This page defines many of the interchange-related terms you may come across. The precise meanings of some terms may vary, depending on which state or agency uses them. Some terms from the Turner-Fairbank Highway Research Center (TFHRC) were defined very well as is; instead of trying to reword them, I simply quoted and attributed them.

If you drive on the left side of the road, which is perfectly okay if everyone else in your country does (grin), swap "left" and "right" in any glossary descriptions.

See the end of this page for links to more interchange resources.

auxiliary lane or operational
An extra lane provided for the entire length of highway between interchanges, giving motorists more time to merge in or out. The lane is created when an entrance ramp meets the highway, and drops out ("exit only") with the exit ramp.

Compare to collector-distributor roads, which also help merging traffic but are separated from the main traffic lanes.

A design feature where two nearly parallel ramps must cross each other and use a grade separation to avoid weaving or crossing. Most often this occurs when an on-ramp from one nearby interchange is braided to avoid interfering with an off-ramp for the next one.
buttonhook ramp or hook ramp
"J-shaped ramp that connects to a parallel or diagonal street or frontage road, which is often well removed from the interchange structure and other ramps." (TFHRC)

"A form of interchange that provides indirect right-turn movements in all four quadrants by means of loops. Generally used where the turning and weaving volumes are relatively low. This type of interchange eliminates all crossing conflicts found in a diamond interchange but requires more area. The cloverleaf type of interchange can have one or two points of entry and exit on each through roadway." (TFHRC)

The complete implementation, using eight ramps, is also called a full cloverleaf. Partial cloverleafs, or parclos, involve fewer ramps. The popular six-ramp version provides two exit ramps (with left turns at the end) and four entrance ramps (all right turns). Many four-ramp parclos may be called folded diamonds, as they serve the same traffic movements as a conventional diamond.

collector - distributor road (C/D)
A one-way road parallel to the main traffic lanes providing access to or from more than one ramp. The C/D road collects traffic from on-ramps or the main lanes, and distributes traffic to off-ramps or back to the main lanes. This minimizes the number of interactions with thru traffic, which can increase capacity and safety. A C/D road may be short (serving two adjacent interchanges, or a single cloverleaf), or may extend for miles in congested or complicated areas.
complete interchange
An interchange providing ramps for all possible traffic movements. A complete freeway to surface street interchange requires four ramps. A freeway ending at another also requires four ramps. Two crossing freeways require eight ramps. A five-way freeway interchange would require 20.
A basic four-ramp interchange between a freeway and a surface street. The four diagonal ramps, one in each quadrant, suggest a diamond shape. See the Diamond page.
Said of a ramp where left-turning movements are handled by left exits instead of flyovers or loops from the right.
directional "T"
A three-way freeway intersection where left-turning movements are handled with directional (left exit) ramps.
double trumpet
An interchange between two freeways where access is provided by an auxiliary divided road, connected to each freeway with a trumpet interchange. Full nonstop access can be provided by this configuration, but typically a double trumpet serves a toll road, with all access to it conveniently served by a tollbooth on the access road.
A directional or semi-directional ramp. Flyovers usually handle left-turning movements (instead of using a loop ramp) and often pass over the main traffic lanes.
In our context (interchange diagrams), a freeway is a road with full access control in the interchange area: the reason an interchange is used instead of an at-grade, signalized intersection. Access is provided only by ramps and merges, with no stopping or turning across opposing traffic. Compare surface street.
The thin triangle of pavement between the thru lanes and the exit lanes as they diverge, (unfortunately, the last resort of drivers unsure to the very end which turn they should be taking). The gore is often diagonally striped to remind traffic to stay off. Eventually unpaved earth, a barrier, or bridge structure must interpose between the thru lanes and the exit ramp; to protect drivers in a crash, collapsible barriers or barrels filled with water or sand are often employed.
grade separation
An intersection that is no longer at-grade (roads cross at the same level) because one road passes over the other on a bridge. Adding ramps to a grade separation, providing access from one road to another, creates an interchange.
half - diamond
An interchange with only two diagonal ramps, one entrance and one exit, in adjacent quadrants. This interchange serves traffic to and from one direction along the freeway, but ignores the other.
high occupancy vehicle (HOV)
Said of lanes or ramps reserved for vehicles with more than one passenger: car pools, van pools, buses, etc. We don't exhaustively cover HOV facilities here, other than to note that separate HOV ramps can lead to very intricate interchanges.
loop ramp
A ramp that serves left-turning movements by instead circling 270 degrees to the right. A full cloverleaf uses four right-turning directional ramps and four loop ramps.
partial cloverleaf (parclo)
An interchange with one or more loop ramps, but fewer than the eight ramps needed to form a full cloverleaf. See cloverleaf.
One of four slices of land created when two roads intersect. This term is used when talking about ramp placement.
A connecting roadway providing access in one direction from one road to another. Some actual ramps may serve two-way traffic, but this practice is generally not used in newer designs. We can conceptually consider such roads as two parallel ramps in opposite directions.
single point urban interchange (SPUI) or single-point diamond
A newer type of diamond where the diagonal ramps are instead placed as close as possible paralleling the freeway, so that ramp traffic in effect meets at a single point on the surface street directly below (or above) the freeway. See SPUI.
My term for a partial cloverleaf providing two exit ramps (with left turns at the end) and four entrance ramps (all right turns). The advantage is that surface street traffic does not need to cross oncoming traffic for access to the freeway.
slip ramp
"A diagonal ramp, more properly called a cross connection, which connects with a parallel frontage road." (TFHRC)
split diamond or three-level diamond
A three-level construction with eight diagonal ramps; crossing and turning movements are handled not on the thru roadways but a one-way traffic square connecting the ramps. See the Volleyball page.
A four-level semi-directional interchange with no loop ramps, typically serving two high-traffic freeways. See Stack.
stacked diamond
A three-level variant of the stack, where left-turning flyover ramps meet at a signalized intersection. See Stacked Diamond.
surface street
In our context (interchange diagrams), a surface street allows access by traffic signal or stop sign, or allows turns across opposing traffic. Compare to freeway
A three-way interchange with no crossing movements, featuring one 270-degree loop ramp opposite the terminating roadway, and a semidirectional ramp following the loop to the outside. See the Trumpet page.
My colloquial term for a split diamond interchange. It's caught on at misc.transport.road, to the perplexment and dismay of traffic professionals. See the Volleyball page.
A type of conflict where traffic veering right and traffic veering left must cross paths within a limited distance. Typically this occurs when an exit ramp closely follows an entrance ramp, between two closely spaced interchanges or two loops of a cloverleaf.

Weaving introduces safety and capacity problems, and is a primary reason some older full cloverleafs are being converted to other types. Loop ramps are either removed outright (diagonal ramps assume their traffic) or replaced by flyovers.