Interchange Design Uniformity and Flexibility

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Interchange Design Uniformity and Flexibility

• CE 453 Lecture 34

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Outline

• Interchange spacing
• Interchange pattern
• Route continuity
• Overlapping routes
• Basic number of lanes
• Lane balance
• Weaving area

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Interchange Design Principles

• Operational uniformity concept interchange
  design should reinforce driver ad hoc
  expectancies that conform with (or reinforce) a
  priori expectancies
• Operational flexibility interchanges should
  accommodate maximum possible variation in traffic
  volumes/patterns

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Operational Uniformity/ Flexibility Benefits

• Reduced number of lane changes (improved
  operations and fewer vehicle conflicts)
• Signing simplified
• Reduced navigation workload
• Ramp traffic impact reduced
• Provide flexibility to serve different traffic
  patterns

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1. Interchange Spacing

• Determined by
• Weaving volumes
• Ability to sign
• Length of speed-change lanes
• Rule of thumb
• Urban 1 mile or more
• Rural 2 miles or more
• If need to be closer
• Use grade separated ramps or CD roads

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2. Uniformity of Interchange Patterns

• Maintain exit/entrance patterns from interchange
  to interchange (single exit before structure,
  single entrance after) See GB Exhibit 10-45
• Avoid two-exit designs use CD roads at
  cloverleafs, exit right then split for
  directional interchanges

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Source A Policy on Geometric Design of Highways
and Streets (The Green Book). Washington, DC.
American Association of State Highway and
Transportation Officials, 2001 4th Ed.
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3. Route continuity

• Give through driver continuous through route
  (i.e., no lane changes to stay on route), see
  Exhibit 10-46
• Interchange configuration should favor through
  movement (even if it is not the highest volume)
• Through route should be on the left of all
  entrances and exits

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Source A Policy on Geometric Design of Highways
and Streets (The Green Book). Washington, DC.
American Association of State Highway and
Transportation Officials, 2001 4th Ed.
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Source A Policy on Geometric Design of Highways
and Streets (The Green Book). Washington, DC.
American Association of State Highway and
Transportation Officials, 2001 4th Ed.
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8. I-235 before construction
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A closer look howd you like to sign this
interchange?
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4. Overlapping Routes

• Avoid overlapping routes especially within
  interchanges Why?
• Concentrated volumes
• Weaving is typical
• Complicated geometry
• Complicated signing
• High driver workload

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Overlapping Routes

• Treatments
• Give preference to higher class route
• Position routes so they dont cross, although
  they may share alignment
• Use CD roads for
• major/minor overlap
• Fully separate routes

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5. Continuity of basic number of lanes

• Basic number of lanes minimum number of lanes
  designated and maintained over a significant
  length of a route, irrespective of changes in
  traffic volume and requirements for lane balance
  or a constant number of lanes assigned to a
  route, exclusive of auxiliary lanes.
• Basic number of lanes f(general volume over
  substantial length of route) see exhibit 10-48

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Source A Policy on Geometric Design of Highways
and Streets (The Green Book). Washington, DC.
American Association of State Highway and
Transportation Officials, 2001 4th Ed.
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Lane drop/addition

• Add basic lane if volume raises enough for a
  significant length of facility
• Drop basic lane where there is a general lowering
  of the volume level
• Dont drop lane that will be needed a short
  distance downstream

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6. Lane Balance Entrance Ramps
Number of lanes downstream should be the same or
one less than upstream freeway lanes plus
entrance lanes Nc Nf Ne 1 or Nc Nf
Ne where Nc number of combined lanes
downstream Nf of freeway lanes Ne of
entrance lanes
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Lane Balance Exit Ramps

• number of lanes on exit and mainline should be
  one more than mainline upstream of exit
• exception is cloverleaf or auxiliary lane 450 m
  or less (then before and after lanes are equal)

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Source A Policy on Geometric Design of Highways
and Streets (The Green Book). Washington, DC.
American Association of State Highway and
Transportation Officials, 2001 4th Ed.
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Auxiliary Lanes

• parking, speed change, turning, storage for
  turning, weaving, truck climbing, and other
• Role mitigate ramp impacts by
• complying with lane balance and basic number of
  lane rules (Exhibits 10-50 and 10-52)
• accommodating speed changes for exiting/entering
  vehicles
• providing same LOS as freeway in ramp and weaving
  areas

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Auxiliary Lanes

• Introduce Auxiliary lanes
• 2500 feet upstream of 2 lane exits
• distance upstream of 1 lane exits that
  accommodates reasonable accel/decel from freeway
  D.S. to ramp proper D.S.

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Auxiliary Lanes

• Drop Auxiliary lanes (Exhibit 10-51)
• as part of isolated 2-lane entrance
• at closely spaced interchanges
• at 2 lane exit
• at single lane cloverleaf exit
• beyond physical nose of single lane exit
• 500-1000 feet beyond single lane exit.

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Various methods of dropping an auxiliary lane
Source A Policy on Geometric Design of Highways
and Streets (The Green Book). Washington, DC.
American Association of State Highway and
Transportation Officials, 2001 4th Ed.
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Using auxiliary lanes to coordinate lane balance
and basic number of lanes
Source A Policy on Geometric Design of Highways
and Streets (The Green Book). Washington, DC.
American Association of State Highway and
Transportation Officials, 2001 4th Ed.
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Lane Reduction

• Reduce basic number of lanes by only one at a
  time
• Beyond point lane is needed (e.g., beyond major
  fork or series of exits)
• At 2-lane exit
• Between interchanges (on right, following the
  exit, 2,000 to 3,000 feet downstream of
  interchange (avoid expectancy problems), where
  DSD is provided)

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7. Weaving Areas

• 2 or more traffic streams crossing paths
• turbulence reduces efficiency, increased driver
  workload, and higher number of vehicle conflicts
• Design parameters
• Configuration
• Length
• weaving/non-weaving volumes
• Solutions
• avoid designs with inherent weaving
• introduce CD roads to remove weaving on mainline
• Increase length or number of lanes

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Type A - Both weaving movements change one lane
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Type B (Major) - One weaving movement does not
change lanes
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Type C (Major) - One weaving movement does not
change lanes, the other changes 2 or more lanes
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Type of Weaving Section Determines Analysis
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I-94/35W Twin Cities
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