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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 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 • Elimination of accident hazard • Road-user benefits (benefits outweigh costs)
Types • Three-legs
Types • Four-legs • Ramps in one quadrant
Four-Leg Intersections • Diamond
Example of DiamondReference: http://www.odot.state.or.us/techserv/engineer/pdu/index.htm
Four-Leg Intersections • Cloverleaf • Partial cloverleaf (parclo)
Example of CloverleafReference: http://www.augustachronicle.com/stories/110899/met_077-0426.000.shtml
Example of ?Reference: http://www.odot.state.or.us/techserv/engineer/pdu/index.htm
Roundabouts http://www.wsdot.wa.gov/safety/roundabouts/
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
Ramps • Usually 1-way (sometimes 2) • Capacity (should be checked)—Could be limited by ramp itself, the entrance terminal or the exit terminal
Ramp Terminal Design • Direct Taper • Parallel Must carefully design where ramp merges/diverges w/ mainline (speed-change lanes, tapers, noses, merging ends, islands)
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
Parallel Adv: Greater distance for a driver to find a gap Dis: Driver’s tend to follow a direct taper.
Distance between Ramps? • ½ mile on a freeway • 600’ between freeway exit and an exit on a collector-distributor road
Control of Access • Should have complete control of access where ramps intersect mainlines
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.
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
Preferred Design Principles (cont) • Exits should Precede Entrances -Congestion is reduced by removing traffic before adding new traffic -Main line weaving is eliminated
Preferred Design Principles (cont) • Provide high ramp speeds -Reduces magnitude of speed changes -Reduces travel time -Improves ramp terminal conditions
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)