Concurrency (road)

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Signs for a concurrency of Interstate 93, U.S. Route 1 and Route 3 in Boston, Massachusetts

A concurrency, overlap or coincidence^[1] on a road network occurs where a single physical road carries two or more different highway, motorway, or other road numbers. Multiplex (and the more specific terms duplex and triplex) is a neologism, albeit well-used, used by roadgeeks for this concept, as an extension of other meanings of the term.

The practice is relatively common; where two roads must pass through a single geological feature, or crowded city streets, it is advantageous for road designers to merely divert them down the same road, saving on the cost of a new road.

1 United States
2 United Kingdom
3 Canada
4 References

[edit] United States

An extreme example: I-40/Business 85/US 29/US 70/US 220/US 421 (also known as Death Valley) in Greensboro, North Carolina

In the United States, highways often form concurrencies in rural areas. As highways in the U.S. are signed with a cardinal direction, it is possible for two highways signed with opposite directions to be running along the same piece of road, a "wrong-way concurrency," such as Interstate 77 and Interstate 81 near Wytheville, Virginia. Most of the time, concurrencies are simply marked by placing signs for both routes on the same post; occasionally a state will instead sign the road as "to" the less major route. Several states don't officially have any concurrencies, instead officially ending routes on each side of one. In these states, concurrencies are typically poorly signed. In the mid-20th century, California had numerous concurrencies, but the California Legislature removed most concurrencies in a comprehensive reform of highway numbering in 1964.

Eastern/Northern terminus of OK-20/AR-43 at MO-43 near Southwest City, Missouri

A particularly unusual concurrency occurs along the Oklahoma/Arkansas state line. At the northern end of this border Oklahoma State Highway 20 concurs with Arkansas Highway 43 and the two roads run north/south on the boundary. In its northernmost mile, the highway curves to the west, at which point OK-20/AR-43 is entirely within the State of Oklahoma.

At least two roads run concurrently with their opposite direction. A short stretch of Broadway in Pawtucket, Rhode Island carries both directions of Route 114, and a short stretch of northbound Interstate 279, as well as the ramps leading to it, in Pittsburgh, Pennsylvania carries both directions of Truck U.S. Route 19.

In some states, a concurrency can occur between an interstate highway and a state toll road. For example, much of the New Jersey Turnpike concurs with Interstate 95, and portions of the New York State Thruway concur with Interstate 87, Interstate 287, Interstate 90, Interstate 84, and Interstate 190. (Actually, I-84 is free except for the one-way toll on the Beacon-Newburgh bridge over the Hudson.)

[edit] United Kingdom

In the United Kingdom, it is common for major through routes to run concurrently with others. Only one road number (typically that of the more heavily used route) is ever shown on road signs however; the other road is either bracketed on the sign, implying that the major route leads to a junction with the minor route (which it will do at the end of the concurrency), or left off altogether. For example, the A82 concurs with the A85 for five miles in western Scotland. Each route-confirmation sign-header gives the road number as "A82 (A85)". A counter-example is the concurrency of the A6 and A591 south of Kendal, where, unusually, a sign gives both roads equal status as "A591/A6".