Three-Leg Interchanges
Three-leg interchanges, also known as T- or Y-interchanges, are usually
provided where major highways begin or end. Three-leg interchanges
should be considered when future expansion to the unused quadrant is
unlikely. This is due in part to the fact that three-leg interchanges are very
difficult to expand, modify, or otherwise retrofit as a four leg facility.
Exhibit 7-1 illustrates examples of three-leg interchanges with several
methods of providing the turning movements. The trumpet type (with a
single structure) is shown in Exhibit 7-1(A) where three of the turning
January 2006 Interchanges 7-9
2006 EDITION
movements are accommodated with direct or semi-direct ramps and one
movement by a loop ramp. In general, the semi-direct ramp should favor the
heavier left-turn movement and the loop the lighter volume. Where both leftturning
movements are fairly heavy, the design of a directional T-type
interchange shown in Exhibit 7-1(B) is best-suited. A fully directional
interchange shown in Exhibit 7-1(C) is appropriate when all turning volumes
are heavy or the intersection is between two access controlled highways.
Construction of the configurations in Exhibit 7-1(B) and Exhibit 7-1(C)
would be the most costly types because of the multiple structures
required in the center of the interchange to accommodate three levels
of traffic. For further examples and design considerations of additional
T- and Y-interchanges, see AASHTO’s A Policy on Geometric Design of
Highways and Streets.