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Interchanges

Interchanges. Ch. 6. 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. Warrants. Freeway development Elimination of bottlenecks/spot congestion

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Interchanges

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  1. Interchanges Ch. 6

  2. 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.

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

  4. Types • Three-legs

  5. Types • Four-legs • Ramps in one quadrant

  6. Four-Leg Intersections • Diamond

  7. Example of DiamondReference: http://www.odot.state.or.us/techserv/engineer/pdu/index.htm

  8. Four-Leg Intersections • Cloverleaf • Partial cloverleaf (parclo)

  9. Example of CloverleafReference: http://www.augustachronicle.com/stories/110899/met_077-0426.000.shtml

  10. Example of ?Reference: http://www.odot.state.or.us/techserv/engineer/pdu/index.htm

  11. Directional

  12. Rotaries

  13. Roundabouts http://www.wsdot.wa.gov/safety/roundabouts/

  14. SPUI,SPI,SPDI (Single Point Interchange-Urban-Diamond) http://www.mainstreetmonroe.com/articles/monroe-spui-is-now-semi-open http://en.wikipedia.org/wiki/Single-point_urban_interchange http://mackinengineering.com/services/15.htm

  15. Diverging Diamond Interchange

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

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

  18. Direct Taper • Driver path is a taper • Driver’s don’t know how to use speed-change lanes • Drivers in right-through lane tend to yield if ramp merges directly into traffic stream

  19. Direct Taper

  20. Parallel Adv: Greater distance for a driver to find a gap Dis: Driver’s tend to follow a direct taper.

  21. Distance between Ramps? • ½ mile on a freeway • 600’ between freeway exit and an exit on a collector-distributor road

  22. Control of Access • Should have complete control of access where ramps intersect mainlines

  23. 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.

  24. Preferred Design Principles (cont) • Provide Single Exits & Single Entrances • No left-hand exits-entrances • Driver’s don’t expect • Decisions & maneuvering occur in high-speed lanes • Trucks must maneuver to LT side

  25. Preferred Design Principles (cont) • Exits should Precede Entrances -Congestion is reduced by removing traffic before adding new traffic -Main line weaving is eliminated

  26. Preferred Design Principles (cont) • Provide high ramp speeds -Reduces magnitude of speed changes -Reduces travel time -Improves ramp terminal conditions

  27. Safety Conditions Design to highest feasible standard (higher accident potential compared to the open freeway) -Exit Ramps (don’t 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)

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