CE562 Lecture 27 Interchanges 3

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CE562 Lecture 27Interchanges (3)
Text A Policy on Geometric Design, pp.555-867.
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Selecting Interchange Type in Rural Areas

• System interchanges (freeway - freeway)
• Traffic volumes Type of interchange
• Strong turning volumes All-directional
• Some turning volumes are strong Semi-directional
• Weak left turns Full cloverleaf
• Moderate left turns Cloverleaf with
  collector- distributor roads.
• Service interchanges (freeway - non-freeway)
• Diamond interchange
• Partial cloverleaf (right-of-way restricted)

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Selecting Interchange Type in Urban Areas

• System interchanges versus service interchanges
• Interchanges in urban areas are closely spaced
• System-wide design instead of individual
  interchanges
• Cloverleaf interchanges are not used

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Design Considerations

• Capacity
• Route continuity
• Uniformity of exit patterns
• Single exits in advance of the structure
• Weaving sections
• Potential for signing
• Costs
• Availability of right-of-way
• Potential for stage construction
• Compatibility with the environment

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Adaptability of Interchanges
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Alignment, Profile, and Cross Section

• Design standard in the interchange area should be
  the same as on the approach section
• Controls for horizontal and vertical alignments
  according to the general rules
• Widening and dividing of non-freeways is
  sometimes required to channelize the at-grade
  intersections with ramps

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Alignment, Profile, and Cross Section
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Sight Distance

• Design of the vertical alignment is the same as
  for the other points on the highway
• The horizontal sight distance in the presence of
  piers and abutments emphasizes the need for
  above-minimum design of curves. Sometimes the
  lateral clearances should be increased.

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Interchange Spacing

• Minimum spacing of interchanges depends on the
  weaving volumes, ability to sign, and required
  length of speed-change lanes.
• A rule of thumb min. 1.5 km in urban areas,
  min.3.0 km in rural areas.

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Route Continuity
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Route Continuity
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Basic Number of Lanes

• The basic number of lanes should be maintained
  over a long length of the freeway.
• The number of lanes is predicated on the design
  hourly volume (DHV) and required level of
  service.

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Basic Number of Lanes
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Lane Balance
Direction of traffic
Two-lane off- and on-ramps are needed
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Lane Balance
Direction of traffic
Ineffective use of the ramp
Visibility obstructed
Any problems?
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Lane Balance
Direction of traffic
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Lane Balance
Direction of traffic
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Lane Balance
Direction of traffic
Only three lanes
Any problems?
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Lane Balance
Direction of traffic
Any problems?
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Lane Balance
Direction of traffic
A?BR-1
BAR-1
Any problems?
NO PROBLEMS !
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Dropping Auxiliary Lanes
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Dropping Auxiliary Lanes
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Dropping Auxiliary Lanes
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Dropping Auxiliary Lanes
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Adding Auxiliary Lanes
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Basic Lanes Reduction

• Lane reduction is allowed
• at interchanges
• between interchanges
• Lane drop design
• If between interchanges, then at least 600-900 m
  from the last ramp intersection (proper signing)
• On right-hand side
• On horizontal tangents
• Before crest curves or on sag curves
• Minimum taper is 501
• Desirable taper is 701

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Collector-Distributor Roads

• Used to remove weaving from the through lanes
• One or two lanes
• Design speed between 60 and 80 km/h
• Equal to or 10 km/h less than the speed on the
  main roadway
• Separation from the main roadway must accommodate
  shoulders and barriers.

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Types of Ramps
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Types of Ramps
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Design Speeds for Ramps
See Exhibit 3-43
Exhibit 10-56
Semidirect and direct connectors
Loops
Ramps for right turns