Use of Modern Roundabouts for Congestion and Safety Improvements
by Stephen G. Metzer, AICPDLZ, Inc.Columbus, OH
What Is A Modern Roundabout?A simple question - but no simple answerRoundabouts are not homogenous
there is great variety (from small mini’s to interchanges)All TRUE roundabouts include:
Yield at entryEntry flare and deflectionLow speedsPrecisely designed diameter and laneage based on volumes
Are not the same as a traffic circle. Most U.S. “round” intersections are neither – they are hybrids!!!
Is It Really A Modern Roundabout?Although Traffic Circles are round, Traffic Circles are NOT roundabouts
They lack the fundamental roundabout characteristicsTraffic circles design and operational characteristics are the opposite of roundaboutsSadly roundabouts are damned by associationWHAT ARE THE DIFFERENCES
In performance?In design?
Traffic Circles and Modern Roundabouts
Medium to HighLow to HighLowCRASH SEVERITY
HighVaries (size dependent)LowSPEED
Low to High (volume dependent)
Low to High (volume dependent)LowVEHICLE DELAY
HighLow to HighLow# CRASHES*
High (up to 8000 vph)
Yes
Determined by traffic volumes
No
Yield
MODERN ROUNDABOUTS
VariesVariesCAPACITY
NoRarelyENTRY DEFLECTION
Determined by amount of space
Determined by amount of spaceDIAMETER
YesOftenWEAVING?
High Speed MergeVaries (Yield, Stop, Merge)ENTRY
TRAFFIC CIRCLESHYBRID/ COMBINATION
MAIN GEOMETRIC DIFFERENCESClassic Traffic Circle
Large Diameter - high circulating speeds Large entry radii - high entry speedsShallow entry angle - high entry speedsENTRY MERGING - HIGH SPEED WEAVING
Modern Roundabouts Small diameter - low entry speedsSmall entry radii - low entry speedsLarger entry angle - lower entry speedsYIELD AT ENTRY - low speed - NO WEAVING
MODERN ROUNDABOUTS Yield At EntrySlow Speed Entry GeometrySmaller Diameter
SLOW SPEEDS (< 25 mph)No weaving High capacity/low delaysLess crashes - lower severityClearly defined priority Simple to useComfortable to drive
Roundabouts are very different from Traffic Circles
WHY USE MODERN ROUNDABOUTS?
Safest type of at-grade intersectionVery high capacity … up to 8000 VPHSuitable for low & high flow intersections Easy to modify if circumstances changeGreat geometric flexibility - ROWBetter turning radii for trucksGood pedestrian safety
WHY USE MODERN ROUNDABOUTS?
Slow down all traffic… Calming effect.Simple for traffic to use. Yield to left.Very simple for pedestrians to use. Low maintenance costs. Aesthetic .. can look superb .. Civic FeatureFocal point or land mark… named by locals
WHY NOT USE ROUNDABOUTS ?Flexibility provides more ways to get it wrongVery difficult to design for high flowsVery difficult with ROW constraintsAt higher flows
‘the devil is in the detail’design aims seriously conflict
Not as safe for bicycles and motorcyclesFew U.S. engineers have necessary experience
CAPACITY COMPARED TO TRAFFIC SIGNALS
Much higher, especially with large left turn flowsGeometry more flexible to fit ROWMuch higher capacity at most intersectionsHigher capacity when more than 4 legs at an intersection
CRASHES COMPARED TO SIGNALS
-
16.0
Fatalities Factor
3,000---55 millionUK
48,0006.251.25 times UK5 times UK275 millionUS
Vehicle Fatalities per
Year
Population x Mileage
Factor
Annual Mileage
Per Driver
Population FactorPopulation
UK has over 11,000 roundabouts Roundabouts have 30-80% less injury accidentsLower accident severityRoundabouts = Signals at very high flows BUT severity much less with Roundabouts
SAMPLECONVENTIONAL MODERNROUNDABOUTS
Large Solid Central Island
MILEHOUSE ROUNDABOUT, UK
Thriving Gas Station
NEW YORK STATE ROUTE 28
New Modern Roundabout
Old Traffic Circle
Old high speed merge lane
AVON, COLORADOWidening of bridge not needed with roundabout – signal required added lanes and bridge widening (big $$$)
AVON, COLORADO
Aesthetics of roundabouts can include landscaping and sculptures to reflect local culture
OKEMOS, MICHIGAN
CASE STUDY – NORTHWESTERN CONNECTOR PROJECT
• NEPA compliance study in Farmington Hills & West Bloomfield Township, Oakland County, MI
• Compared performance of traffic signals and modern roundabouts and the safety benefits of medians
• Results compiled in an Environmental Assessment and Engineering Report
NORTHWESTERN CONNECTOR PROJECT
NORTHWESTERN CONNECTOR PROJECT
HA
GG
ERTY
RO
AD
HA
LSTE
D R
OA
D
DR
AK
E R
OA
D
FAR
MIN
GTO
N R
OA
D
OR
CH
AR
D L
AK
ER
OA
D
NORTHWESTERN
HIGHWAY
W. MAPLE ROAD
14 MILE ROAD
LOS INTERSECTION DELAY
Existing PM LOS<35 SECONDSA - C35 - 55 SECONDSD>55 SECONDSE -
NORTHWESTERN CONNECTOR PROJECT
HA
GG
ERTY
RO
AD
HA
LSTE
D R
OA
D
DR
AK
E R
OA
D
FAR
MIN
GTO
N R
OA
D
OR
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AR
D L
AK
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OA
D
NORTHWESTERN
HIGHWAY
A - CD
<35 SECONDS35 - 55 SECONDS>55 SECONDS
W. MAPLE ROAD
14 MILE ROAD
E -
2020 No-Build PM LOS
LOS INTERSECTION DELAY
NORTHWESTERN CONNECTOR PROJECT
4,9103,3702,7653,5805,3003,2052,8553,355
PM Peak
FFFFFFFF
LOS
2020 No-Build
129181194268140140172137
Delay
66,34045,53037,36048,37071,60043,30038,57545,330ADT
1999 PM Peak/ADT
2020 Signal
Alternative
8A23C14 Mile at Orchard Lk.
4A24C14 Mile at Farmington
3A23C14 Mile at Drake
4A27C14 Mile at Haggerty
8A23CMaple at Orchard Lk.
5A26CMaple at Farmington
5A23CMaple at Drake
14B28CMaple at Haggerty
DelayLOSDelayLOSIntersection
2020 Roundabout Alternative
LOS AND AVERAGE VEHICLE SECONDS OF DELAY OF ALTERNATIVES
NOTE: Both 2020 Build Alternatives included boulevards on West Maple and Orchard Lake Road
NORTHWESTERN CONNECTOR PROJECT
5.9 seconds14.2 secondsEstimated average delay per vehicle per intersection for the year 2020 based on annual delay totals.
Average Delay per Vehicle
408,000 hours
(250,000,000 vehicles)
946,000 hours
(240,586,000 vehicles)
Estimated total annual hours of vehicle delay at project area intersections for the year 2020. An estimate of total annual number of vehicles entering all intersections is provided in parentheses for each alternative.
Total Annual Intersection Delay
12 of 15(3 signals)4 of 14
Number of intersections projected to operate at LOS B or better during pm peak traffic hour for the year 2020.
Intersection LOS
Roundabout Alternative
Signalized Alternative
ExplanationMOE
NORTHWESTERN CONNECTOR PROJECTSignals vs. Roundabouts - Safety Benefits
$8.0 Million$18.1 Million
Estimated annual cost of all crashes projected for year 2020 (in year 2000 dollars). Costs include wage and productivity losses, medical expenses, administrative expenses, motor vehicle damages, employer expenses, and reduced quality of life (based on surveys).
Annual Crash Cost
4.4 crashes/million vehicle miles
5.9 crashes/million vehicle miles
Estimated crash rate in project area for the year 2020.Crash rate
141 crashes262 crashesEstimated number of injury crashes for the year 2020.
Annual injury crashes
Roundabout Alternative
Signalized AlternativeExplanationMOE
NORTHWESTERN CONNECTOR PROJECT
2.9 acres2.7 acresAcres of wetlands impacted by fillingWetland Impacts
5 relocations5 relocationsNumber of residences and businesses requiring relocation.Relocations
28.5 acres29.5 acresAcres of ROW acquisition required.ROW acquisition
$90.3 Million$92.7 MillionTotal estimated cost includes cost of design, ROW acquisition, utility relocation, mitigation, and construction (in year 2000 dollars).
Total Cost
Roundabout Alternative
Signalized AlternativeExplanationMOE
Signals vs. Roundabouts - Cost, ROW Impacts, & Wetland Impacts
NORTHWESTERN CONNECTOR PROJECT
West Maple/Haggerty Road Intersection
Traffic Volumes
1999 Peak – 3,355
2020 Peak – 5,150
Existing and Future Problems Solved by this RoundaboutExisting intersection geometry contributes to crashes.
Reduces projected 2020 peak hour delays by 80-90% compared to no-build.Functions at LOS A during peak hours using 2020 traffic projections.
NORTHWESTERN CONNECTOR PROJECT
West Maple/Orchard Lake Road IntersectionExisting and Future Problems Solved by this Roundabout
Existing signal configuration and congestion contribute to crashes.Reduces projected 2020 peak hour delays by over 90% compared to no-build.
Functions at LOS A during peak hours using 2020 traffic projections.
Traffic Volumes
1999 Peak – 5,300
2020 Peak – 7,150
NORTHWESTERN CONNECTOR PROJECT
Orchard Lake/14 Mile Road/Northwestern Highway Intersection
Traffic Volumes
1999 Peak – 4,910
2020 Peak – 5,075
Existing and Future Problems Solved by this RoundaboutExisting intersection geometry contributes to crashes and congestion.
Reduces projected 2020 peak hour delays by 30-90% relative to no-build.Expected to reduce crashes and delays significantly.
Functions at LOS C or better during peak hours using 2020 traffic projections.
NORTHWESTERN CONNECTOR PROJECTConclusions for the Northwestern Connector Project
Roundabouts will significantly reduce delays during peak hours, when compared to upgraded signals.Roundabouts and divided roadways on W. Maple and O. Lake will substantially reduce the number of injury crashes.Negative impacts of the roundabout alternative are similar to upgraded signals.Cost of the roundabout alternative is similar to the cost for the signalized alternative.Many members of the public are skeptical. Education and information are key.
For more project information, visit the Project Web Site at www.dlzcorp.com/seg
NEED TO PROCEED CAUTIOUSLY WHEN PLANNING PROJECTS!!!!
U.S. new to modern roundaboutsRoundabout design can be very complex Many agencies in the U.S. are understandably cautiousEstimated 50,000 U.S. Roundabouts in next 15 years ?
Cost = $15 - 20 BillionCost of getting it wrong is very high
A few bad designs will do more harm than the good done by many good designs
Use best methods and get help from experiencedexperts (more than just being a P.E.)
validation / good track record / tested by time
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