Roundabouts: Why is Mn/DOT
Building Them?
Traffic Topics
August 5, 2010
Ken E. Johnson
Mn/DOT Office of Traffic, Safety, and Technology
Member of Mn/DOT Roundabout Steering Committee
ken.johnson@state.mn.us
651-234-7386
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
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
Mn/DOT Strategic Directions
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Safety
Mobility
Innovation
Leadership
Transparency
The appropriate application of Roundabouts is in line with 3
of the Strategic Directions of the Department
Roundabouts are …
• Not social engineering
• Not the Europeanization of America
• Not the solution for world peace
• 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.
Intersection Controls
• Why – because that’s where vehicles cross paths
which leads to delay and the possibility of crashes
• Types
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Yield controlled
TWSC (Two way stop controlled)
AWSC (All way stop controlled)
Signalization
Roundabouts
Continuous flow intersections and other non-typical types
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
Introduction to Roundabouts
The image that many American’s have…
Unfortunate
misconception
Video courtesy of Mark Johnson, MTJ Engineering
Not Traffic Circles, Rotaries, etc.
Modern roundabouts vs. traffic circles:
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•
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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
Roundabout Features
Roundabouts: An Informational Guide – FHWA, Exhibit 1-2
Introduction to Roundabouts
This is an example of a modern roundabout…
Types of Roundabouts and Typical
Volumes
Type
Typical Typical Entering
Diameter 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 ADT
Assuming 4 perpendicular legs with balanced flows
In general, can handle similar volumes as signals that are not failing
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
That much traffic?
Roundabout - 4,000 vph
Avon, CO
Video courtesy of Leif Ourston,
Ourston Roundabout Engineering
Signal – 4,400 vph
Loveland, CO
Video courtesy of Bill Hange, City of
Loveland and Mark T. Johnson, MTJ
Engineering
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
Delay Reduction - Signalized
“Average delay per vehicle at the MUTCD peak hour signal
warrant thresholds” for various volumes
Roundabouts: An Informational Guide – FHWA, Exhibit 3-7
Advantages – Increased Safety
Reduced conflict points (8 vs. 32)
Roundabouts: An Informational Guide – FHWA, Exhibit 2-3
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
Simple decision – Wait for opening
(right turn at residential T-intersection)
Safety – National Studies
Maryland – 8 single-lane rural roundabouts studied
Each replaced stop signs or intersection control
beacons
Mean Total
Mean Injury
Crash Rate
Crash Rate
(Crashes/MVE) (Crashes/MVE)
Before Period
After Conversion
% Reduction
1.53
0.48
68%
0.97
0.11
89%
Source – Maryland State Highway Administration, 2001
Safety – National Studies
Insurance 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
Safety – National Studies
Roundabout
Characteristics
Before Condition
# of Sites
Percent Reduction in
Crashes
Total
PDO
Injury
Single 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%
Source – Insurance Institute for Highway Safety and NYSDOT, 2003
NCHRP Report 572Roundabouts in the US (2007)
-35%
SIMILAR
-76%
-60%
-67%
TOO FEW
SIMILAR
SIMILAR
-72%
-32%
-29%
-87%
-71%
-81%
Safety – Increased Ped Safety
Source – Roundabouts: An Informational Guide (FHWA)
Safety – Increased Ped Safety
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•
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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
Safety –Pedestrian Safety
• Disadvantage is that vehicles aren’t necessarily
stopping
• Reduced speeds help
• Possible issues with visually-impaired pedestrians
Safety – Pedestrians
Chance of Pedestrian Death If Hit By a Motor Vehicle
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
100%
80%
40%
5%
20 mph
30 mph
40 mph
50 mph
Literature Review on Vehicle Travel Speeds and Pedestrian Injuries
– Final Report DOT HS 809 021, October 1999
Safety – Pedestrians
United Kingdom Experience
Type of Roundabout
Mini Roundabouts
Small Roundabouts
Conventional (2 lanes)
Large (2+ lanes)
Traffic Signals
Injury Rate/1 million peds
0.31
0.33
0.45
0.72
0.67
Source – TRRL Lab Report 1120, 1984
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
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
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…
Case Study – TH 13 and Scott County Rd 2
Case Study – TH 13 and Scott County Rd 2
• 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 – Golden, CO
• South Golden Road in Golden, Colorado
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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
Case Study – Golden, CO
South Golden Road before improvements
Case Study – Golden, CO
4 roundabouts constructed in 1998 – 99
South Golden Road Experience
Roundabouts 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
South Golden Road Experience
Roundabouts in Series
Total Accidents
Injuries
Average Daily
Traffic
3 years 7 years
prior
after
(96-98) (00-06)
360
150
Accident
Rate
Reduction
- 85%
31
3
- 96%
11,500
15,500
+ 35%
Accident Statistics Provided by Public Works Director
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.
Where are Roundabouts Appropriate?
• 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
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)
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Deflection
Inscribed circle diameter
Roadway width
Approach grades
Where Roundabouts May “Shine”
• 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)
What About Multi-lane Roundabouts?
• 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 Multilane Roundabouts?
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
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All way stop
Signal
Roundabout
Other
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
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
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
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
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
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
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
Questions??
Near Medford, MN - Photo courtesy of Mn/DOT