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.[1] When it is two freeways that share the same right-of-way, it is sometimes called a common section or commons.[2][3]
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 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 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.
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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.[4] 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 not signed 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.
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:
Also, in some cases, two Interstate highways can be concurrent. An example of this is the concurrency of Interstates 20 and 59 between west of Birmingham, Alabama, and east of Meridian, MS. 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.
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 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 run out of sequence: 5, 1, 2, 8, 7, 3.
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 even 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.
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.
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 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, 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.[5]
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.[6]
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, Missouri, 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.[7]
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.[8] Seven of the eight other designations overlap between exits 46 and 47 to create an eight-way concurrency.[9]
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.
Some consolidation schemes involve the use of incorporating two single-digit numbers onto one marker, as along the U.S. Route 1/9 concurrency in northern New Jersey.[10][11]
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.
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