Diverging Diamond Interchange Prepared by: Jim Poorbaugh Bohannan Huston

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Diverging Diamond InterchangePrepared by Jim
PoorbaughBohannan Huston
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What is a DDI?

• Variant of a typical diamond interchange.
• Transposes the opposing directions of crossroad
  traffic through the interchange
• Two-phase signal control rather then multi-phase.

Source Freeway and Interchange Geometric Design
Handbook, ITE
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The One and Only!
Interchange at A13/RD182 near Versailles,
France11 light injury crashes reported in 5
years compared to an average of 23 fatal/injury
crashes of a typical DI in the USA
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Versailles, France DDI in Operation
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Folly or Functional?

• Initial Reactions to Concept
• How many drinks did you guys have at lunch?
• Interesting now please quit messing around and
  design a real interchange?
• People will go the wrong way.
• Just works on paper.
• Are you crazy, crossing traffic like that will
  get someone killed!
• New Mexico drivers have a hard enough time with a
  normal interchange.
• 30mph Design speed through an interchange will
  never work.
• The French thought this up?

• Technical Merits
• Exceptional for exchange movements on/off of
  freeway.
• Two phase signal timing instead of multi-phase
  more green time for each movement.
• Lower Design Speed Lower Severity of Accidents
• Fewer conflict points then a traditional diamond
  interchange
• 25 more capacity lane for lane then a
  traditional diamond
• Missouri Ohio have considered designed DDIs.
• FHWA Research supports DDIs as a viable
  alternate to traditional interchanges.

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Capacity Comparison Between Conventional and DDI
DesignsApproximately, 25 increase in capacity
Source Joe Bared, FHWA
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Diverging Diamond Interchange Vs. Conventional
Diamond at 4-Lane Crossroad
Source Joe Bared, FHWA
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Diverging Diamond Interchange Geometrics

• Intersection spacing is critical!
• FHWA has several layouts based on design speeds
  from 25 to 40mph.
• Intersection spacing varies 400 to 575

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Diverging Diamond Interchange Geometrics

• Intersections use a series of reverse curves.
• Angle of intersection becomes more acute with
  increased design speed.

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San Mateo Preliminary Design
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DDI Simulation
Simulation courtesy of Joe Bared, FHWA
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San Mateo and I-40 Simulations
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Approved Alternative for San Mateo
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Lessons Learned

• Cross Over intersection spacing critical.
• Optimum operation of DDI requires coordinated
  signal timing
• Large through movements degrade performance of
  DDI use at locations with high volume of
  exchange movements.
• 3-d simulations were EXCELLENT tool to get a
  feel going through a DDI

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Conclusions

• Diverging Diamond Interchanges are a viable
  alternative to a Traditional Diamond Interchange.
• Best location for DDI is where heavy exchange
  movements occur.
• Skewed alignments complicate design best to
  have no skew.
• Diamond Interchange can be retrofit to a DDI as a
  means to improve capacity at a reduced cost from
  reconstruction or adding lanes.
• Completion of the first US DDI in Missouri in 07
  will be a major step in acceptance of this
  interchange form.

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Questions?