Use of Modern Roundabouts for Congestion and Safety ...€¦ · Classic Traffic Circle ... UK 55 million - --3,000 6.25 48,000 1.25 times UK US 275 million 5 times UK Vehicle Fatalities

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


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HIGHWAY

W. MAPLE ROAD

14 MILE ROAD

LOS INTERSECTION DELAY

Existing PM LOS<35 SECONDSA - C35 - 55 SECONDSD>55 SECONDSE -


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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


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


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


Signalized AlternativeExplanationMOE


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


Signalized AlternativeExplanationMOE

Signals vs. Roundabouts - Cost, ROW Impacts, & Wetland Impacts


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.


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


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

Use of Modern Roundabouts for Congestion and Safety ...€¦ · Classic Traffic Circle ... UK 55 million - --3,000 6.25 48,000 1.25 times UK US 275 million 5 times UK Vehicle Fatalities

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