2019 diverging diamond design considerations · diverging diamond design considerations mark...
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Diverging Diamond Design Considerations
Mark Doctor, FHWA Resource Center
2019
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FHWADDI
INFORMATIONAL GUIDE
safety.fhwa.dot.gov/intersection/alter_design/pdf/fhwasa14067_ddi_infoguide.pdf
KEY REFERENCES
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Utah DOT DDI Guidelines
Available at:www.udot.utah.gov/main/uconowner.gf?n=14769524027177477
KEY REFERENCES
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https://library.modot.mo.gov/rdt/reports/unnumbrd/or10021rpt.pdf
MISSOURIDOT DDI LESSONS LEARNED
KEY REFERENCES
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The traffic operations at a DDI will greatly influence the appropriate geometric design choices.
Geometric Design
Signal Design Traffic Analysis
Although this is not unique to DDIs, it is of critical importance that the geometric design choices be integrated into considerations of how the signals will be operated.
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Design Development Process
Operational Analysis Geometric Design
Establish Design Hour Volumes (or better a range of DHVs)
Sketch an initial lane configuration
Does the lane configuration satisfy the desired
project operational goals for the DHVs? Revise lane configuration No
Yes Sketch initial alignment for cross
road including crossover
intersection angle, ramp terminal
curvature, and ped/bike facilities
Identify available ROW and constructability
constraints including utility conflicts
Develop initial signal timing
Assess corridor operations if DDI is
close to adjacent intersections
Iterative design revisions may be needed
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Basic DDI Signal Phasing
Option 1 – Alternating Progression on Crossroad
Option 2 – Progression for Exit Ramp Left-Turns
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Key Operational Considerations for Design
• Queue Storage Between Crossovers
• Queue Spillback from Adjacent Intersections
• Lane Utilization / Shared Thru & Turn Lanes
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Queue Storage Between Crossovers
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Queue Spillback
Queue spillback into DDI from downstream adjacent signal
Queue spillback may occur into the DDI and block the departure zone if the downstream signalized intersection cannot handle the increased traffic throughput from the more efficient upstream DDI
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Strategies to mitigate queue spillback risk:
• Remove adjacent signal (grade separate or make right-in/right-out only)
• Move the intersection farther away
• Improve the adjacent intersection (add storage lanes and/or reduce number of signal phases)
Mitigation strategies
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The left turn (into a hospital) was modified to take a right, followed by another immediate right turn that loops under the cross road
Grade Separate
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Grade Separate
I-35 at 95th StLenexa, KS
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Relocate the intersection farther away. This treatment was used at Dorsett Road in Maryland Heights, MO.
Relocate Intersection Farther Away
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Relocate Intersection Farther AwayDorsett Road in Maryland Heights, MO.
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Relocate Intersection Farther AwayI-70 at Woods Chapel Road in Blue Springs, MO.
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I-85 at Poplar Tent Road – Concord, NC (north of Charlotte)
Corridor of “2-Phase” Signals (Signalized RCUTs)
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Lane Utilization
3 Thru Lanes w/ shared Left
2 Thru Lanes w/ Exclusive Left
Developing lanes before the
crossover allows for better
signing and higher capacity
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Shared Left/Thru lane
When considering the left turn capacity for a shared Left/Thru lane, traffic can turn left only until the queue blocks the turn
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Lane utilization
St. George, UT
2 lanes WBShared thru/left
3 lanes EB2 thru + 1 dedicated left-turn lane
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Key Considerations for Design
• Design Speed
• Tangent Through Crossover / Natural Path
• Crossover Angle
• Direct Pass-Through Distance (Eyebrow Offset)
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• Design speed at a DDI affects the reverse curve radii through the two intersection crossovers
• Typically ranging from 25 to 35 mph
Design Speed and Reverse Curvature
Curves approaching and departing crossover
Curves approaching and departing crossover
Curve radii approaching and following the DDI crossover generally range from 200-400 feet
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Source: Utah DOT DDI Guideline
Utah DOT Guidance
• The design speed of the curves in the crossover areas is at least 10 mph less than the design
speed of the approaching crossroad unless the reduction results in a design speed of less than 25
mph
• UDOT has designed crossover areas for travel speeds of 25 to 40 mph
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Avoid abrupt reverse curvature
If the radius leading into the crossover is less than 150 feet, it may cause drivers to take the “fastest path” and encroach into adjacent lanes.
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Alignment AlternativesAlthough the symmetrical alignment is most
common, other options can be advantageous
where:
1) The existing structure can remain in place
while a parallel one is constructed
2) there are ROW constraints on one side of
the cross road
3) There are ROW constraints in opposite
quadrants on either side of the freeway
4) One direction on the crossroad has much
higher volumes and would benefit from less
driver “work load” of traversing the reverse
curvature
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“Natural Path” Through CrossoverVehicle path alignment at the crossovers should direct vehicles into the proper receiving lane. Drivers should be able to drive “straight” through the crossover intersection (tangent between the reverse curves). An insufficient tangent (or no tangent), makes for an awkward driving path that can lead to vehicle path overlap (encroachment into adjacent lane).
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Before, Through & After Crossover
2nd Priority
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Example of Potential Path Overlap
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Tangent Before Crossover
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Case Study: Pleasant Hill Rd, Gwinnett GA
Originally a “Compressed Diamond” Interchange
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Case Study: Pleasant Hill Rd, Gwinnett GA
POTENTIAL PATH OVERLAP
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Case Study: Pleasant Hill Rd, Gwinnett GA
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The “Natural Path” is also influenced by the sharpness of curvatureDesigners should try to “balance” the tangent length and degree of curvature
Example of Potential Path Overlap
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Reverse Curve Option
Although not desirable, if
developing the tangent is
not possible, try to balance
the reverse curvature by
having the point of reverse
curvature near the center
of the crossover.
S o u r c e : U t a h D O T D D I G u i d e l i n e
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A consideration for using a wider lane width between the crossovers is whether there is a single- or
dual-left turn into those lanes and the truck volumes making that turning maneuver. Many truck
drivers are instructed to use the right-most lanes when there are multiple left-turn lanes.
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Crossover Angle• The greater the crossing angle, the less “different” the intersection will
seem and reduce risk of wrong-way movements• Recommended crossover angles of 30-50 degrees
• Several DDIs have angles of 30 degrees or less (and work OK)• Low angles increase crossing distances and increase signal clearance time
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Crossover Angle
S o u r c e : U t a h D O T D D I G u i d e l i n e
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Eyebrows
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Q-Tips
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Wrong Way Direct Path Test
Figure below shows a narrow gap (less than a car width) for a wrong way pass through –GOOD DESIGN
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Example Check
No clear path through
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Wrong Way Direct Path Test
Figure below shows a pass through gap greater than a car width (not desirable)
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Example Check
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Barrier wall along eyebrow
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Vertical curb painted white immediately adjacent to travel lane (no shoulder or bike lane) set against contrasting color of raised channelization island
Eyebrow enhancement options
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Be careful “shaving your eyebrows”
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Consider the “Sum of the Parts”
Consider collectively:- Crossing Angle- Length of Tangent- Setback Distance- Eyebrow Design- Pass Through Test
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Look back angleAn angle close to 90 degrees is preferable, with a maximum of 110 degrees in consideration of drivers
having difficulty turning their neck.
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Sight line obstructions• Median barrier can block driver sight
lines
• Consider truncating the end of the
barrier wall or lower the wall height to
increase sight distance
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Sight line obstructions
The red line shows how the barrier wall could be altered to provide better
sight distance
S o u r c e : M i s s o u r i D O T
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US 65 at SR 248Branson , MO
Curb to Barrier Transition at Throat of Q-Tip
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Missouri US 60 and Kansas Expwy
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• DDI profiles should be relatively flat • Increases driver sight distance
• Crest vs. Sag profile of cross-road• Consider visibility of downstream cross-over intersection
• Drivers don’t like surprises
Vertical Geometry
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The preview distance of the downstream
traffic signals is restricted by the
curvature on the bridge (Pioneer Crossing & I-15, Utah)
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Crest curvature & signal placementWhere traffic signals are not visible to oncoming drivers, supplemental advance signals should be considered
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Key Reference
www.intrans.iastate.edu/smartwz/documents/project_reports/innovative_geometric_design_work_zones_w_cvr.pdf
Survey of industry experts with
knowledge on current practices
pertaining to Maintenance of
Traffic at innovative geometric
design intersection work zones
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QUESTIONS