, or old-style 

rotary to a turbo roundabout reduced injury crashes by 76 percent. The sample size, however, 

was limited to only seven sites and Fortuijn (2009b) noted that the precise reduction in the 

frequency of injury crashes after installing a turbo roundabout is still uncertain.

Another evaluation of crash data at turbo roundabouts investigated the impact of the raised lane 

divider on crash types. Macioszek (2015) compared observed crashes at turbo roundabouts in 

Poland with raised lane dividers with observed crashes at turbo roundabouts in Poland with 

lanes divided by solid white lines. Table 5 provides Macioszek’s (2015) reported differences 

between the distribution of crash types on turbo roundabouts with and without the raised lane 

divider. With no raised lane divider, percentages showed that sideswipe crashes accounted for 

more than half of the crashes at the intersections. While turbo roundabouts with raised lane 

dividers saw a lower proportion of crashes that are sideswipe, those in Macioszek’s (2015) 

sample saw larger percentages of fixed object crashes, which were related to the presence of 

raised lane dividers. Another difference noted by Macioszek (2015) is the different distributions 

of observed injury outcomes. Turbo roundabouts with raised lane dividers showed that 45 

percent of injured persons were classified as “seriously injured” and 55 percent were classified 

as “slightly injured.” At turbo roundabouts with solid painted lines dividing the lanes, 65 percent 

of injured persons were classified as “seriously injured” and 35 percent were classified as 

“slightly injured”. Both intersections had similar distributions of crash contributing factors, with a 

plurality of crashes attributed to failure to yield the ROW. Additionally, while the authors 

presented no supporting data in the paper, their conclusion states that turbo roundabouts 
35

without the raised lane dividers had a higher crash frequency than those with a raised lane 

divider.

Table 5. Differences in percent of total intersection crashes by type on turbo 

roundabouts with raised and painted lane dividers. Table based on Macioszek, 

2015.

Crash Type Raised Lane Divider 

(% of total)

Solid White Line

(% of total)

Sideswipe 24 56

Hit Fixed Object 30 6 

Vehicle-Pedestrian 0 2 

Overturn 5 1 

Rear End 33 34

Other 8 1 

Kiec et al. (2018) used traffic and crash data from nine turbo roundabouts in Poland to estimate 

a safety performance function (SPF). Average annual daily traffic (AADT) volumes at these 

turbo roundabouts ranged from 5,000 to 26,530 vehicles per day. Five of the turbo roundabouts 

had a raised lane divider and four did not. The authors used four years of data to estimate the 

models. They estimated SPFs for all crashes (i.e., all types and severities) and for PDO crashes 

of all types. Figure 29 and figure 30 represent the SPFs estimated from the data. Kiec et al. 

(2018) also tried to include average vehicle speed in the model, but it proved not to be 

statistically significant at the 90-percent confidence level. A review of the equations shows that, 

for a given traffic volume, the lack of a raised lane divider is associated with 59-percent more 

total crashes and 64-percent more PDO crashes when compared to a turbo roundabout with a 

raised lane divider.

Figure 29. Equation. SPF for all crashes on turbo roundabouts in Poland. Image 

based on Kiec et al., 2018. 

Figure 30. Equation. SPF for PDO crashes at turbo roundabouts in Poland. Image 

based on Kiec et al., 2018.

Where:

N PR, All = the number of annual predicted crashes at a turbo roundabout, in crashes per year.

N PR, PDO = the number of annual predicted PDO crashes at a turbo roundabout, in crashes per 

year.

AADT = annual average daily traffic entering the roundabout, in vehicles per day.
36

NoDivider = indicator variable for the lack of raised lane divider; 1 if no raised lane divider, 0 

otherwise.

Safety Surrogates

Some researchers have evaluated turbo roundabout safety performance using alternative 

measures of safety, or safety surrogates. For example, Vasconcelos et al. (2013) used 

microsimulations of roundabouts developed in AIMSUN and the Surrogate Safety Assessment 

Model (SSAM) to compare the safety performance of single-lane, two-lane, and turbo 

roundabouts. SSAM utilizes vehicle trajectories from microsimulation models (such as AIMSUN) 

to predict crash frequency at a location as a function of traffic conflicts per hour (Gettman et al., 

2008). For their turbo roundabout analysis, Vasconcelos et al. (2013) used time-to-collision to 

determine if an event would be classified as a conflict (the threshold was typically less than or 

equal to 1.5 seconds). Once a conflict was identified, potential severity was estimated as a 

function of the difference in vehicle speeds. The three roundabout types proposed for a subject 

intersection in Coimbra, Portugal were modeled using this technique, finding that the turbo 

roundabout had the fewest predicted daily conflicts and a lower percentage of severe conflicts 

than the multilane roundabout.

Bulla-Cruz and Barrera (2016) also used SSAM to compare safety performance between a twolane roundabout and turbo roundabout. The authors built a microsimulation of an existing twolane roundabout and a proposed turbo roundabout at the same intersection in Bogota, Colombia 

using VISSIM. After calibrating the microsimulation to the observed conditions at the existing 

two-lane roundabout, the authors found the turbo roundabout was predicted to have 72-percent 

fewer conflicts than the two-lane roundabout.

Mauro et al. (2015) took a different approach to conflict analysis that they based on the 

“potential conflict” approach developed by Ha & Berg (1995). The process consists of identifying 

all maneuvers required by vehicles to traverse an intersection and identify which maneuvers 

could result in a crash given certain traffic conditions. The method had been previously 

extended to single-lane and multilane roundabouts (Mauro & Cattani, 2004; Mauro & Cattani, 

2005). After reviewing crash data for single-lane and multilane roundabouts, Mauro et al. (2015) 

identified four conflicts at roundabouts and how to calculate the potential conflicts that can 

occur:

1. Failure to yield to circulating traffic with or without stopping.

a. The number of expected conflicts at an entry is a function of entering volume, the 

probability of a driver attempting to enter, circulating traffic volume, and the 

probability of the accepted gap between circulating being “dangerous”, which the 

authors described as being between 3 and 5 seconds, or insufficient, which is 2 

seconds or less.

2. Loss of control by an entering vehicle.

a. The number of expected conflicts at an entry is a function of entering traffic 

volume, the probability of an entering vehicle losing control, and the probability of 

no circulating vehicles being present for the entering vehicle to collide with.

3. Rear-end on an approach.
37

a. The number of expected conflicts at an entry is a function of entering traffic 

volume and the probability of a queuing vehicle failing to stop in time to avoid a 

rear end crash.

4. Crash when exiting a two-lane roundabout from the inside lane.

a. The number of expected conflicts at an exit is a function of circulating traffic in 

the inside and outside lane, exiting traffic volume, and the number of gaps that 

could result in a collision, which are defined as 2 seconds or less.

Determining the number of crashes from the number of conflicts requires estimating a 

calibration factor (ci ) for each conflict type. In the method developed by Ha & Berg (1995), this 

is done by observing traffic flow and collecting traffic data for roundabouts. Observed traffic data 

can be used to tally the number of each conflict type. The number of observed crashes resulting 

from the conflict type divided by the number of conflicts is the calibration factor, as shown in 

figure 31. 

Figure 31. Equation. Calculation of calibration factor for conflict I. Image based 

on Mauro et al., 2015.

Where:

ci = the calibration factor for conflict type i.

N crashes, i = the number of observed crashes at the intersection resulting from conflict type i.

N conflicts, i = the number of observed conflicts of type i. 

Summing the number of predicted crashes, calculated as the product of the calibration factor (ci) 

and the number of conflicts (N conflicts, i) for each conflict type, results in the potential accident rate 

(PAR). Mauro et al. (2015) applied this methodology to compare the potential safety 

performance of two-lane and turbo roundabouts, with traffic volume based on assumed average 

annual daily traffic (AADT) values for each leg (10,000 vehicles per day on major legs and 2,000 

vehicles per day on the minor legs) and using German roundabout capacity equations (FGSV, 

2006). Known capacity is required for estimating the probability of a failure to yield crash 

occurring. Calibration factors for various crash types and severities were estimated using data 

from three single-lane and three multilane roundabouts in Italy; the calibration factors were 

applied to turbo roundabouts. Five combinations of origin-destination were developed to 

evaluate how differences in desired movements can affect safety performance. For the turbo 

roundabout, no conflicts were expected for the exiting conflict as the spiral allows all exits to 

occur from the outside lane. Across the many combinations of traffic distributions, the authors 

found that turbo roundabouts are expected to have 40- to 50-percent fewer total crashes and 

20- to 30-percent fewer injury crashes than two-lane roundabouts. A sensitivity analysis with 

regards to traffic volume found that as traffic volumes increased, this difference is expected to 

increase.
38

Chodur and Bak (2016) used video data to compare driver behavior at seven turbo roundabouts 

in Poland with regards to both lane compliance and speed. For lane compliance, the authors 

reviewed vehicle trajectories to identify vehicles that made an incorrect movement or took an 

incorrect path. As mentioned previously, some turbo roundabouts are built with the raised lane 

divider while others are built without. Where the raised lane divider is present, some movements 

are highly unlikely and are usually limited to areas where the curb is not present. Overall, the 

results indicated noncompliant behaviors at turbo roundabouts are rare, illegal lane changes 

mainly occur at the exit area, and tracking over the lane divider is primarily limited to turbo 

roundabouts without the raised lane divider. The authors assume that fewer intrusions across 

lane lines will result in fewer related crashes within the circulatory roadway.

When analyzing speed, Chodur and Bak (2016) broke vehicle paths into five sections:

1. Approach.

2. Entry. 

3. Conflict.

4. Circulating.

5. Exit.

Comparisons were drawn between speeds in the inside and outside lanes. On average across 

the seven turbo roundabouts, vehicles in the inside lane were observed traveling 5.0 km per 

hour (3.1 mph) faster than vehicles in the outside lane. This difference changed to 1.2 km per 

hour (0.75 mph) slower in the circulatory roadway, as expected given vehicles in the inside lane 

are required to traverse a tighter radius.

Keic et al. (2018) performed additional video analysis of turbo roundabouts in Poland with the 

goal of using vehicle speed as a safety surrogate measure. Kiec et al. measured vehicle speeds 

on the approach, entrance, circulatory roadway, and the exit for the nine study roundabouts.

Average vehicle speeds for turbo roundabouts with raised lane dividers, without raised lane 

dividers, and incorrect vehicle paths when navigating turbo roundabouts without raised lane 

dividers were compared, with the lowest speeds generally occurring for turbo roundabouts with 

raised lane dividers. The authors then used three existing models for roundabouts that relate 

circulating vehicle speed to safety performance at a roundabout approach (see figure 32

through figure 34 and corresponding references). Based on these equations and the fact that 

vehicle speeds are, on average, higher on turbo roundabouts with no raised lane divider than 

those with raised lane divider, the authors concluded that turbo roundabouts without raised lane 

dividers can be expected to experience more crashes.

Figure 32. Equation. Crash prediction model for roundabout approaches in New 

Zealand. Image based on Turner et al., 2009.
39

Figure 33. Equation. Crash prediction model for roundabout approaches in the 

U.S. Image based on Chen et al., 2013.

Figure 34. Equation. Crash prediction model for roundabout approaches in Italy. 

Image based on Chen et al., 2013.

Where:

ventering = the entering vehicle flow at a roundabout approach, in vehicles per hour.

N PR, App = the predicted annual crashes for a roundabout approach, in crashes per day.

vconflict = the conflicting vehicle flow at a roundabout approach, in vehicles per hour.

AADT = annual average daily traffic, in vehicles per day.

ASconflict = the average speed of circulating conflicting traffic at the approach, in km per hour.

AS = the average speed of traffic for the subject roundabout approach, in km per hour.

Summary of Safety Findings

The turbo roundabout was developed outside the U.S. as an alternative to the traditional 

multilane roundabout to deal specifically with crashes observed at those roundabouts. With 

smaller radii and raised lane dividers, the reduced vehicle speeds and elimination of weaving 

was expected to produce fewer and less severe crashes. Though very little research based on 

an analysis of crash data has been done, it appears initial, but limited, findings supports these 

hypotheses. Fortuijn (2009b), Macioszek (2015), and Kiec et al. (2018) all showed turbo 

roundabouts provide improved safety performance compared to multilane roundabouts, 

specifically with regards to the benefits of the raised lane divider. Surrogate measures, whether 

derived from microsimulation (Vasconcelos et al., 2013; Bulla-Cruz and Barrera, 2016; Mauro et 

al., 2015) or field observations (Chodur and Bak, 2016; Kiec et al., 2018), also suggest turbo 

roundabouts are expected to produce fewer and less severe crashes than a comparable 

multilane roundabout. 

Education and Public Involvement

Traditional public engagement typically focuses on the solution before providing the need; 

however, turbo roundabout design projects may benefit from a reverse approach. Given the 

unique geometry and limited knowledge of turbo roundabouts in the U.S., traditional public 

outreach methods for roundabouts will need to be modified for marketing turbo roundabouts. 

For guidance, the project team reviewed the education and outreach efforts as well as examples 
40

of video simulations where turbo roundabouts were initially introduced in the Netherlands and 

other countries. In countries outside the U.S., when turbo roundabouts are proposed, 

transportation agencies educate the public about the design through demonstrated success 

prior to implementation (or during project development) and ongoing education through the use 

of signage after installation. 

Demonstrated Success 

Demonstrating how to navigate a turbo roundabout design is key to gaining the public’s 

acceptance of a turbo roundabout. Practitioners have used real-time video and computergenerated simulations to promote and educate turbo roundabouts to different audiences. 

These strategies are likely best used prior to project installation and throughout the project 

development phases. 

Real-time video demonstrates to the public how different vehicles and traffic flows can navigate 

a turbo roundabout. Videos are typically captured by drone footage or traffic cameras and 

demonstrate real-world situations of different roadway users traveling through a turbo 

roundabout, illustrating the ease of navigation to the viewer. Real-time demonstrations are 

helpful for the public to view how traffic maneuvers in a turbo roundabout. 

Transportation agencies have used real-time videos in Pula, Croatia, Rosmalen, Netherlands 

and Europe to illustrate the design and traffic flow through a turbo roundabout (Zgrablic, 2015; 

Pmverhulst, n.d.). Agencies have also used real-time videos to show to the public what a turbo 

roundabout would look like in their community and demonstrate that other communities have 

had success using a turbo roundabout. To effectively reach the public real-time videos can be 

packaged as marketing pieces to provide more context. 

Simulations, much like real-time videos, are helpful for engineering and public audiences 

because they allow viewers to visualize and analyze a turbo roundabout in greater detail. 

Simulations are capable of providing more context on turbo roundabout capacity and design; 

therefore, the content is easily tailored to either a technical engineering audience or the lay 

person (TU Delft, 2009). Transportation agencies like Royal HaskoningDHV, who specialize in 

engineering on an international scale, have used simulations to combine animated navigational 

viewpoints of a turbo roundabout, such as aerial, driver, and pedestrian perspectives (Royal 

HaskoningDHV, 2015). 

While some simulations are created with both engineering and public audiences in mind, some 

are created solely for an engineering audience. TU Delft, a public technological university 

located in Delft, Netherlands, overlaid an interviewed Fortuijn, who conceptualized the turbo 

roundabout design, with a computer-simulation to explain the different types of turbo 

roundabouts and their respective capacity models for an engineering audience (TU Delft, 2009). 

Signage 

Marktstate (2010) created a guide describing the different types of signs a road user may 

encounter when navigating turbo roundabouts for the Regional Traffic Safety Organization of 

Limburg to distribute to the public. In addition, describing the signage’s importance and 
41

meaning, the guide emphasized the importance of driver awareness to the signs as they 

approach a turbo roundabout so they will choose the correct lane (Marktstate, 2008). One sign 

of particular interest specific to turbo roundabouts warns motorcyclists of the raised dividers 

between the lanes in the turbo roundabout. 

Key Findings 

The key to successful public education on turbo roundabouts is providing key messages 

through the right medium to the right audience. Videos developed for professionals emphasized 

specifics regarding design capacity, operations, and speed and has been successfully shared 

through social media platforms like LinkedIn that allow professionals to engage with one another 

about ideas. These videos are most useful to promote awareness and acceptance of the design 

and to encourage implementation. For the public, it is important to share material that is easily 

digestible and provides a real-life connection. Practitioners have used both computer-generated 

and real-time videos to demonstrate the key concepts at public presentations and posted 

publicly as online resources prior to project installation. 

Once a project is complete, videos are useful for ongoing driver and public education. Signage 

also becomes more important at the project location to direct drivers to select the correct lane.

The next step in this process is a technical summary, where the research team will propose 

ways to update and tailor existing outreach materials to appropriate audiences. 
42

Summary and Next Steps

As turbo roundabouts increase in implementation worldwide, a growing body of literature 

supports the geometric, operational, and safety benefits of the design with relation to other 

alternatives. The specific design features of turbo roundabouts provide the greatest 

improvements over traditional multilane roundabouts. Notable design features include:

• Radial entry which improves sight distance.

• Minimized central island radius and entry radius to reduce vehicle speeds.

• Raised lane divider to prevent weaving in the circulatory roadway.

• Roundabout shields which block the horizon and direct drivers to turn into the 

roundabout.

• Mountable aprons on the central island and the beginning of the raised lane divider to 

ease navigation by heavy vehicles.

• Spiral design so vehicles do not have to cross lanes to exit the roundabout.

In terms of capacity, the turbo roundabout allows for volumes close or equal to a modern 

multilane roundabout while providing similar safety benefits to single lane roundabouts. While 

there are some empirical models based on geometry (Kimber, 1980), most roundabout capacity 

models are based on gap acceptance theory models derived for stop-controlled approaches. 

For all models, capacity is estimated at the approach level, either as a whole or the sum of each 

entrance lane. While much attention has been paid to potential differences in models for turbo 

roundabouts and multilane roundabouts, specifically in relation to the distribution of headways 

within the circulatory roadway, turbo roundabout capacity can be estimated through slight 

adjustments for driver behavior of existing capacity models. As Macioszek (2016) showed, the 

HCM 2010 produced similar capacity estimates of Polish turbo roundabouts as locally estimated 

models. 

Given that safety is possibly the biggest selling point of a turbo roundabout in place of a 

multilane roundabout, it is important to discuss what has been found with regards to safety 

performance of the design. The safety of turbo roundabouts has been evaluated using both 

crash based analyses and safety surrogate analyses. Some notable findings include:

• Conversion from yield-control, signalized, or old-style rotary intersections to turbo 

roundabouts is expected to result in a 72-percent reduction in injury crashes (Fortuijn, 

2009b).

• In Poland, turbo roundabouts with raised lane dividers were found to have fewer total 

crashes, PDO crashes, and a smaller percentage of injury crashes classified as 

“seriously injured” than turbo roundabouts with only a solid white line (Macioszek, 2015; 

Kiec et al., 2018).

• Surrogate analyses suggest turbo roundabouts are expected to have lower crash 

frequency and less severe crashes than multilane roundabouts.
43

Public education and involvement are needed when constructing a unique design such as a 

turbo roundabout. It is important to create outreach materials that focus on signage and how 

different users navigate the roundabout design. The key to determining the most effective type 

of public outreach material is determining the message. There are a variety of platforms 

Federal, State, and local agencies can use when exposing the public to a turbo roundabout

design. One of the most effective forms of outreach is citing examples of demonstrated success, 

through videos—in either real-time or computer-simulations—to show different perspectives and 

how different road users can easily turbo roundabout. 

The next task for this project is the preparation of an annotated outline for a technical summary 

on turbo roundabouts, similar in nature to FHWA’s Roundabouts Technical Summary and Mini 

Roundabouts Technical Summary. The Technical Summary will present turbo roundabouts for a 

U.S. context, drawing on what was learned in this synthesis and merging it with other 

fundamental design and operational principles of roundabouts as contained in NCHRP Report 

672. Comparisons will be drawn with single-lane and multilane roundabouts as well as 

signalized and stop-controlled intersections to highlight potential safety and operational benefits. 

Special attention will be paid to any documented safety analyses, noting both before and after 

conditions as available. It is anticipated the technical summary will have the following structure:

• Section 1: Characteristics of Turbo Roundabouts

• Section 2: Benefits of Turbo Roundabouts

• Section 3: User Considerations

o Motorists

o Pedestrians

o Bicyclists

o Motorcyclists

o Freight

• Section 4: Location Considerations

o Common Site Applications

o Site Constraints

• Section 5: Safety Analysis Methods and Results

• Section 6: Operational Analysis Methods and Results

• Section 7: Design Considerations

o Horizontal Design

o Pedestrian Design Treatments

o Bicycle Design Treatments

o Sight Distance and Visibility

o Vertical Design

o Pavement Markings and Signs

o Lighting

o Landscaping

o Other Design Details and Applications

• Section 8: Costs

• Section 9: References
44

Special attention will be paid to explaining design concerns for pedestrians, cyclists, and 

motorcyclists, all of which are vulnerable in a turbo roundabout. Additionally, because of the 

importance of lane selection on the approach at turbo roundabouts, good signage techniques 

will be highlighted in the summary.

 

 
45

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