Interchange Type. The most commonly used types of interchanges where two routes
cross each other are the diamond, cloverleaf, and directional interchanges. When one
route ends at an interchange with another route, a trumpet or three-leg directional
interchange can be used. Figure 2.40 shows schematic examples of the various types
of interchanges. The trumpet interchange (a) has one loop ramp in its design, which is
a lower-speed ramp. The three-leg directional interchange (b) incorporates all highspeed
ramps in its design. The “one quadrant” interchange (c) has a two-way ramp
with at-grade intersections, all in one quadrant of the interchange. This is used primarily
in urban areas where the routes are both two-way roadways. Typically, this is utilized
as a first stage in a developing area. Right of way in one or more other quadrants
is purchased to allow for future expansion. The diamond interchange (d) is the most
common type where a major facility intersects a minor facility. The capacity is limited
by the at-grade intersections at the minor crossroad. The single-point urban interchange
(SPUI) shown in (e) can be used when the minor road traffic volume increases
and the diamond operation begins to bog down or fail. It allows the use of a single
intersection and usually operates on a three-phase traffic signal. Opposing left turns
from either the ramps or the side road do not cross paths and therefore can run in the
same phase. The third phase is the through traffic on the side road. The partial cloverleaf
interchange (f) can be designed to allow some free-flow right turns from the minor
road or at least eliminate the need for left turns from the minor road. Signals are usually
required to allow access for left-turning vehicles from the ramps onto the minor road.
The full cloverleaf interchange (g) eliminates the need for at-grade signalized intersections
by providing continuous-flow movements for all traffic. This is used when two
major freeways or freeway-style roadways intersect. Under high volumes of traffic,
the short weaving distance between the interior ramp terminals creates congestion
problems. The use of collector-distributor roads can alleviate some of this problem by
separating the through traffic from the entering/exiting traffic. The all-directional
four-leg interchange (h) is the most efficient in terms of handling traffic, but is also
usually the most expensive. It requires the most right of way and the incorporation of
additional bridge structures to accommodate four levels of traffic.