At-grade intersection

An intersection is an junction where two or more roads meet or cross. Intersections may be classified by number of road segments, traffic controls, and/or 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.

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

Another way of classifying intersections is by traffic control technology:

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.

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.

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.

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.

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.

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