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

• Example Conceptual Design

• • Example Example Project Plans