1. Interchange Design Wes Mayberry Transportation Engineering Intern Office of Design, Methods Section Iowa Department of Transportation

2. Interchange • “An interchange is a system of interconnecting roadways in conjunction with on more grade separations that provides movements of traffic between two or more roadways or highway on different levels.” (AASHTO Greenbook, 2004)

3. Interchange Categories • Systems Interchange • Connection between two roadways with full access control (access to and from the roadway only allowed at interchanges) • Service Interchange • Connection between one roadway with full access control and one roadway with partial access control

4. Configuration Service Interchange Systems Interchange Cloverleaf Trumpet • Diamond • Conventional • Compressed • Split • Partial Cloverleaf (Parclo) • Parclo A • Exit Ramps from the major roadway • Parclo B • Exit loops from the major roadway • Parclo AB

5. Type of Ramp • Diagonal • Loop • Semi-Directional • Directional or Outer Connection

6. Capacity Analysis Basic freeway segment Freeway weaving Type A Type B Type C Ramp segment Ramp Junctions Merge Diverge Interchange ramp terminal intersections Unsignalized Sign Roundabout Signalized

7. Ramp Design

8. Components of a Ramp • Connecting Roadway (Ramp Proper) • Terminals • Free Flow • At-grade

9. Connecting Roadway • Design speed • Alignment • Horizontal • Vertical • Length • Diagonal ramp, 1200 ft • Long ramps promote passing

10. Connecting Roadway, cont. • Cross section • Pavement width • Shoulders • Type • Width • Roadside • Fore slopes • Recoverable (4:1 or flatter) • Non-recoverable (slopes between 3:1 and 4:1) • Critical Fore slopes (slopes < 3:1) • Clear zone • Ditches • Back slopes

11. Terminal • Type: • Free flow • Changing speed • Deceleration (Diverging) • Acceleration (Merging) • At-grade • Turning • Intersection control • Signed • Signalized • Circulatory

12. Free Flow Terminal (Exit) • Deceleration Length • Type • Parallel • Taper • Divergence Angle 3°-5° • Route continuity • Gore Source: (AASHTO Greenbook, 2004)

13. Free Flow Terminal (Entrance) • Acceleration Length • Type • Parallel • Taper • Route continuity Source: (AASHTO Greenbook, 2004)

14. At-Grade Terminal • Intersection control • Operational analysis • Turn lanes • Number of turn lanes • Length of turn lanes • Intersection angle • 60° minimum • 75° preferred for older drivers • Departure sight distance (ISD) • Design vehicle • Pedestrian accommodations

15. Art • Tying it all together • Example • Coordinate Geometry

16. Conceptual Design • Example

17. Project Plans • Example • Example