Modern Roundabouts: Safety & Mobility Wrapped in a Pretty Package

Jim Marcinkowski, PE ASCE, APWA & SWE Redding, California May 17, 2010

Short History of Driving in Circles
Some Roundabout Basics
Safety
Users (How To)
Pedestrians
Bicycles
Drivers
Mobility (Congestion)
Before/After Comparisons
Aesthetics

No Controls
Take Your Turn
Follow Rules of Road

Right of Way Rule (Yield to Right)
STOP Signs
Police Control
Traffic Signals
Special Turn Lanes & Signals
Vehicle & Pedestrian Detection
Coordinated Signal Systems

Shape Controls
Movement
View of Columbus circle, Circa 1915 (Courtesy: New York Department of Planning)

Circles, Small and Large
Really Big, High Speed Circles Are Called Rotaries

Yield-To-Right
Circulating traffic yield to the entering vehicles.

Meanwhile in England…

Yield-To-Left
Entering vehicles yield to the circulating traffic.

Roads Are The Most Dangerous
Public Facilities
On The Face Of The Earth
In the U.S., about 800 people are killed each week.

TRAFFIC SIGNALS ARE NOT SAFETY DEVICES

32 Vehicle/Vehicle
24 Vehicle/Pedestrian

8 Vehicle / Vehicle
8 Vehicle / Pedestrian

All crashes All injury crashes Serious injury & fatal crashes

Week by Week reporting of accidents (Feb, 2008)
- no accidents reported since 12/12/07 (source: West Bloomfield Township)

NO INJURY ACCIDENTS
Minor Damage
Majority of Citations
Failure to Yield
Unsafe Lane Change

(17+ seconds curb to curb) (<4 seconds each crossing) Typical Intersection = 60’ vs. Roundabout = 13’ x 2

Goals of Roundabouts:
Yield on entry
Slow, consistent speeds entering/ thru/ exiting
Size as small as practical
Path deflection on entry
Limits the number of vehicle to vehicle & vehicle to pedestrian conflicts

Narrated by
Paul W. Smith

= Delay +
Fuel Wasted +
Pollution + …

Added Delay =
Fuel Consumption
Air Pollution
Noise
Excess Widening =
Resources to Pave
Storm Water Run Off

Expression of both:
Capacity
Driver perception
Ranges from ‘A’ (best) to ‘F’ (worst)
Primarily based on measure of average delay

Best demonstrated through a review of a case study:
US-23/THOMPSON ROAD
Tight Diamond Rural interchange
Thompson Road – 2 lanes
2 lane bridge over US-23
US-23 – 4 lanes
On/off ramps – Single lane w/stop control

Rural Interchange w/ Improvements
Widen Thompson Road to five lanes including bridge
Widen ramps for right and left turn lanes
Install traffic signals
Construction cost = $7 m
Need additional land
NOT SELECTED…

Roundabouts at Ramps
Do Not Widen Thompson Rd
Do Not Widen Ramps
Install Single-lane Roundabout at each ramp termini
Construction Cost = $ 2.5 M
No additional land needed

Roundabouts At Ramps
Selected as the recommended alternative
Best LOS
Lowest delay
Lowest cost

Half mile strip commercial area. 20,000 ADT
No access management = numerous turning conflicts
Existing pavement width = 80’ - “sea of pavement”
Two traffic signals w/third needed
New shopping center was being proposed.

Reduce speed through the section
Improve safety
Improve access for businesses and residential neighborhoods
Create pedestrian friendly environment
Improve aesthetics

Reduce speed through the section
Reduction of 85 th percentile speed from 48 mph to 33 mph
Reduction in average delay time, i.e., it takes less time to drive this segment of road even with a 15 mph reduction in speed

Improve safety
Crash rate reduced 88% even with increased traffic flows from new development
Reduced severity of accidents
Reduce # of injuries.
Before rate = 10/year
After rate = 1/5 years

Improve access for businesses and residential neighborhoods
Over first five years after construction - 60% growth in sales tax revenue for this corridor. Nearly flat sales tax revenue growth for the rest of Golden, CO over same time period
Elimination of large traffic queues from parking lots

Create pedestrian friendly environment

Improve aesthetics

Defined roundabout
Use / location
Benefits
Safety
Traffic calming
Capacity
Environmental
Emissions - Stormwater
Aesthetics

Reduces the number & severity of crashes.
Reduces the number of conflict points.
Increases capacity, usually w/ fewer approach lanes.
Minimizes ROW needed.
Can be closely spaced (to structures, traffic signals or other Rdbts).
Adapts to unusual geometry.

May not be suitable for locations:
Imbalanced traffic flows,
Poor stopping sight distance,
Steep grades (>5%) w/ heavy truck traffic,
Close to railroad crossings,
Within system of coordinated traffic signals.

Existing congestion problems
Known safety problems
Balanced entering traffic volumes, or
High percentage turning movements

Roundabouts: An Information Guide, FHWA 2000
NCHRP 3-78 IMPROVING ACCESS TO ROUNDABOUTS AND CHANNELIZED TURN LANES FOR THE VISUALLY IMPAIRED
www.roundaboutsusa.com
www.ksu.edu/roundabouts/
www.alaskaroundabouts.com
www.roundabout.kittelson.com
West Bloomfield Township
RCOC
Dan Hartman – dhartman@ci.golden.co.us
Northeast Roundabouts

A Coupling of Safety and Mobility
QUESTIONS?

Initial Costs To Build
Surface area paved
Staging construction
Traffic signs
Pavement markings
Traffic & pedestrian signals
Street lighting
Annual Costs To Operate
Electrical & Communication
Relamping
Depreciating equipment
Pavement markings
Crash damage to equipment
Emergency service providers responses to crashes

8 Roundabouts in West Bloomfield Township / Farmington Hills – Joint MDOT, RCOC, City & Twp.

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Modern Roundabouts: Safety & Mobility Wrapped in a Pretty Package

by OHM Advancing Communities

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