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

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

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

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

11. Directional

12. Rotaries

13. Roundabouts

14. SPUI,SPI,SPDI (Single Point Interchange-Urban-Diamond)

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

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

19. 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

20. Direct Taper

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

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

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

25. 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.

26. 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

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

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

29. 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)