Road junction

Road junction

In the field of road transport, a road junction is a place where two or more roads either meet or cross. The main types of road junctions are intersections and interchanges. A road junction may also be called a crossroads. A junction between 3 road segments (arms) is a T junction (two arms form one road) or a Y-junction.

Table of contents

1 Terminology
2 Intersections
3 Interchanges

3.1 Types of interchanges between two freeways
3.2 Types of interchanges between a freeway and a non-freeway road

Terminology

Note: The descriptions of road junctions are for countries, like the United States, where vehicles drive on the right side of the road. For countries where they drive on the left, the junctions are the mirror image (the same except that right and left are reversed).

A ramp is a short section of road which allows vehicles to enter or exit a freeway (motorway).

A directional ramp always tends toward the desired direction of travel.
A non-directional ramp goes in a direction opposite to the desired direction of travel. Many loop ramps (as in a cloverleaf) are non-directional.
A semi-directional ramp exits a road in a direction opposite from the desired direction of travel, but then turns toward the desired direction of travel. Many 'flyover ramps' (as in a stack) are semi-directional.

Weaving is an undesirable situation in which traffic veering right and traffic veering left must cross paths within a limited distance. Weaving creates both safety and capacity problems.

Intersections

An intersection is a road junction at which roads meet at grade. Types of intersections include:

Uncontrolled intersections, without signs or signals (or sometimes with a warning sign). On a 4-way intersection traffic from the right has priority; on a 3-way intersection rules may vary by country: either traffic from the right has priority again, or traffic on the continuing road. For traffic coming from the same or opposite direction, that which goes straight has priority over that which turns off.

Yield-controlled intersections may or may not have specific "YIELD" signs (known as "GIVE WAY" signs in some countries).

Stop-controlled intersections have one or more "STOP" signs. Two-way and four-way stops are common.

Signal-controlled intersections depend on traffic signals, usually electric, which indicate which traffic has the right-of-way at any particular time.
A traffic circle is a special type of intersection at which traffic streams are directed around a circle. Types of traffic circles include roundabouts, 'mini-roundabouts', 'rotaries', "STOP"-controlled circles, and signal-controlled circles.
A box junction can be added to an intersection, generally prohibiting entry to the intersection unless the exit is clear.

Interchanges

A highway interchange is a road junction that utilizes grade separation, and one or more ramps, to permit traffic on at least one road to pass through the junction without crossing any other traffic stream. A complete interchange has enough ramps to provide access from any direction of any road in the junction to any direction of any other road in the junction. A complete interchange between two freeways requires 8 ramps. A complete interchange between a freeway and another road (not a freeway) requires 4 ramps.

Types of interchanges between two freeways

A cloverleaf is a two-level interchange in which left turns are handled by loop ramps. In order to go left, vehicles first pass either over, or under, the other road, then go right 270 degrees on a non-directional loop ramp. The major advantage of cloverleafs is that they are relatively inexpensive. A major problem with cloverleafs is weaving (see definition of weaving, above).
A stack is an interchange in which left turns are handled by semi-directional flyover ramps. In order to go left, vehicles first turn slightly right (on a 'right-turn' ramp), then go left on a ramp which goes over (or under) both freeways and connects to the 'right-turn' ramp in the opposite quadrant of the interchange. Stacks don't suffer from the problem of weaving associated with cloverleafs, but stacks are expensive.
A cloverstack (half cloverleaf, half stack) is an interchange in which left turns are handled by two loop ramps and two semi-directional flyover ramps. A cloverstack avoids the weaving problem associated with cloverleafs, without the expense of a full stack. Cloverleafs are sometimes converted to cloverstacks (by eliminating 2 loop ramps and replacing them with two flyover ramps). Such a conversion improves the capacity and safety of the interchange.
Directional interchange
hybrids, variations and rare types

Types of interchanges between a freeway and a non-freeway road

Diamond

Folded diamond (sometimes called a 4-ramp parclo)

a "six ramp parclo" is the same as a folded diamond, except that there are right-turn ramps in what would otherwise be unused quadrants.

Single Point Urban Interchange (also called "SPUI", "single point diamond", or "X-interchange").

SPDI (Single Point Directional Interchange) - identical to a SPUI except that it uses directional ramps.

Roundabout interchange
Single roundabout interchange (or SRI) - uses one roundabout which spans the freeway (either above or below the freeway)
Roundabout diamond is a conventional diamond except that it uses roundabouts rather than signals or stop signs where the ramps meet the non-freeway road.