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 Diamond Reference: http://www. odot. state. or

Four-Leg Intersections Cloverleaf Partial cloverleaf (parclo)

Example of Cloverleaf Reference: http://www. augustachronicle

Example of. Reference: http://www. odot. state. or

Directional

Rotaries

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

Diverging Diamond Interchange

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

Direct Taper

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)