Interchange Design
Wes Mayberry
Transportation Engineering Intern
Office of Design, Methods Section
Iowa Department of Transportation
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)
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
Configuration
Service Interchange
• 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
Systems Interchange
• Cloverleaf
• Trumpet
Type of Ramp
•
•
•
•
Diagonal
Loop
Semi-Directional
Directional or Outer Connection
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
Ramp Design
Components of a Ramp
• Connecting Roadway (Ramp Proper)
• Terminals
– Free Flow
– At-grade
Connecting Roadway
• Design speed
– Alignment
• Horizontal
• Vertical
• Length
– Diagonal ramp, 1200 ft
– Long ramps promote passing
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
Terminal
• Type:
• Free flow
• Changing speed
– Deceleration (Diverging)
– Acceleration (Merging)
• At-grade
• Turning
• Intersection control
• Signed
• Signalized
• Circulatory
Free Flow Terminal (Exit)
• Deceleration Length
• Type
– Parallel
– Taper
• Divergence Angle 3°-5°
• Route continuity
• Gore
Source: (AASHTO Greenbook, 2004)
Free Flow Terminal (Entrance)
• Acceleration Length
• Type
– Parallel
– Taper
• Route continuity
Source: (AASHTO Greenbook, 2004)
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
Art
• Tying it all together
• Example
• Coordinate Geometry
Conceptual Design
• Example
Project Plans
• Example
• Example