A Comparative Study on Turbo-roundabouts and Spiral Roundabouts

Authors:David Homola; Steven Chan, P.Eng, MBA

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

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

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)

What is aturbo-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.

Benefits of turbo-roundabouts1. 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

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

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

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)

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

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

Comparing spiral roundabouts andturbo-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

22

22

22

Comparing spiral roundabouts andturbo-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

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

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

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

Egg type turbo-roundabout1. 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

Rotor type turbo-roundabout1. 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

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

Closing remarksThis 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

Thank you for your attention!