Motorway junction
In the field of road transport, an interchange is a road junction that typically uses grade separation, and one or more ramps, to permit traffic on at least one highway to pass through the junction without directly crossing any other traffic stream. It differs from a standard intersection, at which roads cross at grade. Interchanges are almost always used when at least one of the roads is a controlled-access highway (freeway or motorway) or a limited-access divided highway (expressway), though they may occasionally be used at junctions between two surface streets.
Note: The descriptions of road junctions are for countries where vehicles drive on the right side of the road. For countries where driving is on the left the layout of the junctions is the same, only left/right is reversed.
Weaving is an undesirable situation in which traffic veering right and traffic veering left must cross paths within a limited distance, to merge with traffic on the through lane.
The German Autobahn system splits Autobahn-to-Autobahn interchanges into two types: a four-way interchange, the Autobahnkreuz (AK), where two motorways cross, and a three-way interchange, the Autobahndreieck (AD) where two motorways merge.
Some on-ramps have a ramp meter, which is a dedicated ramp-only traffic light that throttles the flow of entering vehicles.
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 a freeway and another road (not a freeway) requires at least four ramps. Complete interchanges between two freeways generally have at least eight ramps, as having fewer would considerably reduce capacity and increase weaving. Using U-turns the number for two freeways can be reduced to six, by making cars that want to turn left either pass by the other road first, then make a U-turn and turn right, or turn right first and then make a U-turn. Depending on the interchange type and the connectivity offered other numbers of ramps may be used. For example, if a highway interchanges with a highway containing a collector/express system, additional ramps can be used to strictly link the interchanging highway with the collector and express lanes respectively. For highways with high-occupancy vehicle (HOV) lanes, ramps can be used to service these carriageways directly, thereby increasing the number of ramps used.
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