||The examples and perspective in this article may not represent a worldwide view of the subject. (November 2011)|
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/T intersection or fork, 4-way – a crossroads, or 5-way or more. It may often be controlled by traffic lights, and may have a roundabout (traffic circle in America).
This article primarily reflects 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 of intersections
Intersections are classified as 3-way, 4-way, 5-way, 6-way, etc. depending on the number of road segments (arms) that come together at the intersection.
- 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 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 (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.
- 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, or other cases of flow limitation, certain turns may be not allowed or may be limited by regulatory signs or signals.
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 intersections with large proportions of turning traffic, turn lanes (also known as turn bays) may be provided 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.
Turn lanes may be provided to allow vehicles to turn across oncoming traffic (i.e., a left turn in right-side driving countries, or a right turn in left-side driving countries), or to turn off the road without crossing traffic (i.e., a right turn in right-side driving countries, or a left turn in left-side driving countries). Absence of a turn lane does not normally indicate a prohibition of turns in that direction. Instead, traffic control signs are used to prohibit turns.
Turn lanes may be provided to increase the capacity of an intersection, or to improve safety. 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.
Turn lanes are marked with an arrow bending into the direction of the turn which is to be made from that lane. Multi-headed arrows indicate that vehicle drivers may travel in any one of directions shown.
Traffic signals facing vehicles in turn lanes often have arrow-shaped indications. Green arrows indicate protected turn phases, when vehicles may turn unhindered by oncoming traffic. Red arrows may be displayed to prohibit turns in that direction. Red arrows may be displayed along with a circular green indication to show that turns in the direction of the arrow are prohibited, but other movements are allowed. In some jurisdictions, a red arrow prohibits a turn on red, while in others, it does not.
Disadvantages to turn lanes include increased pavement area, with associated increases in construction and maintenance costs, as well as increased amounts of stormwater runoff. They also increase the distance that pedestrians crossing the street are exposed to vehicle traffic. If a turn lane has a separate signal phase, it often increases the delay experienced by the through traffic going the other way. If it doesn't have a separate phase, the left turn traffic does not get the full safety benefit of the turn lane.
In lieu of turn lanes, there are several different intersection configurations that divert turning traffic elsewhere to increase safety and intersection throughput. These include the Michigan left, "superstreet" and continuous flow intersection.
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 traveler must turn left or right. 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).
- "Manual on Uniform Traffic Control Devices (MUTCD), Part 1" (PDF). U.S. DOT, Federal Highway Administration. December 11, 2009. Retrieved November 28, 2011.
- D.W. Harwood,et al., Safety Effectiveness of Intersection Left- and Right-Turn Lanes, Federal Highway Administration Office of Safety Research and Development, 2002, 
- New York State Driver's Manual, Chapter 4.
- Badger, Emily. "Could These Crazy Intersections Make Us Safer?". The Atlantic Cities. Atlantic Media Company. Retrieved 29 January 2013.
|This article needs additional citations for verification. (January 2008)|
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