is a place where two or more roads either meet or cross. The main types of road junctions are
. A road junction may also be called a
. A junction between 3 road segments (arms) is a
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.
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
- 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.