Interchange Design

Wes MayberryTransportation Engineering InternOffice 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