- Intersection (road)
An intersection is a road junction where two or more roads either meet or cross at grade (they are at the same level). An intersection may be 3-way - a T junction or fork, 4-way - a crossroads, or 5-way or more. It may often be controlled by traffic lights, and may be a roundabout (traffic circle in America).
Types of intersections
Some may classify intersections as 3-way, 4-way, 5-way, 6-way, etc. depending on the number of road segments (arms) that come together at the intersection.[weasel words]
- 3-way intersection - A junction between three road segments (arms) is a T junction (two arms form one road) or a Y-junction.
- 4-way intersections are the most common, because they usually involve 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.
- 5-way intersections are less common but still exist, especially in urban areas with non-rectangular blocks.
- 6-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.
- Seven or more approaches to a single intersection, such as at Seven Dials, London, are rare.
Another way of classifying intersections is by traffic control:
- Uncontrolled intersections, without signs or signals (or sometimes with a warning sign). Priority 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.
- 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 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. These techniques are generally used in conjunction with signal-controlled intersections, although they may also be used at stop-controlled intersections.
At same-grade intersections, turns are usually allowed, but often regulated to avoid interference or collision with other traffic. If the crossing street is a one-way street, a turn into the opposing direction is not allowed, as indicated by "ONE WAY" or "NO LEFT TURN" or "NO RIGHT TURN" signs. In a few other cases, certain turns may be not allowed or limited by regulatory signs or signals, such as a sign saying "NO TURN ON RED" (no turn allowed when a red light is on). In the absence of lane markings indicating otherwise, left turns should be made from the leftmost lane and right turns from the rightmost lane to avoid collision or blocking of traffic going straight.[clarification needed]
At some intersections where vehicles travel on the right side of the road, there are left turn lanes where the street/road approaches the intersection. For example in the intersection shown in the following diagram, there are left turn lanes in the east-west street for traffic approaching the intersection in the eastbound and westbound directions.
These left turn lanes are marked with an arrow bending into the direction of the left turn which is to be made from that lane only. The word "ONLY" in those lanes means that vehicles may only use them to make a left turn from there. In some other cases, a double-headed arrow may indicate vehicles may travel in either one of two directions from that lane. Traffic signals facing vehicles in left turn lanes often have a special green left turn arrow, indicating vehicles may turn left unhindered by oncoming traffic when this green arrow light is on. Even though the north-south street does not have left turn lanes, traffic may still turn left (unless otherwise not allowed) from the leftmost lane facing the intersection northbound or southbound. When there is no green left turn arrow, vehicles from that direction may enter the intersection to turn left only when there is a green light facing them and must yield to all oncoming traffic. There are intersections with no left turn lanes and many major intersections with left turn lanes for traffic from all directions. Streets without left turn lanes usually either have less traffic than streets with left turn lanes or are older streets where it is difficult to widen the street to accommodate the extra lane. Depending on the intersection, many other combinations of traffic signals (such as green, yellow, or red left or right arrows) and left or right turn lanes are possible. In areas where vehicles travel on the left side of the road, the preceding discussion about left turns applies to right turns instead. Often parallel parking on the side of a street is not allowed close to an intersection to allow traffic to flow through better near the intersection.
Turn lanes can have a dramatic effect on the safety of a junction. In rural areas, crash frequency can be reduced by up to 48% if left turn lanes are provided on both main-road approaches at stop-controlled intersections. At signalized intersections, crashes can be reduced by 33%. Results will be slightly lower in urban areas.
A fork (literally "fork in the road") is a type of intersection. When a road splits, the main road steers to the left or right, depending of what side you drive on, and the smaller road heads straight. It is common for 2 lane roads. Heading toward the main road, the traveller must turn left or right. An example of this is the Brevard and Florida Avenue's intersection in Rockledge, Florida, If a road has a curb that sticks out, it is not classified as a fork.
In some places, wider white stop lines (see preceding diagram) indicate where vehicles should stop at an intersection when there is a stop sign or a red light in a traffic signal facing them. Some intersections have pedestrian crosswalks designated on the street pavement. Some possible markings for crosswalks are shown as examples. Note that the stop line is positioned to not allow stopped vehicles to block the crosswalk.
Ghost Island priority junctions are sometimes used in the United Kingdom to provide safer turning areas, which separate turning traffic from through traffic in a similar way to turn lanes (see above).
- ^ D.W. Harwood,et al., Safety Effectiveness of Intersection Left- and Right-Turn Lanes, Federal Highway Administration Office of Safety Research and Development, 2002, 
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