An intersection or an at-grade junction is a junction where two or more roads converge, diverge, meet or cross at the same height, as opposed to an interchange, which uses bridges or tunnels to separate different roads. Major intersections are often delineated by gores and may be classified by road segments, traffic controls and lane design.
This article primarily reflects practice in jurisdictions where vehicles are driven on the right. If not otherwise specified, "right" and "left" can be reversed to reflect jurisdictions where vehicles are driven on the left.
Types
Road segments
One way to classify intersections is by the number of road segments (arms) that are involved. A three-way intersection is a junction between three road segments (arms): a T junction when two arms form one road, or a Y junction, the latter also known as a fork if approached from the stem of the Y. A four-way intersection, or crossroads, usually involves a crossing over of two streets or roads. In areas where there are blocks and in some other cases, the crossing streets or roads are perpendicular to each other. However, two roads may cross at a different angle. In a few cases, the junction of two road segments may be offset from each when reaching an intersection, even though both ends may be considered the same street. Six-way intersections usually involve a crossing of three streets at one junction; for example, a crossing of two perpendicular streets and a diagonal street is a rather common type of 6-way intersection. Five, seven or more approaches to a single intersection, such as at Seven Dials, London, are not common.
Traffic controls
Another way of classifying intersections is by traffic control technology:
Uncontrolled intersections, without signs or signals (or sometimes with a warning sign). Priority (right-of-way) rules may vary by country: on a 4-way intersection traffic from the right often has priority; on a 3-way intersection 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 stops are common, while some countries also employ four-way stops. Signal-controlled intersections depend on traffic signals, usually electric, which indicate which traffic is allowed to proceed at any particular time.
Lane design
A traffic circle is a 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. Some people consider roundabouts to be a distinct type of intersection from traffic circles (with the distinction based on certain differences in size and engineering). A box junction can be added to an intersection, generally prohibiting entry to the intersection unless the exit is clear. Some (unconventional or alternative) intersections employ indirect left turns to increase capacity and reduce delays. The Michigan left combines a right turn and a U-turn. Jughandle lefts diverge to the right, then curve to the left, converting a left turn to a crossing maneuver, similar to throughabouts. These techniques are generally used in conjunction with signal-controlled intersections, although they may also be used at stop-controlled intersections. Other designs include advanced stop lines, parallel-flow and continuous-flow intersections, hook turns, quadrants, seagull intersections, slip lanes, staggered junctions (junctions consisting of two opposing T-junctions where one road intersects two sideroads located diagonally opposite each other; in American English referred to as doglegs), superstreets, Texas Ts, Texas U-turns and turnarounds. A roundabout and its variants like turbo roundabouts, bowties and distributing circles like traffic circles and right-in/right-out (RIRO) intersections.
Turns
At intersections, turns are usually allowed, but are often regulated to avoid interference with other traffic. Certain turns may be not allowed or may be limited by regulatory signs or signals, particularly those that cross oncoming traffic. Alternative designs often attempt to reduce or eliminate such potential conflicts.
Turn lanes
At intersections with large proportions of turning traffic, turn lanes (also known as turn bays) may be provided. For example, in the intersection shown in the diagram, left turn lanes are present in the right-left street.