Interchanges

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Interchanges

• Ch. 6

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Definition

• System of interconnecting roadways in conjunction
  with one or more grade separations, providing for
  the movement of traffic between two or more
  roadways on different levels.

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Warrants

• Freeway development
• Elimination of bottlenecks/spot congestion
• Elimination of accident hazard
• Road-user benefits (benefits outweigh costs)

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Types

• Three-legs

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Types

• Four-legs
• Ramps in one quadrant

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Four-Leg Intersections

• Diamond

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Example of DiamondReference http//www.odot.sta
te.or.us/techserv/engineer/pdu/index.htm
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Four-Leg Intersections

• Cloverleaf
• Partial cloverleaf (parclo)

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Example of CloverleafReference
http//www.augustachronicle.com/stories/110899/met
_077-0426.000.shtml
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Example of ?Reference http//www.odot.state.or.
us/techserv/engineer/pdu/index.htm
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• Directional

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• Rotaries

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Ramps

• Usually 1-way (sometimes 2)
• Capacity (should be checked)Could be limited by
  ramp itself, the entrance terminal or the exit
  terminal

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Ramp Terminal Design

• Direct Taper
• Parallel
• Must carefully design where ramp merges/diverges
  w/ mainline (speed-change lanes, tapers, noses,
  merging ends, islands)

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Direct Taper

• Driver path is a taper
• Drivers dont know how to use speed-change lanes
• Drivers in right-through lane tend to yield if
  ramp merges directly into traffic stream

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Direct Taper
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Parallel

• Adv
• Greater distance for a driver to find a gap
• Dis
• Drivers tend to follow a direct taper.

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Distance between Ramps?

• ½ mile on a freeway
• 600 between freeway exit and an exit on a
  collector-distributor road

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Control of Access

• Should have complete control of access where
  ramps intersect mainlines

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Preferred Design Principles

• Minimize weaving which can reduce speeds, reduce
  capacity and increase accident potential
• Weaving occurs on cloverleaves and where
  entrance/exits are too close together.

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Preferred Design Principles (cont)

• Provide Single Exits Single Entrances
• No left-hand exits-entrances
• Drivers dont expect
• Decisions maneuvering occur in high-speed lanes
• Trucks must maneuver to LT side

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Preferred Design Principles (cont)

• Exits should Precede Entrances
• -Congestion is reduced by removing traffic
  before adding new traffic
• -Main line weaving is eliminated

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Preferred Design Principles (cont)

• Provide high ramp speeds
• -Reduces magnitude of speed changes
• -Reduces travel time
• -Improves ramp terminal conditions

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Safety Conditions

• Design to highest feasible standard (higher
  accident potential compared to the open freeway)
• -Exit Ramps (dont locate beyond crest curve or
  an overpass)
• -Entrance Ramps (provide good visibility)
• -Provide SSD no lower than minimum
• -Gores (gently grade and free of FO)