Cover images by Arcadis

 Turbo Roundabouts, Netherlands

U.S. Department of Transportation

Federal Highway Administration

http://safety.fhwa.dot.gov

FHWA Safety Program

Turbo Roundabouts

Informational Primer
FHWA | Turbo Roundabouts i

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FHWA | Turbo Roundabouts ii

Contents 

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

Section 1: Characteristics of a Turbo Roundabout................................................................. 2

Section 2: Potential Benefits of Turbo Roundabouts............................................................... 6

Section 3: User Considerations.................................................................................................. 8

3.1 Motorists ............................................................................................................................. 8

3.2 Pedestrians......................................................................................................................... 8

3.3 Bicyclists.............................................................................................................................. 9

3.4 Motorcyclists...................................................................................................................... 9

3.5 Freight/Large Vehicles...................................................................................................... 9

Section 4: Location Considerations........................................................................................10

Section 5: Safety Analysis Methods and Results....................................................................11

Section 6: Operational Analysis ..............................................................................................11

Section 7: Design Considerations............................................................................................12

7.1 Horizontal Design.............................................................................................................12

7.2 Sight Distance and Visibility............................................................................................19

7.3 Signage and Pavement Markings.................................................................................19

7.4 Pedestrian Design Treatments .......................................................................................20

7.5 Bicycle Design Treatments.............................................................................................20

7.6 Vertical Design ................................................................................................................21

7.7 Lighting.............................................................................................................................21

7.8 Landscaping....................................................................................................................21

7.9 Other Design Considerations.........................................................................................22

7.10 Comparison to United States Roundabout Design Principles..................................22

Section 8: Costs.........................................................................................................................23

Section 9: Education and Public Involvement......................................................................23

9.1 Key Messages..................................................................................................................23

9.2 Educational Media .........................................................................................................23

9.3 Audiences........................................................................................................................24

9.4 Decision Matrix ................................................................................................................24

Section 10: References ............................................................................................................25
FHWA | Turbo Roundabouts iii

List of Figures

Figure 1. Photograph. Aerial view of turbo roundabout in Delft.(7) ....................................... 2

Figure 2. Graphic. Turbo roundabout features. Image based on Fortuijn, 2009.(6) ............. 3

Figure 3. Graphic. Basic turbo roundabout. Image based on Dzambas et al., 2017 with 

capacity value from Fortuijn, 2009.(6,9) ..................................................................................... 4

Figure 4. Graphic. Egg turbo roundabout. Image based on Dzambas et al., 2017 with 

capacity value from Fortuijn, 2009.(6,9) ..................................................................................... 4

Figure 5. Graphic. Knee turbo roundabout. Image based on Dzambas et al., 2017 with 

capacity value from Fortuijn, 2009.(6,9) ..................................................................................... 5

Figure 6. Graphic. Spiral turbo roundabout. Image based on Dzambas et al., 2017 with 

capacity value from Fortuijn, 2009.(6,9) ..................................................................................... 5

Figure 7. Graphic. Rotor turbo roundabout. Image based on Dzambas et al., 2017 with 

capacity value from Fortuijn, 2009.(6,9) ..................................................................................... 6

Figure 8. Graphic. Conflict point frequency for modern multilane roundabout. Image 

based on Vasconcelos et al., 2014.(11) ..................................................................................... 7

Figure 9. Graphic. Conflict point frequency for turbo roundabout. Image based on 

Vasconcelos et al., 2014.(11)....................................................................................................... 7

Figure 10. Graphic. Sample turbo block. Image based on Overkamp & Van der Wijk, 

2009 and Dzambas et al., 2017.(9,25)........................................................................................14

Figure 11. Photograph. Original design used in the Netherlands for introducing the inner 

lane.(27) .......................................................................................................................................15

Figure 12. Photograph. Revised design used in the Netherlands for introducing the inner 

lane.(28) .......................................................................................................................................16

Figure 13. Photograph. Raised lane divider in a turbo roundabout in the Netherlands. . 17

Figure 14. Photograph. Example introduction of the raised lane divider.(30) .....................17

Figure 15. Photograph. Lane divider for turbo roundabout at Victoria International 

Airport.(31) ...................................................................................................................................18

Figure 16. Graphic. Roundabout directional arrow sign (R6-4b) for central island.(29) .....20

Figure 17. Photograph. Example of a chicane in a splitter island at a turbo roundabout 

in the Netherlands to provide additional time for approaching drivers to identify the 

bicyclist and to encourage bicyclists to perform a two-stage crossing.(32) .......................21

List of Tables

Table 1. Roundabout design principles..................................................................................22

Table 2. Target audience educational/awareness media..................................................24
FHWA | Turbo Roundabouts iv

List of Abbreviations

CG center point of turbo block

FHWA Federal Highway Administration

HCM Highway Capacity Manual 

LED Light Emitting Diode

MUTCD Manual on Uniform Traffic Control Devices

NCHRP National Cooperative Highway Research Program

OS/OW oversize/overweight

Pcu/h passenger cars per hour
FHWA | Turbo Roundabouts 1 

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 

United States. Approximately half of all crashes and half of fatal and serious injury crashes 

occur at or near intersections. In the single year of 2018, 8,858 people were killed in intersection 

and intersection-related crashes.

(1) In stark contrast, there were a total of 46 fatalities at 

roundabouts built in the United States over the nine-year period spanning 2005 to 2013, a time 

period in which the total number of roundabouts in the United States grew from a few hundred 

to a few thousand.

(2) At the individual intersection level, converting a traditional at-grade 

signalized intersection to a modern roundabout is expected to reduce the number of injury 

crashes by 78 percent.

(3) Converting a traditional at-grade minor-road stop control intersection to 

a modern roundabout is expected to reduce the number of injury crashes by 82 percent.

(3) 

Though most roundabouts in the United States are single-lane, multilane roundabouts have 

become more common. There is a tendency for some 2 x 2 multilane roundabouts1 to 

experience higher than expected frequencies of sideswipe – same direction crashes.

(4) Given 

that modern roundabout geometry reduces both the speed and angle of collisions, the 

sideswipe – same direction crashes in 2 x 2 multilane roundabouts tend to be low severity (i.e., 

crashes in which people are not injured, but where vehicles may be damaged).(4) Some other 

countries have implemented a modified version of a multilane roundabout, the turbo 

roundabout, with positive results. Characteristics of the turbo roundabout could potentially be 

effective at influencing driver behavior and reducing lane change conflicts in a way that would 

address the crash types occurring in 2 x 2 multilane roundabouts. First designed and 

implemented in the Netherlands in the 1990s, the turbo roundabout2 (shown in figure 1) has the 

same general operating characteristics as modern roundabouts but utilizes notably different 

geometrics and applications of traffic control devices.

(5) This informational primer seeks to

describe the characteristics of turbo roundabouts, highlighting the design and traffic control 

features, operational capabilities, and potential safety benefits of these roundabout alternatives. 

1 A 2 x 2 roundabout is characterized by two entry lanes approaching two circulating lanes.

2 The turbo roundabout was named by its inventor in the Netherlands in 1996. The descriptor “turbo” is 

meant to symbolize the geometric shape. It is not meant to convey faster vehicle speeds. In fact, its 

geometry was in part developed to reduce vehicle speeds below what might be observed during lowervolume hours of more traditional concentric multilane roundabouts.(6) 
FHWA | Turbo Roundabouts 2 

©2019 Google Earth®.

Figure 1. Photograph. Aerial view of turbo roundabout in Delft.(7)

Section 1: Characteristics of a Turbo Roundabout

Based on a review of international experience(8), features (illustrated in figure 2) that

characterize turbo roundabouts include the following:

(6) 

• A second circulatory 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, circulatory lanes in the roundabout.

• Smooth flow is encouraged by a spiral alignment.

• Lane dividers discourage lane changing within the roundabout. Drivers, therefore, select 

the proper lane prior to entering the roundabout. Internationally, options for lane 

separation have included raised, mountable lane dividers; flush lane dividers; or solid 

pavement markings.

• Each segment of the roundabout includes one circulatory 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 and entry are typically at right angles to the roundabout.

• Roundabout directional arrow signs direct drivers and increase conspicuity of the central 

island. 

• Mountable aprons offer sufficient maneuvering space for longer vehicles.
FHWA | Turbo Roundabouts 3 

Source: FHWA.

Figure 2. Graphic. Turbo roundabout features. Image based on Fortuijn, 2009.(6)

There are different types of turbo roundabouts, including the basic, egg, knee, spiral, and rotor 

turbo roundabouts.(6) These options differ with respect to central island design, number of 

circulating lanes, and number of approach lanes, as described below:

• Basic – inside lane added on major approaches, two lanes on each approach (see figure 

3). 

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

approaches (see figure 4). 

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

figure 5). 

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

approaches with three lanes and two approaches with two lanes (see figure 6). 

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

each approach (see figure 7). 

The variations in turbo roundabout designs differ in terms of total capacity available, so the type 

selected may be dictated by intersection demand. The capacity values provided in figure 3 

through figure 7 represent capacity in the Netherlands and are not necessarily reflective of 

expected capacity values elsewhere.
FHWA | Turbo Roundabouts 4 

Source: FHWA. 

Figure 3. Graphic. Basic turbo roundabout. Image based on Dzambas et al., 

2017 with capacity value from Fortuijn, 2009.(6,9)

Source: FHWA. 

Figure 4. Graphic. Egg turbo roundabout. Image based on Dzambas et al., 2017 

with capacity value from Fortuijn, 2009.(6,9)
FHWA | Turbo Roundabouts 5 

Source: FHWA. 

Figure 5. Graphic. Knee turbo roundabout. Image based on Dzambas et al., 2017 

with capacity value from Fortuijn, 2009.(6,9)

Source: FHWA. 

Figure 6. Graphic. Spiral turbo roundabout. Image based on Dzambas et al., 

2017 with capacity value from Fortuijn, 2009.(6,9)
FHWA | Turbo Roundabouts 6 

Source: FHWA. 

Figure 7. Graphic. Rotor turbo roundabout. Image based on Dzambas et al., 2017 

with capacity value from Fortuijn, 2009.(6,9)

Section 2: Potential Benefits of Turbo Roundabouts

An international crash-based safety evaluation suggests conversion of 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.

(6) In addition, the geometric characteristics of the turbo 

roundabout result in operational outcomes that should help address lane selection, lane 

changing, and entering and exiting behaviors that can lead to the lower severity, multiple-vehicle

crashes in 2 x 2 multilane roundabouts. The spiral road geometry and lane dividers 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 8 and figure 9 show that the turbo 

roundabout eliminates some of the conflicts associated with the common crash types in modern 

2 x 2 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.

(10) 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.

(6) The geometry of turbo roundabouts also helps to manage the speeds

of vehicles entering, navigating, and exiting the roundabout. Operationally, the capacity of a 

turbo roundabout is expected to be similar to other modern multilane roundabouts. 
FHWA | Turbo Roundabouts 7 

Source: FHWA. 

Figure 8. Graphic. Conflict point frequency for modern multilane roundabout. 

Image based on Vasconcelos et al., 2014.(11) 

Source: FHWA. 

Figure 9. Graphic. Conflict point frequency for turbo roundabout. Image based 

on Vasconcelos et al., 2014.(11) 
FHWA | Turbo Roundabouts 8 

Section 3: User Considerations

It is important to consider how various user groups are accommodated at turbo roundabouts 

given the intersection type’s key features. Five primary user groups – motorists, pedestrians, 

bicyclists, motorcyclists, and freight/large vehicles – are discussed in this section. 

3.1 Motorists

Turbo roundabouts rely on more direct entry geometry and enhanced delineation of lanes that 

can make it easier for motorists to successfully navigate them. Signage and supplemental 

pavement markings are provided in advance on approaches so drivers are given enough time to 

select their desired lane. When locating signs, designers should consider decision and stopping 

sight distance as well as potential queue lengths to provide drivers with adequate advance 

notice. At the entrance to the roundabout, drivers are required to identify acceptable gaps in no 

more than two conflicting lanes. A roundabout directional arrow sign placed directly in the 

drivers’ field of view directs drivers to enter the circulatory roadway in the appropriate direction.

Internationally, these signs are also recommended to increase the conspicuity of the central 

island and communicate to drivers the need to slow and turn into the roundabout.

(6) The 

Roundabouts Informational Guide also recommends using landscaping to increase central 

island conspicuity.

(12) Finally, the spiral geometry and enhanced delineation reinforce the 

appropriate maneuvers from each lane inside the roundabout. 

One notable difference between turbo roundabouts and other modern roundabouts is the ability 

to complete U-turns. Modern roundabouts allow vehicles from all approaches to complete U-turn 

maneuvers. The lane arrangement of a turbo roundabout prohibits vehicles that enter on some 

approaches from completing U-turns. The approaches and lanes from which vehicles can and 

cannot perform U-turns vary based on the type of turbo roundabout. For instance, vehicles 

entering from the inside lane of the left and right approaches (the major road approaches) in 

figure 9 (a “Basic” turbo roundabout) can complete a U-turn; while vehicles approaching from 

the top and bottom approaches (the minor road approaches) cannot. As a result, it is important 

for analysts to consider the frequency of U-turn maneuvers at an intersection when evaluating 

turbo roundabouts as a potential alternative.

3.2 Pedestrians

The navigation through a turbo roundabout by a pedestrian does not differ from single-lane and 

multilane roundabouts. As a result, designers can follow guidance for pedestrian facilities at 

roundabouts proposed in the Roundabouts Informational Guide and National Cooperative 

Highway Research Program (NCHRP) Report 834:

(12, 13)

• Keep sidewalks along the perimeter of the roundabout, separated from the edge of the 

circulatory roadway with a landscaped strip or buffer. 

• Where crosswalks are provided, locate them for pedestrian convenience and safety, 

where drivers can be expected to yield the right-of-way, and where the crossing will be 

less likely to be blocked by queued vehicles.

• Provide a splitter island sufficiently wide to accommodate a crossing that is accessible to 

pedestrians with disabilities as well as wide enough for comfortable queueing.
FHWA | Turbo Roundabouts 9 

3.3 Bicyclists 

The decision of whether to provide separated bicycle facilities at turbo roundabouts depends on 

context, considering factors such as bicycle volume, the presence of existing bicycle facilities, 

motor vehicle volume, complexity of the roundabout, adjacent infrastructure and land use, and 

right-of-way availability. Bicycle features at turbo roundabouts are not expected to differ from 

traditional roundabouts, and features designers can consider to better accommodate bicyclists 

include:

(12) 

• Keeping radii small to reduce vehicle speeds, which can make bicyclists more 

comfortable if they ride in the roundabout. 

• Terminating bicycle lanes before the edge of the circulatory roadway and crosswalks

with enough length remaining for bicyclists to merge into traffic. 

• Introducing bicycle lanes on exit legs downstream of crosswalks.

• If bicyclists are required to utilize the sidewalk, designing sidewalks to meet shared use 

path width requirements. 

• If the intent is for bicyclists to cross at-grade on approaches, whether on a designated 

crossing or on a pedestrian crosswalk, a pavement-level cut-through of the splitter island 

can be provided.

(14) The cut-through can be designed to include a chicane to encourage 

a two-stage crossing for bicyclists and provide more time for approaching drivers to 

identify crossing bicyclists. This is a commonly used treatment in the Netherlands.

3.4 Motorcyclists

While fatal crashes at roundabouts are much less likely than at traditional three- and four-leg 

intersections, motorcyclists are overrepresented in those fatal crashes. Motorcyclists were

involved in 21 of the 46 fatal crashes that occurred at roundabouts in the United States between 

2005 and 2013.

(15) Roadway features that can have a significant impact on motorcycle safety 

performance at roundabouts include the presence and location of raised lane dividers and

curbing, surface friction, pavement markings, drainage, sight distance (especially rider 

conspicuity), radii, the roadside environment, and surface conditions. Specific concerns for 

motorcyclists in turbo roundabouts are the truck apron and lane divider options that are raised.

Sloped curbing with minimal vertical reveal can provide a more forgiving environment to 

motorcycles compared to vertical or rolled curbing. Designers can also provide supplemental 

signage alerting motorcyclists to these elements of turbo roundabouts. Potential alternatives to 

the raised lane dividers include striping and colorized and/or textured pavement, which are 

discussed in Section 7.1.4 Lane Divider. 

3.5 Freight/Large Vehicles 

The design of some turbo roundabout features is influenced by the physical dimensions and 

turning characteristics of the larger vehicles that will use the intersection. The lane widths of 

turbo roundabouts are determined with consideration of the design vehicle, typically the largest 

vehicle anticipated to regularly navigate the intersection. European turbo roundabout design 

guidance includes discussions on selecting lane widths so that design vehicles do not track into 

adjacent lanes.

(9) However, the dimensions of European design vehicles are often smaller than 

design vehicle dimensions in the United States. Designing turbo roundabouts in the United 

States to prevent, for example, a WB-67 from tracking into an adjacent lane it not feasible within 
FHWA | Turbo Roundabouts 10

a reasonably sized roundabout. However, this characteristic is not limited solely to turbo 

roundabouts. The Roundabouts Informational Guide states that “multilane roundabouts are 

designed either to allow large vehicles to track across more than one lane while entering, 

circulating, and exiting or to stay within their lane” [Pages 2-19].(12) This concept has also been 

adopted by some State departments of transportation as well. The Washington State 

Department of Transportation Design Manual informs designers to “assume a truck’s travel path 

will [straddle] parts of two adjacent lanes” in multilane roundabouts [Pages 1320-18].

(16) The

South Carolina Department of Transportation allows large vehicles to “track across the whole 

width of the circulatory roadway to negotiate the roundabout” [Pages 9.7-11].

(17) Given this 

allowance for multilane roundabouts, it is reasonable for agencies to allow design vehicles to 

track across multiple lanes within turbo roundabouts. In these situations, a raised lane divider is 

unlikely to be a sustainable option due to repeated strikes by the larger vehicles.

Starting the lane divider of a turbo roundabout as near as possible to the vehicle entry point is 

necessary to prevent vehicles circulating in the outside lane from changing to the inside lane at 

these locations. However, large vehicles entering the inside lane from an approach need a 

wider opening to account for their larger swept paths. Where a raised lane divider option is 

used, a traversable, demarcating feature can be provided at the origin of the raised divider to 

ease the entrance of larger vehicles. 

A central truck apron is provided in turbo roundabouts to help accommodate larger vehicles that 

need to navigate the intersection. Aprons can also be provided on the perimeter of the 

roundabout to provide additional turning space for large vehicles. Finally, agencies can work 

with the State Oversize/Overweight (OS/OW) Load Permit Office to determine if the intersection 

is commonly used by OS/OW vehicles, and if so, obtain the applicable length and width 

requirements for those vehicles to develop strategies for accommodation.

Section 4: Location Considerations

Modern roundabouts can be among the safest feasible intersection alternatives in a wide variety 

of settings and contexts – low-speed urban, high-speed rural, at isolated intersections, as 

corridor treatments, and even at interchange ramp terminal intersections. Relevant site 

characteristics that can influence whether a roundabout is a feasible alternative include right-ofway limitations, intersection skew, winter maintenance needs, adjacent traffic generators or 

sites that require pre-emption, and downstream bottlenecks. Additional detail on roundabout 

applications commonly found to be feasible and advantageous can be found in the 

Roundabouts Informational Guide3.

(12)

 

Turbo roundabouts may be considered at any intersection where a roundabout is a potential 

alternative, particularly where traffic demand indicates the need for a multilane roundabout.

Their design provides similar capacity to multilane roundabouts while reducing conflict points, 

3 At the time of publication of this informational primer, the 2nd Edition of the Roundabout 

Informational Guide was published under NCHRP Report 672. The 3rd Edition is being 

developed under NCHRP Project 03-130.
FHWA | Turbo Roundabouts 11

discouraging lane changes, and maintaining the speed reduction characteristics of single-lane 

roundabouts.

Section 5: Safety Analysis Methods and Results

Given the brief history of turbo roundabouts, international safety studies based on an analysis of 

crash data are limited, and not yet available in the context of a United States driving population. 

Dutch research analyzed crash data at seven intersections—including signalized, yield-control, 

and old-style rotary types—that were converted to a turbo roundabout and found a 76-percent 

reduction in the number of injury crashes.

(6) Polish research found that turbo roundabouts with a 

raised lane divider experience a lower crash frequency than those with paint stripes only. 

However, the research observed lower severity crash outcomes in both cases. Only 7 percent of 

crashes on turbo roundabouts without a raised lane divider resulted in an injury, compared to 4 

percent of crashes with a raised lane divider.

(18) Safety surrogate measures resulting from

microscopic traffic simulations or field observations (e.g., time-to-collision, vehicle speeds, 

vehicle conflicts, incorrect movements, and incorrect paths) have also indicated that turbo 

roundabouts are likely to experience less frequent and less severe crashes than multilane 

roundabouts due to the reduction of conflict points within the roundabout and the lower speeds 

required to navigate the smaller radii4. [See references 19, 20, 21, 22, and 23.] 

Section 6: Operational Analysis

For a turbo roundabout to be successful, it is important to verify the design can accommodate 

the projected traffic volumes at the intersection. At modern multilane roundabouts in the United 

States, the capacity of one entry lane ranges from 300 to 1,100 passenger cars per hour (pc/h), 

depending on conflicting flow in the circulatory roadway, implying a total approach capacity 

ranging from approximately 600 to 2,200 pc/h for a two-lane approach.

(12, 24) As with modern 

roundabouts, turbo roundabout capacity is measured at the approach level. Operational 

performance models for turbo roundabouts have not yet been developed for, or adapted to, the 

context of a United States driving population. International research suggests basic turbo 

roundabouts have similar capacities as multilane roundabouts with two entry and two circulating

lanes. One such study from the Netherlands estimated a capacity for a basic turbo roundabout 

design of approximately 3,500 pc/h for all entries combined, assuming conflicting traffic volumes 

between 1,900 and 2,100 pc/h.

(25) However, roundabout capacity in the Netherlands is likely to 

be higher than in the United States given broader driver familiarity with roundabouts.

Gap-acceptance models that consider critical headway, critical follow-up time, and conflicting 

traffic appear adequate for estimating turbo roundabout capacity. Research in Poland found the 

Highway Capacity Manual (HCM) capacity models for roundabouts produced capacity estimates 

for Polish turbo roundabouts that were comparable to estimates from Polish-specific turbo 

roundabout capacity models.

(26) The roundabout capacity models of the HCM are likely to 

represent reasonable capacity estimates for turbo roundabout approaches with up to two lanes. 

As with single and multilane roundabouts, analysts would apply the HCM models to each lane of 

4 The ability to reliably link safety surrogates to crash frequency and severity remains a topic of ongoing 

research and debate. 
FHWA | Turbo Roundabouts 12

each approach, given the specific characteristics of the lane and approach (e.g., number of 

entry lanes, number of conflicting lanes, conflicting flow).

Section 7: Design Considerations

The geometric design of a turbo roundabout is driven by the desired capacity and the desired 

characteristics of a design vehicle’s horizontal swept path. The projected demand and cross 

sections on the approach roadways inform the number of lanes/lane arrangement decisions, 

which dictate the type of turbo roundabout to be built (see Section 1: Characteristics of a Turbo 

Roundabout). Once the type is selected, a horizontal swept path analysis of the design vehicle 

informs lane width decisions along with other lane width-related considerations (e.g., right-ofway, performance for all vehicle types and users). The turbo roundabout type and lane widths 

are combined to construct the turbo block, which guides the geometric design of the circulatory 

roadway. 

7.1 Horizontal Design

7.1.1 Turbo Block 

The spiral alignment of a turbo roundabout is generated from the “turbo block,” a series of

circular arcs with centers located at various points along a reference line known as a “translation 

axis.” The turbo block consists of arcs that represent the inner and outer edges of each lane. 

The inner radius of the turbo block, which represents the radius of the central island, is selected 

based on the anticipated size of the turbo roundabout. The shift along the translation axis from 

the center is the width of the lane represented by the arc. The turbo block and angle of the 

translation axis differs for each turbo roundabout type. Figure 10 is a sample turbo block for a 

basic turbo roundabout with the major roadway oriented horizontally. 

The turbo block is defined by the characteristics shown in figure 10. First is the center point 

(CG), which is the intersection of the approach centerlines. Second is the orientation of the 

translation axis, which is defined in relation to the major road approaches. Assuming the major 

road is oriented with the x-axis in figure 10, the right side of the translation axis is rotated 57.5

degrees around the center below the x-axis for a four-leg intersection, and the left side of the 

translation axis is rotated 65 degrees around the center below the x-axis for a three-leg 

intersection.

(9,25) The angle of rotation for the translation axis can be tweaked to provide smooth, 

spiraled vehicle paths for all vehicle movements. Third are the radii of the circles (TR1, TR2, 

TR3, and TR4). TR1 defines the radius of the inside edge of the inside roadway. TR2 defines 

the outside edge of the inside roadway; with the difference between TR2 and TR1 equal to the 

width of the inside travel lane plus additional width for the edge lines delineating the raised lane 

divider. TR3 defines the inside edge of the outside roadway. The difference between TR2 and 

TR3 is the width of the lane divider. TR4 defines the outside edge of the outside roadway. 

The fourth key set of dimensions defining the turbo block is the distances between the center 

points of the arcs. The circles corresponding to the four radii are split along the translation axis, 

and the resulting arcs are slid along the translation axis in opposing directions by half the

distance defined as the shift. The shift is the distance between the centers of the arcs. The shift 

can differ for the TR1 centers and the TR2/3/4 centers if the inside roadway width is different 

than the outside roadway width. The shift for the TR1 centers (Δʋ in figure 10) is equal to the 
FHWA | Turbo Roundabouts 13

difference between the inside edge of the inside roadway and the inside edge of the outside 

roadway (also the difference between the values used for TR3 and TR1). The shift for the TR1 

centers is achieved by sliding the two arcs defined by TR1 in opposing directions away from 

CG, each by Δʋ/2. In international practice, Δʋ/2 ranges from between 8.5 and 9.5 feet (for total 

shifts ranging between 17 and 19 feet), as shown in figure 10. The shift for the TR2/3/4 centers 

(Δu in figure 10) is the distance between the outside edge of the inside roadway and the outside 

edge of the outside roadway (also the difference between the values used for TR4 and TR2). 

The shift for the TR2/3/4 centers is achieved by sliding the arcs defined by TR2/3/4 in opposing 

directions away from CG by Δu/2, as shown in figure 10. This value (Δu/2) typically ranges from

between 7.5 and 8.5 feet (for a total shift of 15 to 17 feet). If the inside and outside roadways 

are the same width, the shift value for all radii are the same (Δʋ = Δu). 

Internationally, the radii (TR1, TR2, TR3, and TR4) for basic turbo roundabouts have ranges as 

follows:

• 34 to 66 feet for TR1.

• 52 to 82 feet for TR2.

• 53 to 83 feet for TR35. 

• 70 to 100 feet for TR4.

With the offset arcs making up the turbo roundabout, the nominal diameter of the turbo

roundabout is twice the value TR4 plus the width of the TR2/3/4 shift, Δu. Assuming a shift of 15 

feet, the inscribed circle for basic turbo roundabouts ranges from 155 feet to 215 feet.

5 The one-foot difference between the minimum and maximum TR2 and TR3 values implies an average 

width of one foot for the lane divider.
FHWA | Turbo Roundabouts 14

Source: FHWA. 

Figure 10. Graphic. Sample turbo block. Image based on Overkamp & Van der 

Wijk, 2009 and Dzambas et al., 2017.(9,25)

7.1.2 Lane and Roadway Width 

Determining the width of each lane of a turbo roundabout is informed by a horizontal swept path 

analysis of the design vehicle. The inside lane is often wider than the outside lane to 

compensate for the design vehicle maneuvering a smaller radius. Internationally, inside lane
FHWA | Turbo Roundabouts 15

width ranges from between 14 and 16 feet, while outside lane width ranges from between 13 

and 14.5 feet. The inner roadway width, defined as the distance from the central island to the 

lane divider (TR2 minus TR1), including the inside and outside edge line pavement markings

ranges from between 16 and 18 feet. The outer roadway width, defined as the distance from the 

lane divider to the outer edge of the roundabout (TR4 minus TR3), again including the inside 

and outside edge line pavement markings ranges from between 15 and 16.5 feet.

(9, 25) 

7.1.3 Central Island 

The central island is defined by the innermost radius of the turbo block (TR1) and consists of a 

traversable portion (mountable apron) and a non-traversable portion. The non-traversable 

portion is typically used for signage, specifically a roundabout directional arrow sign. There are 

cutouts in the central island to introduce the inside lane of the turbo roundabout on the 

applicable approaches. There are two developed methods for design of these cutouts and 

beginning the inner lane. A curved entry, shown in figure 11, provides a smooth path for 

approaching vehicles, but may result in a greater chance of circulating vehicles entering the 

inside lane. A flat entry, shown in figure 12, helps to discourage this movement from circulating 

vehicles. Designers should check that objects on the central island do not restrict sight distance 

along the circulatory roadway.

©2019 Google Earth®.

Figure 11. Photograph. Original design used in the Netherlands for introducing 

the inner lane.(27)
FHWA | Turbo Roundabouts 16

 ©2019 Google Earth®.

Figure 12. Photograph. Revised design used in the Netherlands for introducing 

the inner lane.(28)

7.1.4 Lane Divider 

One important feature of the turbo roundabout is a lane divider between each circulating lane. In 

the Netherlands, this lane divider is raised but mountable, designed with little vertical profile and 

a rather flat slope to provide forgiveness for errant vehicles (as shown in figure 13). Often, the 

raised lane divider is introduced with a traversable, demarcating feature to allow tracking by 

large vehicles (see figure 14). Some countries (including Poland, Germany, and Canada) have 

implemented turbo roundabouts without raised lane dividers, in part due to possible challenges 

these dividers present to motorcyclists and snow plowing operations.

(9) In the United States, a 

roundabout in Alta, Utah6 has a raised, mountable lane divider separating lanes for a two-lane 

portion of the roundabout.

Alternatives to the raised lane divider include striping and colorized or textured pavement, as 

shown in figure 15 from a turbo roundabout in Canada. While these options do not provide a 

physical barrier to lane changing, they still communicate this message to the driver both 

visually, and in the case of textured pavement, through audible and tactile mediums. Other 

alternatives to consider include: 

• Milled rumble strips or rumble stripes, which provide more intense feedback to drivers 

than textured pavement. 

• A double solid white lane, which the Manual on Uniform Traffic Control Devices 

(MUTCD) describes as a standard approach when crossing the lane lines are 

prohibited.

(29) For one example, two roundabouts in Conway, Arkansas7 use solid wide 

white thermoplastic lines to separate its two lanes within the circulatory roadway.

• Raised pavement markers which can provide visual and tactile feedback and be snowplowable where needed.

6 Located at Latitude 40.645758 Degrees North, Longitude 111.494956 Degrees West.

7 Located at Latitude 35.066366 Degrees North, Longitude 92.414523 Degrees West.
FHWA | Turbo Roundabouts 17

© 2020 Arcadis.

Figure 13. Photograph. Raised lane divider in a turbo roundabout in the 

Netherlands. 

 ©2020 Google Earth®. 

Figure 14. Photograph. Example introduction of the raised lane divider.(30)
FHWA | Turbo Roundabouts 18

 ©2020 Google Earth®. 

Figure 15. Photograph. Lane divider for turbo roundabout at Victoria International 

Airport.(31)

7.1.5 Approach Geometry 

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

changes to the alignment along the approach roadway and maintaining exit curvature that

encourages drivers to maintain slower speeds through the exit of the roundabout. Additionally, 

turbo roundabouts are built with little or no flare or deflection and smaller entry radii. The angle 

between entering traffic and circulating traffic is therefore larger (closer to a perpendicular entry) 

for a turbo roundabout than for other modern multilane roundabouts. These approach features 

differ from modern multilane roundabouts in the United States, which typically include flare to 

gain some capacity increase and deflection to align entering vehicles “to the right of” the central 

island in the desired direction of travel. The entry geometry of a turbo roundabout generally 

does not channelize drivers into the circulatory roadway to the right of the central island and the 

splitter islands generally do not have enough curvature to block a direct path of approaching 

vehicles to the central island. This approach geometry is based on the premise that it will be 

clearer to drivers that they are approaching an intersection that should be negotiated at lower 

speeds.

(6) Potential disadvantages include drivers errantly hitting the central island, making 

wrong-way left turn maneuvers to enter the roundabout, and making wrong-way exit maneuvers 

into entrance approach lanes.

(12) International literature emphasizes the importance of a 

roundabout directional arrow sign, placed in the central island in the line of sight of approaching 

drivers, that directs drivers to turn right and increases the conspicuity of the central island 

(discussed in Section 7.3). It also emphasizes the need for a forgiving design of the central 

island and sign in the case that either is struck. 
FHWA | Turbo Roundabouts 19

Internationally, turbo roundabout entry radii range from 39 to 50 feet.

(9, 25) For comparison, 

modern multilane roundabouts in the United States are designed with entry radii exceeding 65 

feet, and even single-lane roundabouts have entry radii ranging from 50 to 100 feet.

(12) 

7.2 Sight Distance and Visibility

Adequate stopping and decision sight distance should be provided for all users of the turbo 

roundabout. Stopping and intersection sight distance should be provided at all approaches. The 

Roundabouts Informational Guide provides guidelines for evaluating sight distance and visibility 

at roundabouts.(12)

7.3 Signage and Pavement Markings

There are a few differences in the traffic control devices within the circulatory roadway of turbo 

roundabouts compared to modern multilane roundabouts. For modern multilane roundabouts, 

lanes are separated using either a single dashed or solid white line. As discussed in section

7.1.4 Lane Divider, these are replaced with lane dividers in turbo roundabouts. Potential 

advantages of the lane divider compared to single dashed or solid white lines include less 

ambiguous and more intuitive messaging to drivers on lane selection, lane keeping, and the 

appropriate maneuvers from each lane. 

Given the operational characteristic of prohibited lane changes within the circulatory roadway of 

a turbo roundabout, signage and pavement markings on the approaches, especially for lane 

selection, are critical for motorists to identify and select their desired lane before entering the 

roundabout. Chapter 2 of the MUTCD, as well as the Roundabouts Informational Guide, 

describe applications of lane control signage for roundabout approaches.

(12, 29) Lane control 

signage can be supplemented using pavement marking arrows. 

A version of the roundabout directional arrow sign (R6-4, R6-4a, or R6-4, as shown in figure 16) 

in the central island directs drivers to the right and increases the conspicuity of the central 

island. Signage can also direct pedestrians and bicyclists to designated facilities, drivers to their 

desired lanes, and communicate the presence of raised curbing, such as a raised lane divider (if 

one is used). If the lane divider includes grooved, textured, or brick pavements, consideration 

can be given to including sign W8-15 to warn road users of its presence. Pavement markings 

shall be used to delineate the edges of the approach and circulatory lanes. Additionally, 

supplemental delineation can be achieved using reflectors or light emitting diodes (LEDs) to 

illuminate the edges of the apron and lane dividers.

(29) Finally, given the important role signage 

and pavement markings play for all users of turbo roundabouts, it is important that all traffic 

control devices are compliant with the MUTCD and for agencies to establish consistent 

maintenance practices that sustain the visibility and retroreflectivity of traffic control devices. 
FHWA | Turbo Roundabouts 20

Source: FHWA. 

Figure 16. Graphic. Roundabout directional arrow sign (R6-4b) for central 

island.(29)

7.4 Pedestrian Design Treatments

Pedestrian accommodations for turbo roundabouts do not differ from modern roundabouts. 

Crossings should be kept at the perimeter of the intersection, with crosswalks and splitter 

islands on the approaches to provide two stage crossings. All sidewalks, crosswalks, and curb 

ramps should be accessible to and usable by pedestrians with disabilities. The crosswalk should 

be placed far enough (minimum of 20 feet, or one vehicle-length) from the circulatory roadway 

so a motorist can exit the roundabout and then stop before reaching any potential pedestrians in 

the crosswalk.

(12) 

7.5 Bicycle Design Treatments

Bicycle guidance for turbo roundabouts is the same as for modern roundabouts. A bicyclist 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 bicyclist volume, 

motor vehicle volume, complexity of the roundabout, adjacent infrastructure and land use, and 

available right-of-way. In the Netherlands, separate bicycle paths outside of the roundabout are 

recommended where possible, including for turbo roundabouts.

(25) Dutch guidance recommends 

adding curb cuts with chicanes in splitter islands for bicycle crossings (figure 17). The curb cuts 

encourage bicyclists to use the crossing, while the chicane encourages the crossing to be taken 

in two stages. 
FHWA | Turbo Roundabouts 21

©2019 Google Earth®.

Figure 17. Photograph. Example of a chicane in a splitter island at a turbo 

roundabout in the Netherlands to provide additional time for approaching 

drivers to identify the bicyclist and to encourage bicyclists to perform a twostage crossing.(32) 

7.6 Vertical Design

Vertical alignment considerations are the same as other modern roundabouts. The geometry 

should not restrict sight distance throughout the intersection area, including decision sight 

distance on the approaches when selecting lanes, stopping sight distance on the approach and 

on the circulatory roadway, and intersection sight distance at the entrances to the circulatory 

roadway. 

7.7 Lighting

The use of proper lighting is encouraged to improve the visibility of the middle island and raised 

lane divider.

(25) Lighting should also be provided to give adequate visibility for pedestrian and 

bicycle facilities, especially crossings, though it is important that designers are careful to avoid

creating negative contrast lighting and shadowing.

(12)

 

7.8 Landscaping

Landscaping should be limited to the non-traversable portion of the central island and not hinder 

stopping sight distance around the circulatory roadway. If sprinklers are used to maintain 

landscaping, designers should consider the impacts of irrigation runoff onto the circular 

roadway, as unexpected wet pavement can introduce another potential risk to users of the 

intersection.

(33)

 
FHWA | Turbo Roundabouts 22

7.9 Other Design Considerations

Other design considerations, such as bypass lanes, access management, at-grade rail 

crossings, evacuation routes, and bus stops, should be addressed the same as they are for 

modern roundabouts. Specific guidelines for these issues are available in the Roundabout 

Informational Guide.

(12)

 

7.10 Comparison to United States Roundabout Design Principles

The Roundabouts Informational Guide describes six overarching principles that inform the 

design of roundabouts.

 (12) Table 1 describes the principles and the manners in which they are 

addressed in turbo roundabouts.

Table 1. Roundabout design principles. 

Design Principles from the Roundabout 

Informational Guide(12)

 

Addressed in Turbo Roundabouts

“Provide slow entry speeds and consistent 

speeds through the roundabout by using 

deflection.”

International practices of a perpendicular entry 

and smaller radii of the right turns on entry are 

intended to slow vehicle entry speeds.

“Provide the appropriate number of lanes 

and lane assignment to achieve adequate 

capacity, lane volume balance, and lane 

continuity.”

Turbo roundabout variants are available for a 

range of traffic demand. International research 

suggests basic turbo roundabouts have similar 

capacities as multilane roundabouts with two 

entry and two circulating lanes.

“Provide smooth channelization that is 

intuitive to drivers and results in vehicles 

naturally using the intended lanes.”

The spiral lane markings and lane dividers 

provide intuitive messaging to drivers on lane 

selection, lane keeping, and the appropriate 

maneuvers from each lane. 

“Provide adequate accommodation for the 

design vehicles.”

As with modern multilane roundabouts, lane 

width decisions for turbo roundabouts are 

informed by a horizontal swept path analysis 

of the design vehicle along with other lane 

width-related considerations (e.g., right-ofway, performance for all vehicle types and 

users). Additionally, aprons are provided on 

the central island and as necessary on the 

perimeter of the roundabout to provide 

additional space.

“Design to meet the needs of pedestrians 

and cyclists.”

Pedestrian and bicycle accommodations for 

turbo roundabouts do not differ from modern 

multilane roundabouts.

“Provide appropriate sight distance and 

visibility for driver recognition of the 

intersection and conflicting users.”

Signage is placed far enough in advance of 

the roundabout so road users are aware of the 

approaching intersection and the need to 

select their lane before entering the 

roundabout. The roundabout directional arrow 

sign on the central island increases driver 

recognition of the roundabout.
FHWA | Turbo Roundabouts 23

Section 8: Costs

As of this writing, no turbo roundabouts have been constructed in the United States, meaning 

there is no local data related to turbo roundabout costs. However, turbo roundabouts are similar 

to multilane roundabouts, and are therefore expected to have similar types and magnitudes of 

costs. Turbo roundabouts may vary slightly from multilane roundabouts in required right-of-way. 

A radial entry with no flare and smaller entrance radii requires a larger swept path for large

vehicles. The circular roadway may therefore be wider in some cases than for a comparable 

multilane roundabout. However, significant changes to the alignment of the approach roadway 

are generally unlikely given the entry geometry of the turbo roundabout. 

Section 9: Education and Public Involvement

Given the unique geometry and limited knowledge of turbo roundabouts in the United States, 

traditional public outreach methods for roundabouts will need to be modified for educating the 

public about turbo roundabouts. Below is a discussion on various messages and approaches 

that may benefit education efforts, drawing on successful methodologies used in Europe and 

other roundabout strategies implemented in the United States.

9.1 Key Messages

During initial public outreach, agencies may find it helpful to place emphasis on the safety 

benefits of roundabouts in general, along with additional emphasis on the reduction of conflict 

points and the intuitive lane selection and channelization associated with turbo roundabouts. 

Agencies can also emphasize the key differences between multilane roundabouts and turbo 

roundabouts, including the lane divider and the spiral lane markings. 

As the project develops, highlighting previous turbo roundabouts can be important to 

demonstrate the positive impact made on traffic flows and to create a greater sense of familiarity 

with how to navigate the roundabout. These messages can reemphasize the importance of lane 

selection on the approach and the principle of no lane changing in the circulatory roadway. The 

lack of a track record for turbo roundabouts in the United States may present challenges to 

convey these key messages in the short term; international success can be discussed here 

instead. As turbo roundabouts are opened throughout the United States, it is important to 

incorporate feedback from those projects into messaging on future projects. After installation, 

agencies can continue providing information on how specific user types are intended to navigate 

the turbo roundabout. 

9.2 Educational Media 

Real-time video or simulations are appropriate media for educating engineering audiences and 

the public alike at the beginning of a project, as they provide a clear depiction of how users are 

meant to navigate the turbo roundabout. Though video will be difficult to obtain until after early 

United States installations, agencies can take this form of media into consideration as more 

turbo roundabouts are built. Other suggested media include social media, flyers and fact sheets, 

slide decks, and educational guides. 
FHWA | Turbo Roundabouts 24

9.3 Audiences

It is important for agencies to consider all relevant audiences for public involvement and 

education efforts, not just the general driving population. Other important target audiences 

include:

• New/young drivers. 

• Large vehicle/freight drivers. 

• Motorcyclists. 

• Bicyclists and pedestrians. 

• Local and State roadway personnel, including maintenance crews and land use 

planners. 

9.4 Decision Matrix

Transportation agencies need to consider the audience’s key needs and issues, the appropriate 

method for reaching the targeted audience, and the agency’s capabilities and budget to 

implement the selected education/awareness approaches. A decision matrix, similar to that in 

table 2, can be useful for identifying audiences and developing appropriate marketing and 

communication materials.

Table 2. Target audience educational/awareness media. 

Audience Organization 

Informational 

Primer

Real-time 

Video/Simulatio

Signage 

Slide Decks

Social Media

Education 

Guide 

Fact Sheets and 

Flyers 

Webinars 

Local and 

State 

Transportation 

Agencies

Roadway Designers & 

Engineers X X X X X X 

Maintenance Crews X X X 

Land Use Planners X X X X X X 

User Groups

Drivers X X X X X X 

Large Vehicle/Freight Drivers X X X X X X 

Motorcyclists X X X X X X 

Bicyclists & Pedestrians X X X X X X 
FHWA | Turbo Roundabouts 25

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FHWA | Turbo Roundabouts 26

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For More Information:

Visit https://safety.fhwa.dot.gov/intersection/

FHWA, Office of Safety

Jeffrey Shaw

jeffrey.shaw@dot.gov

202-738-7793

FHWA-SA-20-019