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.

Often times when two routes with exit numbers overlap (concurrency), one of the routes has its exit numbers dominate over the other and can sometimes result in having two exits of the same number, albeit far from each other for the same route number.

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 or adjacent posts; occasionally a state will instead sign the road as "to" the less major route. An example of the latter is the concurrency of Maryland Route 290 and Maryland Route 291 in Kent County, Maryland, where MD 290, the less major route, is signed as "to" MD 290 along MD 291, the more 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, 276, and 476 concur with the Pennsylvania Turnpike, and Interstate 76 concurs with a part of the Ohio Turnpike. The rest of the Ohio Turnpike is part of Interstate 80, and much of that stretch is also part of Interstate 90. Interstate 80/90 continues as such onto the Indiana Toll Road, with I-80 leaving that toll road in the Chicago area and I-90 staying on it all the way to Illinois.

Some Interstate highways are concurrent with a non-Interstate designation in their entirety. Often times, the reason for the Interstate ending with a simultaneous continuation with an non-Interstate designation on a transition from freeway to semi-freeway (expressway) or backroad/surface street.

Here is a list of examples:

• Interstate 27 in Texas, concurrent with U.S. Route 87 in its entirety.
• I-194 in Michigan, concurrent with M-66 in its entirety.

Also, in some cases, two interstate highways can be concurrent. A example of this is the concurrency of Interstates 20 and 59 between west of Birmingham, AL and west of Meridian, MS.

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

Concurrencies are also found in Canada. In Manitoba, the Trans-Canada Highway from Winnipeg to Portage La Prairie is concurrently signed with Yellowhead Highway. In Ontario, the Queen Elizabeth Way and Highway 403 run concurrently between Burlington and Oakville, forming the province's only concurrency between two 400-series highways. Kings Highways in Ontario have many concurrencies, as well as county roads that often share concurrent termini or run concurrently for short sections.

Wrong-way concurrency

As highways in the United States and Canada are usually 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 and 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.

Canada also possesses at least three wrong-way concurrencies: an 11 km (7 mi) stretch of Saskatchewan Highways 2 and 11 between Chamberlain and Findlater, a 7 km (4 mi) stretch of British Columbia Highways 5 and 97 in Kamloops, and a 1.5 km stretch of Highways 12 and 99 in Lillooet.

Concurrency extremes

Some concurrencies have extreme examples such as triple, and even quadruple concurrencies. Also, Interstate 25 is concurrent for US-87 for 400 miles, and runs through the entire state of Colorado.

Consolidation plans for concurrencies

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 efforts to consolidate concurrencies along with simultaneous consolidation of route numbering involves U.S. Route 10 in Michigan which used to multiplex with I-75/US-23 and also multiplexed U.S. Route 24. During that time US-10 had its terminus scaled back to Bay City, MI so US-10 on the Lodge Freeway would derive into M-10 and M-4 would be deprecated as an extension to "M-10". Despite the consolidation overhaul, trailblazers in the Metro Detroit Area for "Route 10" still depict it as "US-10" on some signs.

Other consolidation schemes involve the use of incorporating 2 single digit numbers into one shield, for instance U.S. Route 1/9 in New Jersey is a fairly long route which saves signage doing it this way.

Other miscellaneous concurrencies

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.

References

1. State Highway Routes Selected Information, 1994 with 1995 Revisions (PDF) - see Route 3 for instance]
2. Star Tribune, Freeway flaws; Fixing them may take decades, June 3, 2005: defines "common sections" as "2 freeways share a single right-of-way"
3. Minnesota Department of Transportation, I-494 and I-35W Interchange Reconstruction, accessed October 2007: gives the AADT at several such interchanges, calling them "commons"

External links

• Highway Multiplex Photo Gallery