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

Nov 18, 2022 •181 likes •677 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

  2.  What is a DDI  Recommended Practices  Traffic Operations Analysis  Geometric Design  FHWA Review  Conclusions & Lessons Learned

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

  8.  Developed by Access Utah County (AUC), I-15 CORE and the TOC  Reviewed by UDOT Technical Committee  For DDI feasibility on other projects

  10. 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

  11.  Operational Failure when:  A + B > 1900 vph

  12.  Operational Failure when:  A + B > 1900 vph  A + C > 1900 vph

  13.  Operational Failure when:  A + B > 1900 vph  A + C > 1900 vph  B + C > 1900 vph

  14.  Operational Failure when:  A + B > 1900 vph  A + C > 1900 vph  B + C > 1900 vph  A + B + C > 2100 vph

  15.  Operational Failure when:  A + B > 1900 vph  A + C > 1900 vph  B + C > 1900 vph  A + B + C > 2100 vph  Transition from LOS D to LOS F is very short . For this reason, UDOT defines LOS C as a minimum for acceptable operation.

  16.  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

  17.  Considerations:  Traffic & Pedestrian Volumes  Multi-Lane Turning Movements  Merging & Weaving Operations within the Interchange  Non-traditional Signal Warrant Analysis

  22.  Proximity of side streets, railroads, etc.  Accommodation of pedestrians  Existing geometric constraints in retrofit projects

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

  24. 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.

  26.  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.

  29.  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.

  31.  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 other appropriate design vehicle should be applied

  34.  500 East, American Fork  Why a DDI  Crossover geometry & glare reduction  Side street & ramp terminal separation

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

  45.  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.

  46. When compared with SPUI or Diamond interchanges: DISADVANTAGES ADVANTAGES  Contrary to driver expectation  Higher left-turn capacity  Operation sensitive to signal and  No exclusive left-turn lanes street spacing  Smaller structures  Rapid transition from LOS D to LOS F  Two-phase signal provides flexibility in timing  ROW impacts at crossover intersections  Smaller ROW footprint at the interchange  Integration of pedestrian traffic

  47.  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 operational life of the interchange for much less than reconstruction.

  48. Thank you for your time.

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