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
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
Cloverleaf
Trumpet

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
Capacity analysis is used to determine some of the gore geometric features of interchange ramps. Things such as number of lanes, length of turn lanes, intersection control, etc. Capacity analysis determines the type of free flow entrance or exit taper necessary for an interchange ramp, and establishes need for auxillary lanes on the freeway facility.

7. Ramp Design

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

9. Connecting Roadway Design speed Length 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 At-grade 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 Intersection angle
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