A concurrency in a road network is an instance of one physical road bearing two or more different highway, motorway, or other route numbers. When two freeways share the same right-of-way, it is sometimes called a common section or commons. Other terminology for a concurrency includes overlap, coincidence, duplex (two concurrent routes), triplex (three concurrent routes) and multiplex (any number of concurrent routes).
Concurrent numbering can become very common in countries that allow it. Where multiple routes must pass through a single mountain crossing, or through a major city, it is often economically and practically advantageous for them all to be accommodated on a single physical road. In some countries, however, concurrent numbering is avoided by posting only one route number on road signs; these routes disappear at the start of the concurrency and reappear when it ends. Criticism of concurrencies include environmental intrusion, as well as being considered a factor in road accidents.
Most concurrencies are simply a combination of two route numbers on the same physical road. This is often practically advantageous as well as economically advantageous; it may be better for two route numbers to be combined into one along riverways or through mountain valleys. Some nations allow for concurrencies to occur, however some nations specifically do not allow it to happen. In those nations which do permit concurrencies, it can become very common. In these countries, there are a variety of concurrences which can occur.
An example of this is the concurrency of I-70 and I-76 on the Pennsylvania Turnpike in western Pennsylvania. Interstate 70 merges with the Pennsylvania Turnpike so the route number can ultimately continue into eastern Maryland; instead of having a second physical highway built to carry the route, it is combined with the Pennsylvania Turnpike and the Interstate 76 designation. A triple Interstate concurrency is found north of Madison, Wisconsin, with I-39, I-90, and I-94.
I-25 is concurrent with US 87 for approximately 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 County Road 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.
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.
In the United States, concurrencies are simply marked by placing signs for both routes on the same or adjacent posts. Several states do not officially have any concurrencies, instead officially ending routes on each side of one.[a] There are several circumstances where unusual concurrencies exist along state borders. One example 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.
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.
Europe and the Middle East
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 (8.0 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".
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.
In Sweden and Denmark, the most important highways use only the European route numbers, which have cardinal directions. In Sweden the E6 and E20 go concurrent for 280 kilometres (170 mi). In Denmark the E47 and E55 go concurrent for 157 kilometres (98 mi). There are more shorter concurrencies. There are two stretches in Sweden and Denmark where three European routes go concurrent. These are E6, E20 and E22 in Sweden, and E20, E47 and E55 in Denmark. Along all these concurrencies, all route numbers are signposted.
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.  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.
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 (although in one case the number 96 is only six miles apart). 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, Georgia—where I-75 is dominant—the exit numbers range from 242 to 251 while I-85's highest mile marker in Georgia is 179.
Some brief concurrencies in the past have been eliminated by reassigning the designations along the roadways. This can involve scaling back the terminus of one designation 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.
Between states, 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 1999, 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. MDOT's stated reason for the modification was to "reduce confusion along the US 27/US 127 corridor". After US 27's signage was removed, the highway north of the Lansing area was renumbered US 127, and the US 27 designation was removed from I-69.
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. In the mid-20th century, California had numerous concurrencies, but the California Legislature removed most of them in a comprehensive reform of highway numbering in 1964.
- Staff (March 4, 2014). "Realigning Concurrent Routes". ArcGIS Help 10.2 & 10.2.1. Esri. Retrieved April 8, 2014.
- "Freeway flaws; Fixing them may take decades". Star Tribune (Minneapolis). June 3, 2005. "common sections ... 2 freeways share a single right-of-way"
- Staff (December 19, 2012). "Realigning Overlapping Routes". ArcGIS Resource Center. Esri. Retrieved April 8, 2014.
- 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"
- Reichard, Timothy. "Guide to Highway Multiplexes". Central PA/MD Roads. Retrieved April 8, 2014.
- "Reducing Sign Clutter". Traffic Advisory Leaflet 01/13. Department for Transport. January 2013.
- Ho, Geoffrey; Scialfa, Charles T.; Caird, Jeff K. & Graw, Trevor (Summer 2001). "Visual Search for Traffic Signs: The Effects of Clutter, Luminance, and Aging". Human Factors 43 (2): 194–207. doi:10.1518/001872001775900922. ISSN 0018-7208. PMID 11592661.
- Pennsylvania Department of Transportation (2010). Tourism & Transportation Map (Map). Cartography by Geographic Information Section, Bureau of Planning and Research. Section E10–L11.
- Office of Public Affairs (August 17, 2010). "Wisconsin Highway Trivia". Wisconsin Department of Transportation. Retrieved April 8, 2014.
- Special Committee on U.S. Route Numbering (1989) (PDF). United States Numbered Highways (Report). American Association of State Highway and Transportation Officials. p. 134. http://route.transportation.org/Documents/USRN-page101_186.pdf. Retrieved April 8, 2014.
- Staff (December 31, 2013). "Table 1: Main Routes of the Dwight D. Eisenhower National System Of Interstate and Defense Highways as of December 31, 2013". Route Log and Finder List. Federal Highway Administration. Retrieved April 8, 2014.
- 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.
- Indiana Department of Transportation (2007). Indiana Transportation Map (Map). Cartography by INDOT (2007–08 ed.). Indianapolis inset.
- Planning and Research Division (April 2010). State Highways 2009 (ZIP) (Database). Arkansas State Highway and Transportation Department. Retrieved April 11, 2011.
- Arkansas State Highway and Transportation Department (2010). State Highway Map (Map). 1 in=15 mi. Cartography by Planning and Research Division. Section A1.
- Manitoba Ministry Infrastructure and Transportation (2012) (in English and French). Official Highway Map / Carte Routière Officielle (Map). Cartography by MIT. Section D2–E2.
- Ministry of Transportation of Ontario (2010) (in English and French). Official Road Map / Carte Routière (Map). 1:250,000. Cartography by Geomatics Office (2010–11 ed.). Section R25.
- Virginia Department of Transportation (2012). Official State Transportation Map (Map). 1 in≈13 mi. Cartography by VDOT (2012–14 ed.). Section F6–G6.
- Staff. Sign for US 30, Route 47 and Route 72 (Highway guide sign). Starks, IL: Illinois Department of Transportation. Retrieved March 7, 2012.
- Michigan Department of Transportation (2013). State Transportation Map (Map). 1 in:15 mi / 1 cm:9 km. Cartography by MDOT. Section J10, M11.
- Georgia Department of Transportation (2011). Official Highway and Transportation Map (Map). Cartography by GDOT (2011–12 ed.). Main map, section B1, I2; Atlanta inset, section E5.
- Zink, Ray (April 17, 1999). "Report of the Special Committee on Route Numbering to the Standing Committee on Highways" (PDF). American Association of State Highway and Transportation Officials. Retrieved May 24, 2008.
- Ranzenberger, Mark (April 27, 2008). "US 127 Signs Getting Updated". The Morning Sun (Mount Pleasant, MI). pp. 1A, 6A. OCLC 22378715. Retrieved August 23, 2012.
- Debnar, Kari & Bott, Mark (January 14, 2002). "US 27 Designation Soon To Be Deleted from Michigan Highways" (PDF) (Press release). Michigan Department of Transportation.
- Staff. Signage for US 1/9, NJ 21, US 22, and I-78 (Highway guide sign). Newark, NJ: New Jersey Department of Transportation. Retrieved December 5, 2009.
- "Route Renumbering: New Green Markers Will Replaces Old Shields" (PDF). California Highways and Public Works 43 (1–2): 11–14. March–April 1964. ISSN 0008-1159. Retrieved March 8, 2012.
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