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Concurrency (road)
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A concurrency, overlap, or coincidence in a road network is an instance of one physical road bearing two or more different highway, motorway, or other route numbers. When it is two freeways that share the same right-of-way, it is sometimes called a common section or commons.
Road enthusiasts often use the term multiplex – as well as the more specific duplex and triplex – to refer to such instances although those type are the more common instances.
Concurrency is a relatively common phenomenon: where two routes must pass through a single geological feature, or crowded city streets, it is often both economically and practically advantageous for them both to be accommodated on one road.
he United States, highways often form concurrencies in rural areas.
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Encyclopedia
A concurrency, overlap, or coincidence in a road network is an instance of one physical road bearing two or more different highway, motorway, or other route numbers. When it is two freeways that share the same right-of-way, it is sometimes called a common section or commons.
Road enthusiasts often use the term multiplex – as well as the more specific duplex and triplex – to refer to such instances although those type are the more common instances.
Concurrency is a relatively common phenomenon: where two routes must pass through a single geological feature, or crowded city streets, it is often both economically and practically advantageous for them both to be accommodated on one road.
Concurrencies by nation
United States
In the United States, highways often form concurrencies in rural areas. 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.
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 along the boundary.
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 Interstates 87, 287, 90, 84, and 190 (actually, I-84 is free except for the one-way toll on the Beacon-Newburgh bridge over the Hudson). Also, Interstates 70, 76, and 476 concur with the Pennsylvania Turnpike, and Interstate 76 concurs with a part of the Ohio Turnpike.
Some brief concurrencies in the past have been eliminated by scaling back the terminus of a state trunkline at the route it was formerly concurrent with, and at the same time can have an upgrade of a road segment to state highway standards to replace its designation with the other one; for instance, M-47 in Michigan used to run concurrent with M-46 for only a few miles. Meanwhile a gap between M-52's northern terminus at M-36 and M-47's southern terminus at I-96 during the 1960s was filled in to replace much of M-47 with an extension of M-52 thereby eliminating M-47's concurrency with M-46 in 1969. Incidentally, M-47's current routing is not part of any of its original alignment.
Other concurrencies can involve a special unnumbered tourist route with a unique route shield being concurrent with numbered highways in its entirety. Examples of this include Lake Superior Circle Tour, Lake Huron Circle Tour, Lake Michigan Circle Tour and Lake Erie Circle Tour. However Lake Erie's circle tour route is an exception since it is a lone designation for the Ambassador Bridge. All of the Great Lakes Circle Tour routes have a small percentage of concurrency with Interstate highways. Interstate 75 is the most common example of an Interstate highway being concurrent with the routes since all of the circle tour highways are concurrent with it. However, Lake Ontario is an exception since it doesn't even touch Michigan, nor does it even have a circle tour route.
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".
Canada
In Canada, it is common for a highway to carry more than one designation. In Manitoba the Trans-Canada Highway from Winnipeg to Portage La Prairie carries both Trans-Canada Highway and Yellowhead Highway signage. In the Toronto area, the Queen Elizabeth Way carries Highway 403 east-west from Hamilton at the junction of the QEW and 407 ETR to Oakville.
Wrong-way concurrency
As highways in the U.S. are signed with a cardinal direction, it is possible for two highways signed with opposite, conflicting directions to be running along the same stretch of physical roadway. The road itself is likely to be actually pointed in a third direction.
For example, near Wytheville, Virginia, there is a concurrency between Interstate 77 (which runs primarily northwest-southeast but is signed north-south) and Interstate 81 (which runs primarily northeast-southwest but is also signed north-south). The road itself is oriented east-west and carries the two Interstates signed in opposite directions. So one might simultaneously be on I-77 North and I-81 South, while actually traveling due west.
At least two roads run concurrently with their own 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 U.S. Route 19 Truck.
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