national single lane roundabouts r geometric design in ... · 33 components (geometric elements)...
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Single Lane RoundaboutsGeometric Design in Context
-Urban versus Rural
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ICD
GEOMETRIC PARAMETERSAffecting Capacity
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Effective Geometry
V = Approach Road half width
E = Entry Width
L’ = Effective Flare Length
D = Inscribed Circle Diameter
R = Entry Radius
Phi = Entry Angle
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Empirical EvidenceN
ational Roundabout C
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Empirical EvidenceN
ational Roundabout C
onference 2005 DR
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Empirical Evidence
Entry angle (phi)Gap models do not include phi, blind to effect
If designs uses large phi• Like R, the large loss in capacity not
predicted
• Large phi increase crashes into central island
• Uncomfortable for drivers, additional capacity reduction
Phi best between 200 – 350 on MLRs
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Entry Angle & Entry Radius
Tangent approaches:• Small entry angle
• Large entry radius
• Not much deflection
This can result in:– High capacity– Poor observance of yield
and potential for high speeds and entry-circulating crashes
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oundabout Conference 2005 D
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Entry Angle & Entry Radius
Perpendicular approaches:• Large entry angle
• Small entry radius
• Lots of deflection
Combined Net Effect:– Low capacity– Abrupt braking at entries
and potential for rear-end crashes (especially in high-speed locations)
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oundabout Conference 2005 D
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Geometric Parameters Affecting Safety
Geometric Parameters in the Predictive Relationship:
•Entry Path Curvature(Ce)•Entry Width (E)•Approach lane(s) width (v)•Angle between arms (phi)•Inscribed Circle (IC)•Diameter/Central Island Diameter (CI)
•(19 Others less significant e.g. sight distance to the left
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oundabout Conference 2005 D
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Safety Explicit in Design:
UK Graph of crashes versus EPC for Entry Deflection
EPC is a surrogate for entry angle and other speed related parameters
Optimum100’< EPC < 300’
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Vehicle Entry Path
• Determines the design speed of a roundabout• Fastest path allowed based on geometry is
drawn• Fastest path possible for a single vehicle
Absence of other traffic,
Ignore all lane markings
Traverse thru entry, around central island, out the exit
Fastest path is the thru movementCheck Right turns for skewed intersections
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oundabout Conference 2005 D
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Fastest Path Through a Single Lane Rdbt
5 feet5 ft off curbs
Curb Splitter IslandsPage 134
FHWA
5 ft off curbs
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Application of EPC requires assessment of traffic flows…
LARGEFLOW
SMALL CIRCULATING
FLOWS
Optimum entry safety depends on this combination
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A1bMatching the design to the context
Outside Truck Aprons?
•Same intersection•Different design•Very different operations
•Focus was peds./bikes •Required outside truck apron
Virtually no effect of tight radii
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Controlling entry speed•R1 should be used to control speed - Not entry angle
•PHI amongst other geometrics is simply a means to an end NOT the end itself – an outcome not a criteria.
•With a small PHI for improved capacity you can use other geometrics to compensate and get a small R1 so that speed is controlled.
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oundabout Conference 2005 D
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FTN
ational Roundabout C
onference 2005 DR
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Entry Deflection Urban Case
40 km/h
20 km/h
Simple to draw – poor composition = POOR performance
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oundabout Conference 2005 D
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Entry Deflection with Roundabout
30 km/h
35 km/h
Complex to draw but expect optimal safety and efficiency
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oundabout Conference 2005 D
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DESIGN EXAMPLEOriginal – Re-design
More entry curvature
Flatter exit
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Rural Context Design Elements
•Provide a minimum SSD to the entry.
•Align approach roadways
•Set vertical profiles to make the central island conspicuous.
•Splitter islands should extend to initiate deceleration.
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Rural Context Design Elements
•Use landscaping on extended splitter islands and roadside to create a tunnel effect.
•Provide illumination in transition to the roundabout.
•Use signs and marking effectively to advise of the appropriate speed and path for drivers.
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Central Island Delineation
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Australian Researched Method of Achieving Speed Reduction National R
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Speed Differential Alternative – FHWA Guide
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Speed Differential Alternatives for Rural DesignLong medians N
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• Safety:
– Transition between high-speed rural and low-speed urban environments
50mph
30mph
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Proof: Predicted Vs. Measured Speeds - Data CollectionN
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Speed StudiesN
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Additional Treatment of High Speed Entry
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Original SLR N
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Empirically Based Re-DesignN
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Entry Path Curvature
Entry Curvature =Slow entry(R1 & R2)
Entry curvature = tangential entry
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Components (geometric elements) vs. Composition (functionality)
•The design isn’t functional unless it passes the test of the driver interface
•It’s not enough to have knowledge of the components
•Composition based on principles is what determines the functionality
•If you only focus on the components the final assembly may be totally overlooked
•Adhering to the manual using data, figures and tables does not guarantee a sound design.
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