One-way system

One-way traffic (or uni-directional traffic) is traffic that moves in a single direction. A one-way street is a street either facilitating only one-way traffic, or designed to direct vehicles to move in one direction. One-way streets typically result in higher traffic flow as drivers may avoid encountering oncoming traffic or turns through oncoming traffic. Residents may dislike one-way streets due to the circuitous route required to get to a specific destination, and the potential for higher speeds adversely affecting pedestrian safety. Some studies even challenge the original motivation for one-way streets, in that the circuitous routes negate the claimed higher speeds.

Signs are posted showing which direction the vehicles can move in: commonly an upward arrow, or on a T junction where the main road is one-way, an arrow to the left or right. At the end of the street through which vehicles may not enter, a prohibitory traffic sign "Do Not Enter", "Wrong Way", or "No Entry" sign is posted, e.g. with that text, or a round red sign with a white horizontal bar. Sometimes one portion of a street is one-way, another portion two-way. An advantage of one-way streets is that drivers do not have to watch for vehicles coming in the opposite direction on this type of street.

The abstract "No Entry" sign was officially adopted for standardization at the League of Nations convention in Geneva in 1931. The sign was adapted from Swiss usage, derived from the practice of former European states that marked their boundaries with their formal shield symbols. Restrictions on entry were indicated by tying a blood-red ribbon horizontally around the shield. The sign is also known as C1, from its definition in the Vienna Convention on Road Signs and Signals.

The European "No Entry" sign was adopted into North American uniform signage in the 1970s, replacing a previous white rectangular sign bearing only the English text "Do Not Enter". In addition to the standardized graphic symbol, the US version still retains the wording "Do Not Enter", while the European and Canadian versions typically have no text.

Since Unicode 5.2, the Miscellaneous Symbols block contains the glyph ⛔ (U+26D4 NO ENTRY), representable in HTML as ⛔ or ⛔.

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