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A one-way pair, one-way couple, or just couplet is a pair of parallel, usually one-way streets that carry opposite directions of a signed route or major traffic flow, or sometimes opposite directions of a bus or streetcar route.
The usual purpose is to provide higher capacity by increasing the number of lanes in each direction. It also allows easier creation of a green wave by adjusting traffic lights on the through route, because fewer phases are needed at each intersection.
"One-way pair" refers specifically to a single major route, divided across two streets; more extensive is when there is a grid of multiple alternating one-way streets, which is common is city centers. It is also common for frontage roads on either side of a freeway to be one-way pairs.
On a one-way pair, traffic usually flows the same way as on a normal divided highway – meaning opposing traffic follows the usual left/right convention of the country, e.g., in the United States and most of the world, northbound traffic on east side, southbound traffic on west side, following the "drive on the right" convention – though occasionally flow is switched. Following this convention allows a one-way pair to be more easily integrated into an existing network of two-way streets, as a single two-way street is effectively split into the two sides of the pair, as in the diagram below:
|/ ← ← ← ← ← ← ← /|
|/ ← ← ← ← ← ← ← /|
|← ← ←||/ → → → → → → → /||← ← ←|
|→ → →||/ → → → → → → → /||→ → →|
I-78 travels along a one-way pair of surface streets, 12th Street and 14th Street, in Jersey City, New Jersey, between the end of the New Jersey Turnpike Newark Bay Extension and the Holland Tunnel, which leads into New York City.
There are literally hundreds of one-way pairs among the streets and avenues of New York City. One famous example is Fifth Avenue with Madison Avenue. Others include First Avenue with Second Avenue; Third Avenue with Lexington Avenue; and Seventh Avenue with either Sixth Avenue or Eighth Avenue.
The east side of Portland, Oregon features a number of one-way pairs, both north-south and east-west, with the east-west pairs being associated with bridges; these all follow the usual flow convention – see Transportation in Portland, Oregon for more details. By contrast, the Portland Transit Mall, which is a public transportation (bus and rail) corridor, has the opposite flow, with the westernmost component (6th Ave) running north, with the eastern component (5th Ave) running south.
There are a number of one-way pairs in Downtown Los Angeles. These include 3rd and 4th Streets, 5th and 6th Streets, 8th and 9th Streets, 11th and 12th Streets, and Main and Spring Streets.
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