Culture
interchanges in movies, computer games, etc.
Diamond
and other 4-ramp designs
Six-ramp
partial cloverleaf
Cloverleaf
Trumpet
and other 3-way interchanges
Stack
and other heavy-duty 4-way interchanges
Volleyball
an odd 3-level 4-way treatment
SPUI
Single-Point Urban Interchange
Oddities
Some strange or fictional interchanges
The Full Cloverleaf
The classic cloverleaf allows "non-stop" full access between two busy roads.
Traffic merges and weaves, but does not cross at-grade; unless the interchange is
too congested, no stopping is required.
The colloquial "cloverleaf" is the same as
the more technical "full cloverleaf", as you can omit ramps to get a partial one.
Typically a cloverleaf is used where a freeway intersects a busy surface street, though many older freeway-freeway interchanges are also cloverleafs. As we'll see, the full cloverleaf is not considered as applicable in some situations now as it might have been a few decades ago; in several places cloverleafs have been replaced with either signalized interchanges or higher-capacity directional interchanges with flyovers.
Design Notes
The cloverleaf is (on paper) the simplest way to connect two freeways.
The only bridges required are to separate the two roadways. If land is expensive,
so too can be the cloverleaf, which becomes a choice between tight turning radii
(and lower design speed) or lots of consumed land. You'll notice that most loop ramps
are banked to counteract centrifugal forces.
A small advantage that "falls out of the design" is the "second chance:"
if you miss the first ramp to the right, you can simply take three loops in a
row to get back on track. (I've also used two loops of a cloverleaf to make a
U-turn.)
Weaving
A disadvantage to the plain cloverleaf is the "weaving" process,
where drivers exiting one loop have to
merge and cross other drivers entering the next one. Weaving, which causes
bottlenecks and accidents, is the primary reason cloverleafs are now deprecated
in designs for new or revamped interchanges. In several instances, California is
replacing old full cloverleafs with 6-ramp partial cloverleafs.
C/D roads
One way to improve a cloverleaf is to add collector/distributor (C/D) roads, which
run parallel to the freeway and isolate it from the weaving action at the
loops. Traffic exits the freeway onto the C/D road, and then can decide which
direction to take. Likewise, onramps from both directions of the other road
merge together first, then merge onto the primary road. The weaving problem
still exists, but has been moved to a lighter-traffic side road; and the
freeway now has two ramps (entrance and exit) to deal with instead of four.
Where two freeways intersect, each one may have C/D roads.
Variations
Another modification (shown), at the I-395/King St (VA 7) interchange in Alexandria,
unrolls two of the loops to remove the weaving action and simplify the interface
(in each direction, the driver first chooses to get to the other road, then chooses
which direction (e.g. north or south) to take. The remaining two loops could be unrolled
as well, creating a petal-like arrangement I've seen called a Catherine Wheel.
Topologists (in the mathematical sense) will see how ramps can be stretched, flipped, moved
and so on, without changing the basic function of the interchange.
Another variation (not pictured yet) replaces two diagonally opposing loops with semidirectional flyovers, creating a half-cloverleaf half-stack interchange. Lacking a catchy name for this (Caltrans calls it the Type F-3), fellow roadgeek Nathan Perry arrived at the name "Cloverstack." Since the two loops in a cloverstack are diagonally opposite from each other, there is no weaving.
History
The first cloverleaf opened in New Jersey (at routes 4 and 25, which are now US 1/9 and NJ 35)
in 1929; it is scheduled to be replaced with a diamond interchange around 2004.
The interchange was under consideration for the National Register of Historic Places,
but was declined, primarily because of the many safety and capacity-related alterations
to it over the years: "widening
and curb replacements for both roadways, revised
geometry for one ramp, placement of a center barrier
on Rt.US 1&9, removal of bridge pilasters, and the
addition of modern lights and guide rails."
Thanks to Raymond C. Martin for this information.