This document compares spiral roundabouts and turbo-roundabouts. Spiral roundabouts use spiral lane markings to guide traffic, while turbo-roundabouts use nested spiral lanes to eliminate lane changing in the circulatory area. Turbo-roundabouts have reduced conflict points and allow for higher capacity. The document recommends standardizing terminology, applying the "spiral block" concept to roundabout design, and providing turbo-roundabout guidelines for North American road design standards.

2. A Comparative Study on Turbo-roundabouts and
Spiral Roundabouts
Authors:
David Homola; Steven Chan, P.Eng, MBA

3. Outline
• Purpose of the paper
• Background
• What is a turbo-roundabout?
• What is a spiral roundabout?
• Comparing spiral roundabouts and turbo-roundabouts
• Recommendations
• Further studies

4. Purpose of the Paper
• Primary focus:
– To clarify the differences between spiral roundabouts and turbo-
roundabouts in the geometric design
• Secondary focus:
– To facilitate the enhancement of the North American roundabout
guidelines to provide guidance on turbo-roundabouts
– To regionalize the US turbo-roundabout design practice to suite the local
conditions*
* Study only focused to the geometric design

5. Background
• Roundabouts provide benefits over regular junctions, such as
– Safety,
– Continuous traffic flow,
– Traffic calming effect,
– Vehicle speed reduction
• Multi-lane roundabouts miss some of these, as
– The inner lane is often under-utilized – results lower capacity
– High number of conflict points – accident probability increases
– Wide circulatory area allows vehicles to deflect from the desired curvature –
high speed movements cause more serious accidents
• Solution: the turbo-roundabout (alternative type of multi-lane roundabout)

6. What is a
turbo-roundabout?
1. Nested spiral lane at one or more entries.
2. Yield to no more than two lanes (deviation
might be possible).
3. Smooth spiral markings.
4. Raised and mountable lane dividers.
5. At least one lane offers a choice for direction.
6. At least two exit legs have two exit lanes.
7. Approach legs are aligned perpendicularly to
the circulatory lanes.
8. Roundabout signage cuts off the horizon for
optimal recognition.
9. Aprons in central island and armpit are
established to keep narrow lane width for
passenger vehicles but provide additional
driving surface for HGVs.
10. Circulatory roadway kept narrow and deflected
enough to keep fastest vehicle speeds low.

7. Benefits of turbo-roundabouts
1. Nested spiral lanes, eliminated lane
changing on the circulatory -
Reduced number of conflict points
2. Motorist channeled into narrow
areas - Vehicle speed kept lower
3. Equal lane utilization, protected exit
maneuvers –
Higher capacity volumes
4. Nested spiral lanes and
perpendicular approaches -
Minimized footprint

8. The turbo block concept
• Chan, Livingston – Design Vehicle’s Influence to the geometric design of turbo-
roundabouts, 4th International Roundabout Conference, April, 2014
– A turbo-roundabout has a strong connection to the design vehicle(s) and the
swept path of this/these
– To provide a general guidance on the required lane width, turbo-blocks were
prepared to construct the circulatory carriageway along

9. What is a spiral roundabout?
• Spiral roundabout: unofficial term for roundabouts with spiral markings
– Multi lane roundabouts with spiral markings (DMRB, TD 16/07)
UK terminologies:
• Spiral markings: applied on complex roundabouts to improve lane discipline
– “Concentric spiral” – directs off the outermost circulating lane
– “Spiral” - a series of lane additions and lane drops around the circulatory
carriageway so that drivers enter in the lane appropriate for their desired
exit

10. What is a spiral roundabout?
Examples for multi lane roundabouts with spiral markings in the British design
guideline (DMRB, TD 16/07)
Concentric spiral
markings
(DMRB)
Spiral markings
(DMRB)

11. What is a spiral roundabout?
• Spiral roundabout: multi-lane roundabouts with spiral transitions or spiral
markings (FHWA, NCHRP Report 672)
US examples in the road marking section:
• These refer closely to the use of concentric spiral and spiral markings at
multi-lane roundabouts

12. What is a spiral roundabout?
Multi-Lane Roundabouts with Spiral Transitions from the FHWA
guideline

13. Comparing spiral roundabouts and
turbo-roundabouts
Similarities
• Can have the same lane
configurations
• Both use spiral marking and spiral
transitions
• Both adopt to the traffic
circumstances
• Both operate on a high level of
capacity
1-1 1-1
1-1 1-1
2
2
2
2
2
2

14. Comparing spiral roundabouts and
turbo-roundabouts
Differences
• Constructed along different geometric
elements
• Length, which on the spiral transition
occurs
• Lane separation, lane-discipline
• Better lane utilization and therefore
higher capacity
• Narrower circulatory lanes block vehicles
to deviate from the desired curvature

15. Recommendations
• Standardize the terminology
– Define the following terminologies:
• Turbo-roundabout
– Implement the existing Dutch criteria set for turbo-roundabouts
into the North American design publications
• Turbo-like roundabout
– Roundabouts, which follows the turbo-roundabout concept, but not
meeting all the desired criteria
• Spiral roundabout
– Roundabouts constructed along concentric circles and uses spiral
lane markings to guide traffic from inside to outside

16. Recommendations
• Apply the spiral block concept
– Similarly to the turbo-block concept, provide the industry with “spiral roundabout
blocks”, to achieve the desired lane configurations easier and ensure design vehicle
swept path needs are met

17. Recommended turbo-roundabout
layouts for FHWA spiral roundabouts
Knee type turbo-roundabout 1. Entries from major traffic
flow yield to one lane only
2. Vehicles are channeled
into a narrower area
3. Perpendicular entry angles
4. Less conflict points and
better driving experience
5. Smaller land-requirement

18. Egg type turbo-roundabout
1. Vehicles are channeled
into a narrower area
2. Perpendicular entry angles
3. Less conflict points and
better driving experience
4. Smaller land-requirement
Recommended turbo-roundabout
layouts for FHWA spiral roundabouts

19. Rotor type turbo-roundabout
1. Yield only to two lanes the
maximum
2. Vehicles are channeled
into a narrower area
3. Less conflict points and
better driving experience
4. More effective land use
with a smaller footprint
Recommended turbo-roundabout
layouts for FHWA spiral roundabouts

20. Further studies
• Regionalized design
– To provide coherent and comprehensive overview for the North American
roundabout practice
• Dimensional guidance for the FHWA turbo-roundabouts
– To adopt the turbo-roundabout concept, it is vital to adjust the Dutch turbo-
roundabout types so those are able to accommodate North American AASHTO
vehicles
• Elaborate whether to apply or not raised lane separators
• Research and design turbo-roundabout specific signage

21. Closing remarks
This study:
• Reveals the main characteristics, benefits and drawback of turbo- and spiral
roundabouts
• Lays down the necessary background information
• Proposes recommendations for:
– A standardized and distinct terminology
– The adaptation of the spiral-block concept
– The implementation for turbo-roundabouts in the FHWA guidelines
• The paper provides equivalent turbo-roundabout layouts following the
examples in the current guide

22. Thank you for your attention!