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Interchange Design Uniformity and Flexibility

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Title: Interchange Design Uniformity and Flexibility


1
Interchange Design Uniformity and Flexibility
  • CE 453 Lecture 34

2
Outline
  • Interchange spacing
  • Interchange pattern
  • Route continuity
  • Overlapping routes
  • Basic number of lanes
  • Lane balance
  • Weaving area

3
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

4
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

5
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

6
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

7
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.
8
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

9
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.
10
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

21
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

22
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.
23
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

24
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
25
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)

26
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.
27
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

28
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.

29
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.

30
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.
31
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.
32
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)

33
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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