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.
A 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. Though in some cases, concurrencies are avoided through various means.
Examples by nation 
United States 
In the United States, 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 this is the concurrency between 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. Another extreme example exists in North Dakota and Minnesota. At Jamestown, North Dakota, U.S. Route 52 begins a concurrency with Interstate 94 for almost 350 miles, ending at St. Paul east of Interstate 35E. Despite being signed on maps, US 52 is signed sporadically until the exit at either end of the concurrency.
A particularly unusual concurrency occurs along the Oklahoma–Arkansas state line. At the northern end of this border Oklahoma State Highway 20 runs concurrently with Arkansas Highway 43 and the two roads run north–south along the boundary.
Another unusual concurrency runs along the southern border of Delaware. Near the southwestern corner of the state, the eastbound lane of Delaware Route 54 is physically located in Maryland and is signed as Maryland Route 54, while the westbound lane is in Delaware and is signed as Delaware Route 54. Route 54 at its western end eventually enters Maryland entirely and both directions are signed as a Maryland route at that point.
In some states, a concurrency can occur between an Interstate highway and a state toll road. For example, much of the New Jersey Turnpike is concurrent with I-95, and portions of the New York State Thruway overlap with Interstates 87, 287, 90, and 190. The Massachusetts Turnpike is concurrent with I-90 for its entire length.
Some Interstate highways are concurrent with a non-Interstate designation in their entirety. Often, the reason is that at the ending of the Interstate highway, the road transitions from freeway to semi-freeway (expressway) or surface street, and the non-Interstate designation continues on the changed road. Sometimes it can be within a segment of a long-distance freeway that zips through a major city.
Here is a list of examples:
- Interstate 99 in Pennsylvania, concurrent with U.S. Route 220 in its entirety.
- I-194 in Michigan, concurrent with M-66 in its entirety.
- Not an actual Interstate, but (future) Interstate 22 is concurrent with U.S. Route 78 in its entirety in Mississippi and Alabama.
- Interstate 27 is concurrent with U.S. Route 87 in its entirety between Amarillo, TX and Lubbock, TX.
In the reverse example, there are sometimes non-Interstate highways that are concurrent with an Interstate highway in its entirety. Examples of this include Georgia's 400-series highways.
Also, in some cases, two Interstate highways can be concurrent. An example of this is the concurrency of Interstates 20 and 59 between Birmingham, Alabama, and Meridian, Mississippi. In other cases, instances of arc-shaped bypasses of medium-sized cities can involve concurrencies of Interstate Highways and other route types.
A unique concurrency can be found in Greensboro, North Carolina, where Interstate 40 runs concurrent with Business 85. Business I-85 runs along the former alignment of I-85, which has since been rerouted along the Greensboro Urban Loop. I-40 was also rerouted along this loop in 2008, but reverted to its original routing shortly afterward.
United Kingdom 
In the United Kingdom, it is common for major through routes to run concurrently with others. Only one road number—usually that of the more important 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 runs concurrently with the A85 for five miles (8 km) 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".
In central Scotland, the M876 starts from the M80 and shares its middle section with the M9, so that the junction numbers on the M876 appear to run out of sequence: 8, 1, 2, 8, 7, 3—with only 1, 2 and 3 being actual M876 junction numbers.
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. King's Highways in Ontario have many concurrencies, as well as county roads that often share concurrent termini or run concurrently for short sections. These are often not signed as such, with the parent route being signed in place of both.
Secondary highways in Ontario rarely ever form a concurrency, even with their own kind. Highway 518 instead parallels the Highway 11 freeway in less than a kilometer of it on Highway 11's former surface road as Ontario's secondary highways generally avoid freeway features, except interchanges at them, though some King's Highways run parallel to avoid concurrencies as well.
Certain primary highways in New Brunswick form concurrencies. Route 7 runs concurrent with the Trans-Canada Highway between Exits 294 and 306. Route 11 officially terminates at its junction with Route 15 near Shediac, but exit signs at Exit 467 of Route 2 show Route 11 running concurrent with Route 15 in both directions.
Two freeways, the Trans-Israel Highway (Highway 6), and Highway 1 run concurrently just east of Ben Shemen Interchange. The concurrency is officially designated "Daniel Interchange", providing half of the possible interchange directions. It is a 1-mile (1.6 km) segment consisting of eight lanes providing high-speed access between the two highways. Access from Highway 1 west to Highway 6 south and Highway 6 north to Highway 1 east is provided via Route 431, while access between Highway 1 east to Highway 6 north and Highway 6 south to Highway 1 west are provided at Ben Shemen Interchange. The other movements are provided through the concurrency.
East of Ashdod, Highway 3 and Highway 40, both running north–south, converge and cross each other, running concurrently for 2.5 miles (4.0 km) between Re'em Junction and Malakhi Junction. Southwest of Haifa, the north–south Highway 70 and the east–west Highway 75 run concurrently for 2.5 miles (4.0 km) between Yagur Junction and HaAmakim Junction.
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. Another well-traveled example is the I-580 and I-80 concurrency in the Bay Area of California, known locally as the Eastshore Freeway, which travels north-south between the eastern foot of the Bay Bridge and Albany, CA along the shoreline of San Francisco Bay.
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,[dubious ] and a short stretch of eastbound Interstate 376, as well as the ramps leading to it, in Pittsburgh, Pennsylvania, carries both directions of U.S. Route 19 Truck.
An unusual example of a three-directional concurrency occurs near the town of Starks, Illinois. To take advantage of an underpass beneath a railroad, US Route 20, Illinois Route 72 and Illinois Route 47 all converge. The net result is that a driver can be traveling east on US 20, west on IL 72, and south on IL 47 (the actual compass direction) all at the same time.
Canada also possesses at least four wrong-way concurrencies: an 11 km (6.8 mi) stretch of Saskatchewan Highways 2 and 11 between Chamberlain and Findlater, a 7 km (4.3 mi) stretch of British Columbia Highways 5 and 97 in Kamloops, a 10 km (6.2 mi) segment of New Brunswick Route 8 and Route 117 in Miramichi and 5.5 km (3.4 mi) segment of Highway 6 and Highway 21 west of Owen Sound, Ontario.
Some concurrencies have extreme examples such as triple, and even quadruple concurrencies. A triple Interstate concurrency is found north of Madison, WI, with I-39, I-90, and I-94. Also in Madison, there is a quadruple concurrency with four US Highways: US 12, US 14, US 18, and US 151 along the Beltline south of the city. St. Louis has an example of a quadruple Interstate concurrency, with I-44, I-55, I-64, and I-70, plus US 40 running concurrently for a brief run downtown. I-25 is concurrent with US 87 for 400 miles (640 km), running from Raton, New Mexico through the entire state of Colorado and all the way to I-25's northern terminus at I-90 in Buffalo, Wyoming, although US 87 is unsigned for most of this distance. The longest Interstate highway concurrency is I-80 and I-90 for 265 miles (426 km) across Indiana and Ohio. One of the shorter examples is the M-35 concurrency with CR 492 in Marquette County, Michigan. The two roads overlap for 153 feet (47 m) to cross a rail line, with M-35 yielding to CR 492 traffic in the process. Another short duplex is between US 49 and US 64 near Fair Oaks, Arkansas, a concurrency of 150 feet across a railroad.
I-465 around Indianapolis currently has the most concurrent routes. Portions of the 53-mile (85 km) highway overlap with I-74, US 31, US 36, US 40, US 52, US 421, SR 37 and SR 67 -- a total of eight other routes. Seven of the eight other designations overlap between exits 46 and 47 to create an eight-way concurrency.
Consolidation plans 
Some brief concurrencies in the past have been eliminated by reassigning the designations along the roadways. This can involve scaling back the terminus of a state highway to the end of a concurrent section. At the same time, there could be an extension of another highway designation that is used to replace the newly shortened designation with another one. For instance, US 27 in Michigan previously ran concurrently with I-69 from the Michigan–Indiana state line to the Lansing, Michigan, area. From there it turned northwards to its terminus at Grayling. In 2000, the Michigan and Indiana departments of transportation petitioned the American Association of State Highway and Transportation Officials for permission to truncate US 27 at Fort Wayne, Indiana. In 2002, Michigan removed the US 27 designation from I-69 and extended the US 127 designation from Lansing to Grayling.
Other plans to avoid concurrencies involve rerouting an older designation off a highway when a new moniker is being applied. When I-795 was applied to the US 117 freeway in North Carolina, instead of forming a concurrency, US 117 was instead returned to its original parallel alignment. There are even instances of a surface-street and freeway less than 5 miles in parallel where they both implement concurrencies; for instance, Interstate 40 is concurrent with U.S. Route 74 on the freeway part and U.S. Route 19 is concurrent with U.S. Route 23 on the surface street part in parallel between Clyde, NC and Asheville, NC. Then, U.S. Route 23 forms yet another concurrency when it diverts off the east-west surface street onto the "physically" north-south Interstate 26 to the north.
Some routes avoid concurrencies altogether by having their corridors physically cross or deviate within less than 500 feet (150 m), but the intersecting numbered routes ricochet their trajectory off the junction to avoid a concurrency. One example involves North Carolina Highway 111 and US 117 north of Goldsboro. I-80 and I-76 both switch from non-toll freeway to the Ohio Turnpike near Youngstown. Ontario Highway 403 goes northwest and diverts northeast where Ontario Highway 407 forms connector ramps as that highway curves from northeast to northwest on the same survey line; this was a civil engineering tactic that the MTO used to avoid having a cloverleaf junction at a four-way freeway crossing. These types of deviations are called "bumps".
Effect on exit numbers 
Often when two routes with exit numbers overlap, 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 along the same highway. An example of this is from the concurrency of I-94 and US 127 near Jackson, Michigan. The concurrent section of freeway has an exit with M-106, which is numbered Exit 139 using I-94's mileage-based numbers. US 127 also has another Exit 139 with the southern end of the US 127 business loop in Mount Pleasant, Michigan. However, there are also instances where the dominant exit number range is far more than the secondary route's highest exit number, for example the concurrency of I-75 and I-85 in Atlanta where I-75 is dominant—the exit numbers range from 242 to 251 while I-85's highest mile marker in Georgia is 179. I-40 in Tennessee (a neighboring state of Georgia), which is also concurrent with I-75, has the instance of its exit numbers in the concurrency range being higher than I-75's highest exit number in the state.
- Office of Highway System Engineering (August 1995). "State Highway Routes Selected Information, 1994 with 1995 Revisions" (PDF). California Department of Transportation. Route 3. Archived from the original on March 16, 2007. Retrieved March 7, 2012. "Conincident with Rte 299"
- "Freeway flaws; Fixing them may take decades". Star Tribune. June 3, 2005. "common sections ... 2 freeways share a single right-of-way"
- Reichard, Timothy. "MD 290/MD 291 Multiplex". Central PA/MD Roads. Retrieved March 7, 2012.
- Reichard, Timothy. "I-279/US 19/Truck US 19/US 22/US 30 Multiplexes". Central PA/MD Roads. Retrieved March 7, 2012.
- Staff. Sign for US 30, Route 47 and Route 72 (Highway guide sign). Starks, IL: Illinois Department of Transportation. http://fc02.deviantart.net/fs11/i/2006/243/6/6/Navigational_Conundrum_by_Sylderon.jpg. Retrieved March 7, 2012.
- Michigan Department of Transportation (2009). MDOT Physical Reference Finder Application (Map). Cartography by Michigan Center for Geographic Information. http://www.mcgi.state.mi.us/prfinder/. Retrieved April 13, 2010.
- "Interstate 465 Indiana". Interstate Guide. AARoads. January 18, 2012. Retrieved March 7, 2012.
- Indiana Department of Transportation (2007). Indiana Transportation Map (Map) (2007–08 ed.). Indianapolis inset.
- Staff. Signage for US 1/9, NJ 21, US 22, and I-78 (Highway guide sign). Newark, NJ: New Jersey Department of Transportation. http://commons.wikimedia.org/wiki/File:NJ_guide_sign.jpg. Retrieved December 5, 2009.
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