roundabouts: why is mn/dot building them? traffic topics august 5, 2010 ken e. johnson mn/dot office...
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Roundabouts: Why is Mn/DOT Building Them?
Traffic TopicsAugust 5, 2010
Ken E. JohnsonMn/DOT Office of Traffic, Safety, and Technology
Member of Mn/DOT Roundabout Steering [email protected]
651-234-7386
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Ken’s RA Biography
• Experienced RAs in Ireland in 1999 & 2001– The only intersection I didn’t need to think about
• Roundabouts: It’s Time for Minnesota– Capstone project for Masters degree– Coauthored with John Gorder and Brian Malm
• Member of Mn/DOT’s Roundabout Steering Committee and Roundabout Review Team
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Introduction to Roundabouts• Why consider roundabouts?• What is a roundabout – what is a traffic circle?• What’s so great about them anyway?
– Advantages and Goals• Location considerations• What’s happening at Mn/DOT?
– How do we decide?– Changes to traffic control devices
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Mn/DOT Strategic Directions• Safety• Mobility• Innovation• Leadership• Transparency
The appropriate application of Roundabouts is in line with 3 of the Strategic Directions of the Department
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Roundabouts are …• Not social engineering
• Merely one type of intersection control made of concrete, asphalt, gravel, dirt, etc.
• One of the tools that traffic engineers use to control crossing traffic.
• Not the Europeanization of America• Not the solution for world peace
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Intersection Controls• Why – because that’s where vehicles cross paths
which leads to delay and the possibility of crashes• Types
– Yield controlled– TWSC (Two way stop controlled)– AWSC (All way stop controlled)– Signalization– Roundabouts– Continuous flow intersections and other non-typical types
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TWSC Intersections• Mn/DOT’s good old default
– Mobility is our major goal
• But what if there are problems…– Crashes– Side street traffic gets heavy enough that excessive delay occurs to the
minor road traffic
• AWSC – usually very safe, but bad for delay• Signals – can help with delay, but safety can be issue• Interchanges - $$$$$
• Roundabout – in appropriate application, very safe and has less delay than other intersection types
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Introduction to Roundabouts
The image that many American’s have…
Unfortunate misconception
Video courtesy of Mark Johnson, MTJ Engineering
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Not Traffic Circles, Rotaries, etc.Modern roundabouts vs. traffic circles:• Yield on entry – first rule (UK 1966)• Counter-clockwise circulation• Lower speeds necessary - deflection
– Splitter islands– Central island
• Pedestrian movements restricted to crossing legs• No parking
First RB in USA – Summerlin, NV early ‘90s
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Roundabout Features
Roundabouts: An Informational Guide – FHWA, Exhibit 1-2
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Introduction to Roundabouts
This is an example of a modern roundabout…
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Types of Roundabouts and Typical Volumes
Type Typical Diameter
Typical Entering Volumes
Mini-roundabout
45-90’ 15,000 ADT
Single lane 90-180’ 25,000 ADT (34,000 in FL)
Multi-lane 150-300’
45,000 – 70,000 ADTAssuming 4 perpendicular legs with balanced flowsIn general, can handle similar volumes as signals that are not failing
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Rough Entering Volumes
Intersection 2005 Entering Volumes
Co Rd B2 and Dale 12,000 ADT
Larpenteur and Dale 27,150 ADT
Co Rd B and Lexington
24,000 ADT
TH 51 and Larpenteur Ave
53,000 ADT
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That much traffic?Signal – 4,400 vph
Loveland, CORoundabout - 4,000 vph
Avon, CO
Video courtesy of Bill Hange, City of Loveland and Mark T. Johnson, MTJ Engineering
Video courtesy of Leif Ourston,
Ourston Roundabout Engineering
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Advantages – Reduced Delay• Delay is the difference between
– Travel time with no intersection control– Travel time with intersection control
• Includes slow down, stop, and speed up time
• Always better than AWSC• Usually better than TWSC if minor road
volumes are close to major road volumes• 50/50, 60/40, 70/30
• Usually better than signal
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Delay Reduction - Signalized
“Average delay per vehicle at the MUTCD peak hour signal warrant thresholds” for various
volumesRoundabouts: An Informational Guide – FHWA,
Exhibit 3-7
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Advantages – Increased Safety
Reduced conflict points (8 vs. 32)Roundabouts: An Informational Guide – FHWA,
Exhibit 2-3
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Advantages
• Increased safety (continued)– Shallow angle of conflict– Slower speeds
• Better reaction time• Reduced severity of crashes
– Energy = ½ x mass x speed x speed
– Simplified decision making• Right turn at residential T-intersection
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Simple decision – Wait for opening(right turn at residential T-intersection)
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Safety – National Studies
Mean Total Crash Rate
(Crashes/ MVE)
Mean Injury Crash Rate
(Crashes/ MVE)
Before Period 1.53 0.97After Conversion 0.48 0.11% Reduction 68% 89%
Source – Maryland State Highway Administration, 2001
Maryland – 8 single-lane rural roundabouts studied
Each replaced stop signs or intersection control beacons
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Safety – National StudiesInsurance Institute for Highway Safety - 2000
• Crash Reductions Following Installation of Roundabouts in the United States
• 24 intersections in 8 states converted to roundabouts
• 39% Reduction in All Crashes• 76% Reduction in Injury Crashes
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Safety – National StudiesRoundabout Characteristics Before Condition # of Sites
Total PDO InjurySingle Lane,Urban Stop Controlled 12 69% 67% 80%Single Lane,Rural Stop Controlled 9 65% 63% 68%Multi Lane,Urban Stop Controlled 7 8% 0% 73%Urban Signalized 5 37% 31% 75%All Sites 33 47% 41% 72%
Percent Reduction in Crashes
Source – Insurance Institute for Highway Safety and NYSDOT, 2003
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NCHRP Report 572-Roundabouts in the US (2007)
-76%-35%
-87%-72%-71%-32%-81%-29%
-67% TOO FE W
S IMIL A R S IMIL A R
-60%SIMILAR
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Safety – Increased Ped Safety
Source – Roundabouts: An Informational Guide (FHWA)
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Safety – Increased Ped Safety• Shorter crossing distance• Pedestrian only looks one direction• Drivers at signals watch the light• Drivers at typical intersections tend to look left and
turn right• Pedestrian crossing separated from intersection –
allows driver to pay more attention to pedestrian
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Safety –Pedestrian Safety• Disadvantage is that vehicles aren’t necessarily
stopping• Reduced speeds help
• Possible issues with visually-impaired pedestrians
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Safety – Pedestrians
20 mph 30 mph 40 mph 50 mph0%
10%20%30%40%50%60%70%80%90%
100%
5%
40%
80%
100%
Chance of Pedestrian Death If Hit By a Motor Vehicle
Literature Review on Vehicle Travel Speeds and Pedestrian Injuries – Final Report DOT HS 809 021, October 1999
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Safety – Pedestrians
Type of Roundabout Injury Rate/ 1 million pedsMini Roundabouts 0.31Small Roundabouts 0.33Conventional (2 lanes) 0.45Large (2+ lanes) 0.72Traffic Signals 0.67
Source – TRRL Lab Report 1120, 1984
United Kingdom Experience
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Safety – Pedestrians• Pedestrian Study in Melbourne,
Victoria (Australia)– Source: Andrew O’Brien & Associates Pty
Limited– Roughly the same size/population as 7
County Twin Cities Metro area– ~4000 roundabouts– 1996-2000
– 57 total pedestrian crashes from 1371 overall crashes
– 0 fatalities– 32% required hospitalization
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Safety – Pedestrians
Victoria, Australia anecdotal experience• Source: Andrew O’Brien & Associates Pty
Limited• Black Spot Program – fix intersections with
serious crash rates – to be eligible, there must be 3 injury/fatal crashes in a 3 year timeframe
• Over 5000 roundabouts in Victoria• Roughly the same size/population as
MN• All pedestrian black spot intersections in
Victoria are signals
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Advantages
• Reduced environmental impacts– Less fuel consumption– Vehicle emissions reduced
• 33% less hydrocarbons• 36% less CO• 21% less nitric oxides
– More pleasant to live next to…
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Case Study – TH 13 and Scott County Rd 2
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• Rural TWSC Intersection of High-Speed Roads• 2000-2004 (5 year period)• 26 injury, 9 property damage, 2 fatal
crashes• 50 injuries and 2 fatalities
• Roundabout opened to traffic in Sept 2005• Sept 2005 - August 2010• 5 reported crashes• 3 injuries and no fatalities
Case Study – TH 13 and Scott County Rd 2
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Case Study – Golden, CO• South Golden Road in Golden, Colorado
– Before condition– ¾ mile segment of 4-lane expressway with TWLTL– Almost indiscriminate private business driveways– Two signals – Ulysses and Johnson– Proposed additional signal in corridor– Corridor serve several residential neighborhoods, many
businesses including fast food restaurants and gas stations, and other retail outlets
– High and middle schools located ¼ mile from corridor
Source: Alex Ariniello, LSC Transportation Consultants
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South Golden Road before improvements
Case Study – Golden, CO
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4 roundabouts constructed in 1998 – 99
Case Study – Golden, CO
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South Golden Road ExperienceRoundabouts in Series (2004
information)• Travel Time through Corridor Reduced
• 78 sec (expected to go to 103 sec) to 68 sec
• Intersection Delay Reduced• Access to Businesses Delay Reduced
• Before – average 28 seconds, maximum 118 seconds
• After – average 13 seconds, maximum 40 seconds
• Right turn/U-turn Safer and Quicker than Left Turn In
• 85th percentile Speed Reduced from 47 mph to 33 mph
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3 years prior
(96-98)
7 years after
(00-06)
Accident Rate
Reduction
Total Accidents 360 150 - 85%
Injuries 31 3 - 96%
Average Daily Traffic
11,500 15,500 + 35%
Accident Statistics Provided by Public Works Director
South Golden Road ExperienceRoundabouts in Series
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Good for Businesses
In 2004, sales tax revenues had increased 60% since roundabouts constructed – only portion of city that had seen increase each year, even during an economic slowdown. Plus, over 75,000 sq ft additional retail/office space added.
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• Entering volumes less than 8,000 vph• Roughly same functional classification
• Arterial/arterial, collector/arterial, collector/collector
• Entry volumes balanced = higher capacity• The upper capacity limits of a roundabout
will be higher with balanced flows.• However, this guideline depends on overall
volume.• Heavy mainline travel (ie 90/10) – signal
may outperform a roundabout, but needs to be analyzed on a case by case basis.
• All levels of pedestrian volumes
Where are Roundabouts Appropriate?
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Locations Not Recommended for a
Roundabout • Corridors with a well operating coordinated
signal system• Where Satisfactory geometric designs can’t
be provided (doesn’t have to be ideal)– Deflection– Inscribed circle diameter– Roadway width– Approach grades
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• Locations with unusual geometry or number of legs
• Rural intersections with high crashes or high delay
• Intersections with high amounts of left turns on one or more legs
• Interchange locations with right of way, bridge width, or bridge crest restrictions
• Near schools or playgrounds• Through small towns or commercial
corridors (combined with access management)
Where Roundabouts May “Shine”
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• Concern that MN drivers will find them confusing
• Nationwide experience• Similar amount of crashes as signals• Still ~75% reduction in injury crashes• Most typical crashes are caused by not
following the signs• Failure to yield to both lanes at entry• Exit conflict
• Some education efforts underway• Best advice – follow the signs and pavement
markings
What About Multi-lane Roundabouts?
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What About Multilane Roundabouts?
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What’s Mn/DOT Doing?• Developed Roundabout Chapter in Road Design
Manual• Intersection Control Evaluation
– Any intersection that requires more than a TWSC will require ICE to choose correct intersection application• All way stop• Signal• Roundabout• Other
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What’s Mn/DOT Doing?• Roundabout projects will have extra reviews
– Same level of reviews as freeways– Roundabout Review Team
• Updated State Aid Roundabout Design Checklist– Guidance to cities and counties
• Updating Minnesota Manual of Traffic Control Devices– Based on Federal MUTCD (2009)– Many changes for roundabouts
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What’s Mn/DOT Doing?• Education efforts
– Roundabout webpage – google mndot roundabouts– “Roundabout on a stick” – handed out last year at State
Fair– Minnesota State Map– Per project– Washington County Roundabout “U”
- google roundabout u
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Roundabouts in MN• State Highway System (existing)
– I-35 and Steele County Road 12 near Medford Outlet Mall (2 at ramp intersections)
– TH 13 and Scott County Road 2 near New Prague– Elm Creek Roundabout in Maple Grove (connected to
future TH 610)– TH 7 with TH 25 and Carver County Road 10– TH 61 and Jamaica Avenue in Cottage Grove (2 at ramp
intersections)– TH 3 and 190th Street in Farmington/Empire Township
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Roundabouts in MN• State Highway System (existing)
– TH 3 and 80th Street East in Inver Grove Heights– TH 52 and Wentworth Avenue in West St. Paul and
South St. Paul (2 at ramp intersections)– TH 284 and 15th Street in Waconia– TH 19 and Chalupsky Ave in New Prague– TH 19 and Alton Ave in New Prague
• All roads – State Aid Office List (self-reporting)– 80 constructed and in operation as of Feb 2010– http://www.dot.state.mn.us/stateaid/ - Roundabout List
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Roundabouts in MN• State Highway System (under construction)
– TH 5 and Jamaca Ave/Stillwater Blvd in Lake Elmo– TH 61 and Washington County Road 2 in Forest Lake
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Is this the future?
• Victoria, Australia– Pop ~5 million– Area ~238,000 km2
– Density ~22 / km2
• Melbourne, Victoria– Pop ~3.7 million– Area ~7700 km2
– Density ~480 /km2
• Minnesota, US– Pop ~5 million– Area ~225,000 km2
– Density ~24 / km2
• 7 Co Metro Area, MN– Pop ~3.1 million– Area ~ 7700 km2
– Density ~400 /km2
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Is this the future?• Victoria, Australia
– 1973 – 3 roundabouts in Victoria– 2006 ~5000 roundabouts in Victoria– In this timeframe, removed 3
• Melbourne, Victoria– 2006 ~4000 roundabouts in Melbourne– Largest RB in Melbourne
• Andrew O’Brien, Intersection Designer in Victoria• ~50,000 ADT on one road• ~20,000 ADT on crossing road
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Questions??
Near Medford, MN - Photo courtesy of Mn/DOT