Intersection (road)

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A crossroads in Kharkiv
An intersection in Makati City

An intersection is the junction at-grade (that is to say, on the same level) of two or more roads either meeting or crossing. An intersection may be three-way (a T junction or Y junction – the latter also known as a fork if approached from the stem of the Y), four-way (often in the form of a crossroads), or have five (a 5-points) or more arms. Busy intersections are often controlled by traffic lights and/or a roundabout.

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 of intersections

Road segments

One way to classify intersections is by the number of road segments (arms) that are involved.

  • 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.
Intersection along the Veterans Memorial Parkway, an at-grade limited-access road in London, Ontario.

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

Turns

At intersections, turns are usually allowed, but 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)[2] may be provided. For example, in the intersection shown in the diagram, left turn lanes are present in the right-left street.

Turn lanes allow vehicles to cross oncoming traffic (i.e., a left turn in right-side driving countries, or a right turn in left-side driving countries), or to exit a 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 specific turns.

Turn lanes can increase the capacity of an intersection and/or 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 are slightly lower in urban areas.[3]

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 the directions pointed to by an arrow.

Turn signals

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,[4] 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 over which 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 oncoming through traffic. Without a separate phase, left crossing traffic does not get the full safety benefit of the turn lane.

Lane management

Alternative intersection configurations can manage turning traffic to increase safety and intersection throughput.[5] These include the Michigan left, "superstreet" and continuous flow intersection.

Diagram of an example intersection of two-way streets as seen from above (traffic flows on the right side of the road). The East-West street has left turn lanes from both directions, but the North-South street does not have left turn lanes at this intersection. The East-West street traffic lights also have green left turn arrows to show when unhindered left turns can be made. Some possible markings for crosswalks are shown as examples.

Pedestrians

Intersections generally must manage pedestrian as well as vehicle traffic. Pedestrian aids include crosswalks, pedestrian-directed traffic signals ("walk light") and over/underpasses. Walk lights may be accompanied by audio signals to aid the visually impaired. Medians can offer pedestrian islands, allowing pedestrians to divide their crossings into a separate segment for each traffic direction, possibly with a separate signal for each.

Cyclists

Intersections can be made more bicycle-friendly with various interventions such as bike boxes, protected intersections or grade separation.

See also

References

  1. ↑ Steyn, Hermanus. (2014). Displaced Left-turn Intersection Informational Guide. Washington, D.C.: U.S. Department of Transportation, Federal Highway Administration.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ D.W. Harwood,et al., Safety Effectiveness of Intersection Left- and Right-Turn Lanes, Federal Highway Administration Office of Safety Research and Development, 2002, [1]
  4. ↑ New York State Driver's Manual, Chapter 4.
  5. ↑ Lua error in package.lua at line 80: module 'strict' not found.

External links

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