What is a DDI Recommended Practices Traffic Operations Analysis - - PowerPoint PPT Presentation

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What is a DDI Recommended Practices Traffic Operations Analysis - - PowerPoint PPT Presentation

Nov 18, 2022 181 likes •678 views

What is a DDI Recommended Practices Traffic Operations Analysis Geometric Design FHWA Review Conclusions & Lessons Learned Its not about you, its about the public Developed by Access Utah County (AUC), I-15

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 What is a DDI  Recommended Practices

  • Traffic Operations Analysis
  • Geometric Design
  • FHWA Review

 Conclusions & Lessons Learned

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It’s not about you, it’s about the public

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 Developed by Access Utah County (AUC), I-15

CORE and the TOC

 Reviewed by UDOT Technical Committee  For DDI feasibility on other projects

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Main goal is to reduce delay:

  • Accommodate the left turn movements
  • Reduce the number of signal phases

Only two signal phases are required, compared with 3 for a SPUI, or 4 with a diamond

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 Operational Failure when:

  • A + B > 1900 vph
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 Operational Failure when:

  • A + B > 1900 vph
  • A + C > 1900 vph
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 Operational Failure when:

  • A + B > 1900 vph
  • A + C > 1900 vph
  • B + C > 1900 vph
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 Operational Failure when:

  • A + B > 1900 vph
  • A + C > 1900 vph
  • B + C > 1900 vph
  • A + B + C > 2100 vph
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 Transition from LOS D

to LOS F is very short.

For this reason, UDOT defines LOS C as a minimum for acceptable operation.

 Operational Failure when:

  • A + B > 1900 vph
  • A + C > 1900 vph
  • B + C > 1900 vph
  • A + B + C > 2100 vph
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 Considerations:

  • Weaving and merging
  • Signal timing
  • Driver behavior
  • Intersection spacing

 Requires use of micro-simulation software

  • UDOT requires use of VISSIM (PTV America) for analysis
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 Considerations:

  • Traffic & Pedestrian Volumes
  • Multi-Lane Turning Movements
  • Merging & Weaving Operations within the

Interchange

  • Non-traditional Signal Warrant Analysis
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 Proximity of side streets, railroads, etc.  Accommodation of pedestrians  Existing geometric constraints in retrofit

projects

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 Ramp terminal separation distance  Crossover intersection layout and grading  Signing and Striping  Lane and Shoulder Widths  Glare Reduction  Roadway Cross-Section

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To provide for queue storage and weaving space, crossover intersections should be spaced

850’ apart, with adjacent

signalized intersections 1000’ from the crossover intersections.

Right-in/Right-out access points may be placed according to the UDOT Access Management rules.

Placement of accesses closer than 1000’ should be analyzed using VISSIM to determine potential operational effects.

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 Design speed through the crossover is

reduced at least 10 mph, with a minimum speed through the curves of 25 mph.

 Utilize reverse curves without tangents to

minimize length of crossover zones.

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 Placement of glare screen

is specific to each DDI design, and may affect horizontal alignments.

 Line-of-sight calculations

should be used.

 Additional study required

to identify appropriate design parameters.

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 Shoulder widths should NOT be reduced

across structure

 Lane widths should be at least 12 feet  Turning templates for side by side WB-67 or

  • ther appropriate design vehicle should be

applied

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 500 East, American Fork

  • Why a DDI
  • Crossover geometry & glare reduction
  • Side street & ramp terminal separation
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 SR-92, Lehi

  • Why a DDI
  • Crossover geometry & glare reduction
  • Side street & ramp terminal separation
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 Access Justification Report (AJR)

  • Used by FHWA for all modifications to the interstate

system.

  • The AJR is not a substitute for the NEPA process, and the

NEPA process does not eliminate the need for a AJR document.

  • Addresses the eight “policy points” defined by FHWA as a

type of warrants process.

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ADVANTAGES

 Higher left-turn capacity  No exclusive left-turn lanes  Smaller structures  Two-phase signal provides

flexibility in timing

 Smaller ROW footprint at the

interchange

DISADVANTAGES

 Contrary to driver expectation  Operation sensitive to signal and

street spacing

 Rapid transition from LOS D to

LOS F

 ROW impacts at crossover

intersections

 Integration of pedestrian traffic

When compared with SPUI or Diamond interchanges:

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 Design/Build contractors may submit the DDI

interchange type as an alternate concept; RFP should be written to address the possibility.

 Operational analysis of a DDI requires specific

software and requires more review time.

 Retrofit of existing structures may extend the

  • perational life of the interchange for much less than

reconstruction.

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Thank you for your time.

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