A concurrency in a road network is an instance of one physical roadway bearing two or more different route numbers.  When two roadways 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), multiplex (any number of concurrent routes),  dual routing or triple routing.  
Concurrent numbering can become very common in jurisdictions that allow it. Where multiple routes must pass between a single mountain crossing or over a bridge, or through a major city, it is often economically and practically advantageous for them all to be accommodated on a single physical roadway. In some jurisdictions, however, concurrent numbering is avoided by posting only one route number on highway signs; these routes disappear at the start of the concurrency and reappear when it ends. However, any route that becomes unsigned in the middle of the concurrency will still be signed on most maps and road atlases.
Most concurrencies are simply a combination of at least two route numbers on the same physical roadway. This is often practically advantageous as well as economically advantageous; it may be better for two route numbers to be combined into one along rivers or through mountain valleys. Some countries allow for concurrencies to occur, however, others 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 Interstate 70 (I-70) and I-76 on the Pennsylvania Turnpike in western Pennsylvania. I-70 merges with the Pennsylvania Turnpike so the route number can ultimately continue east into Maryland; instead of having a second physical highway built to carry the route, it is combined with the Pennsylvania Turnpike and the I-76 designation.  A triple Interstate concurrency is found in Wisconsin along the five-mile (8.0 km) section of I-41, I-43, and I-894 in Milwaukee, Wisconsin.  The concurrency of I-41 and I-43 on this roadway is an example of a wrong-way concurrency.
The longest Interstate highway concurrency is I-80 and I-90 for 265 miles (426 km) across Indiana and Ohio, while the longest Interstate highway concurrency with three interstates is I-39, I-90, and I-94 in Portage, Wisconsin for over 29 miles (47 km). 
There are at least two examples of eight-way concurrencies. The first example is in Indianapolis, between exits 46 and 47 of the 53-mile (85 km) I-465 beltway, where the highway is concurrent with U.S. Route 31 (US 31), US 36, US 40, US 52, US 421, State Road 37 (SR 37), and SR 67.  Once I-69 is extended south of Indianapolis, this segment will have a nine-way concurrency. The second example is in downtown Athens, Georgia, between exits 4 and 8 of Georgia State Route 10 Loop, where the highway is concurrent with US 29, US 78, US 129, US 441, State Route 8 (SR 8), SR 15, and SR 422. 
In the United States, concurrencies are simply marked by placing signs for both routes on the same or adjacent posts. The federal Manual on Uniform Traffic Control Devices prescribes that when mounting these adjacent signs together that the numbers will be arranged vertically or horizontally in order of precedence. The order to be used is Interstate Highways, U.S. Highways, state highways, and finally county roads, and within each class by increasing numerical value. 
Several states do not officially have any concurrencies, instead officially ending routes on each side of one. [lower-alpha 1] 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 highways run north–south along the boundary. 
Concurrencies are also found in Canada. British Columbia Highway 5 continues east for 12 kilometres (7.5 mi) concurrently with Highway 1 and Highway 97, through Kamloops. This stretch of road, which carries Highway 97 south and Highway 5 north on the same roadway (and vice versa), is the only wrong-way concurrency in British Columbia. Concurrencies are also very common in Quebec. Most notably, the Samuel-de-Champlain Bridge features a concurrency with three Autoroutes: A-10, A-15, and A-20. Another example is A-55, which runs concurrently with A-10, A-20, and A-40, all of which are major highways.
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.  The concurrency was not in the original plan which intended for both the QEW and Highway 403 to run parallel to each other, as the Hamilton–Brantford and Mississauga sections of Highway 403 were initially planned to be linked up along a corridor (later planned to be tolled) now occupied by Highway 407. To avoid forcing drivers to pay tolls to use a section of a continuous Highway 403, the new link was designated as a western extension of the tolled Highway 407, with the Mississauga section of Highway 403 planned to be renumbered as Highway 410. The renumbering to 410 never came to pass,  and consequently Highway 403 was signed concurrently along the Queen Elizabeth Way in 2002, remedying the discontinuity. Nonetheless, many surface street signs referring to that section of freeway with the QEW/Highway 403 concurrency still only use the highway's original designation of QEW, although the MTO has updated route markers on the QEW to reflect the concurrency. 
At the national level, the Trans-Canada Highway, which does not bear a uniform number in the eastern provinces, follows various provincial highways. In the Atlantic Provinces the main designated TCH route either follows a single numbered route across each province (an exception being the switching of the designation between Nova Scotia Highways 104 and 105), or has branches that are signed exclusively as TCH routes. In Ontario and Quebec, The TCH follows a series of provincial highways, and also has branches that follow sections of others that have concurrencies with it, signed with TCH shields alongside the provincial number.
In the United Kingdom, routes do not run concurrently with others. Where this would normally occur, the roadway takes the number of only one of the routes (usually, but not always, the most important route), while the other routes are considered to have a gap and are signed in brackets (the equivalent of "to" signs in North America). An example is the meeting of the M60 and the M62 northwest of Manchester: the motorways coincide for the seven miles (11 km) between junctions 12 and 18 but the motorway between those points is only designated as the M60 (although in this case the same junction numbers would also apply to the M62). European route numbers as designated by UNECE may have concurrencies (for instance E15 and E30 around Greater London), but since the E-route numbers are unsigned and unused in the UK, the existence of these concurrencies is purely theoretical.
In Sweden and Denmark, the most important highways use only the European route numbers that have cardinal directions. In Sweden the European route E6 and E20 run concurrently for 280 kilometres (170 mi). In Denmark the E47 and E55 run concurrently for 157 kilometres (98 mi). There are more shorter concurrencies. There are two stretches in Sweden and Denmark where three European routes run concurrently; these are E6, E20 and E22 in Sweden, and E20, E47, and E55 in Denmark. Along all these concurrencies, all route numbers are posted with signs. 
In the Czech Republic, the European route numbers are only additional, and they are always concurrent with the state route numbering, usually highways or first-class roads. In the state numbering system, concurrences exist only in first-class and second-class roads; third class roads do not have them. The local term for such concurrences is peáž (from the French word péage). In the road register, one of the roads is considered the main ("source") road and the others as the péaging (guest) roads. The official road map enables a maximum of five concurrent routes of the intrastate numbering system.  Cycling routes and hiking routes are often concurrent.
In Israel, 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 one-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. 
Since highways in the United States and Canada are usually signed with assigned cardinal directions based on their primary orientation, it is possible for a stretch of roadway shared between two highways to be signed with conflicting, even opposite, cardinal directions in a wrong-way concurrency.[ citation needed ] For example, near Wytheville, Virginia, there is a concurrency between Interstate 77 (which runs primarily north–south, as it is signed) and Interstate 81 (which runs primarily northeast–southwest, but is also signed north–south). Because of the way they intersect, the section of Interstate where they overlap has the two roads signed for opposite directions, leading to the town's nickname of "Which-Way-Ville".  A vehicle might simultaneously be on I-77 northbound and I-81 southbound, while actually traveling due westbound.  An unusual example of a three-directional concurrency occurs southeast of Rhinelander, Wisconsin, where US 8 westbound (the actual compass direction) converges with southbound Wisconsin Highway 17 and northbound Wisconsin Highway 47.[ citation needed ]
An example of a letter-suffixed wrong-way concurrency which is technically in four cardinal directions, is in Knoxville, Tennessee, where southbound US 11E converges with northbound US 25W and westbound US 70 (which is the real compass direction).[ citation needed ]
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. (US 127's mile markers in Michigan reflect the cumulative distance north of the Ohio state line; the numbers resume north of the I-94 overlap and reflect the distance accumulated on that concurrency.) 
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 independent 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 US 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. 
M-89 is an east–west state trunkline highway in the US state of Michigan that runs from near Ganges to Battle Creek. M-89 starts at an interchange with Interstate 196/US Highway 31 (I-196/US 31) and passes through Allegan, Kalamazoo County, Michigan, and Calhoun counties. The highway also briefly crosses the southwest corner of Barry County before it terminates an at intersection with Business Loop I-94 on the northwestern side of downtown Battle Creek. In between the trunkline runs parallel to the Kalamazoo River through rural southwestern Michigan farmlands while also running through the middle of several smaller towns in the area.
US Route 223 or US Highway 223 (US 223) is a diagonal (northwest–southeast) United States Numbered Highway lying in the states of Michigan and Ohio. The southernmost section is completely concurrent with the US 23 freeway, including all of the Ohio segment. It connects US 23 in the south near Toledo, Ohio, with US 127 south of Jackson, Michigan. The highway passes through farmland in southern Michigan and woodland in the Irish Hills. Including the concurrency on the southern end, US 223 is 46.34 miles (74.58 km) in total length.
Interstate 496 (I-496) is an auxiliary Interstate Highway that passes through downtown Lansing in the US state of Michigan. Also a component of the State Trunkline Highway System, the freeway connects I-96 to the downtown area. It has been named the R.E. Olds Freeway for Ransom E. Olds, the founder of Oldsmobile and the REO Motor Car Company. I-496 runs east–west from I-96/I-69 near the downtown area and north–south along a section that runs concurrently with US Highway 127 (US 127). The trunkline also passes a former assembly plant used by Oldsmobile and runs along or crosses parts of the Grand and Red Cedar rivers.
M-5, commonly referred to as Grand River Avenue and the northern section as the Haggerty Connector, is a 27.9-mile-long (44.9 km) state trunkline highway in the Metro Detroit area of the US state of Michigan. The highway runs through suburbs in Oakland and Wayne counties in addition to part of Detroit itself. It starts in Commerce Township as a north–south divided highway and freeway called the Haggerty Connector and connects with Interstate 96 (I-96) in Novi. The freeway then turns southeasterly to bypass the suburb of Farmington as an east–west highway. The freeway ends on the southeast side of Farmington, and M-5 follows Grand River Avenue as boulevard into Detroit. The eastern terminus is at the five-way intersection between Grand River Avenue, Cass Avenue, and Middle Street in Downtown Detroit, where it had been extended to in May 2016 from an interchange with I-96 on the northwest side of the city. The trunkline passes between suburban residential subdivisions and along urban commercial areas while serving 17,200–68,800 vehicles on average each day.
M-79 is an east–west state trunkline highway in the central portion of Lower Peninsula of the U.S. state of Michigan. The western terminus is about three miles (4.8 km) southeast of Hastings at the junction with M-37 and the eastern terminus is in downtown Charlotte at the junction with M-50 and Business Loop Interstate 69 (BL I-69). It passes through Quimby and Nashville, where there is a junction with M-66. The entire highway is undivided surface road. It has no direct access with Interstate 69 (I-69), although a sign for the highway is located on southbound I-69 at exit 61.
M-50 is a state trunkline highway in the US state of Michigan. Although designated as an east–west highway, it is nearly a diagonal northwest–southeast route. The western terminus is at exit 52 along Interstate 96 (I-96) near Alto a few miles east of the metro Grand Rapids area, and its eastern terminus is in downtown Monroe at US Highway 24. In between the trunkline runs through seven counties of the southern part of the Lower Peninsula mostly through rural farm fields and small communities. The highway also runs through downtown Jackson to connect between two freeway sections of US 127. In the Irish Hills area of the state southeast of Jackson, M-50 runs next to Michigan International Speedway.
M-15 is a north–south state trunkline highway in the US state of Michigan. The southern terminus is a junction with US Highway 24 (US 24) just south of Clarkston on the northwestern edge of the Detroit metropolitan area. The trunkline is a recreational route running north and northwest to the Tri-Cities area. The northern terminus is the junction with M-25 on the east side of Bay City. The total length is about 73+2⁄3 miles (118.6 km) between the two regions.
M-108 was a state trunkline highway in the US state of Michigan. The highway followed Nicolet Street, although some maps also labeled it as Mackinaw Highway. The road was on the boundary between Emmet and Cheboygan counties.
M-45 is a state trunkline highway in the US state of Michigan that is also called Lake Michigan Drive. The highway runs from Agnew near Lake Michigan to the west side of Grand Rapids in the western Lower Peninsula. Lake Michigan Drive continues in each direction from M-45's termini, extending west of US Highway 31 (US 31) and east of Interstate 196 (I-196). In between, the road runs through rural and suburban areas of Ottawa and Kent counties, including the main campus of Grand Valley State University in Allendale. Lake Michigan Drive was originally part of M-50 until the mid-1960s. Previously in the 1920s and 1930s, the M-45 number was designated along a highway in the Upper Peninsula (UP).
US Highway 27 (US 27) is a part of the US Highway System that now runs from Miami, Florida, to Fort Wayne, Indiana. In the US state of Michigan, it was a north–south state trunkline highway that entered the state south of Kinderhook and ended south of Grayling. Its route consisted of a freeway concurrency with Interstate 69 (I-69) from the state line north to the Lansing area before it followed its own freeway facility northward to St. Johns. From there north to Ithaca, US 27 was an expressway before continuing as a freeway to a terminus south of Grayling.
M-54 is a north–south state trunkline highway in the US state of Michigan that bypasses the city of Flint. It is named Dort Highway for much of its length, in honor of Flint carriage and automobile pioneer Josiah Dallas Dort. The portion from the north end of Dort Highway to Clio Road is part of the historic Saginaw Trail, and was also part of the old Dixie Highway. The modern highway runs for 30.276 miles (48.724 km) through Genesee and Saginaw counties from connections with Interstate 75 (I-75) near Grand Blanc on the south to Birch Run on the north. The highway serves mostly suburban and urban sections of the Flint area. Outside of the city, it also passes through agricultural areas in northern Genesee County and southeastern Saginaw County. It also shares a short east–west section with M-83 near Birch Run.
M-19 is a north–south state trunkline highway in the US state of Michigan. The trunkline begins northeast of Detroit at a junction with Interstate 94 (I-94) near New Haven and runs northward to a junction with M-142 just east of Bad Axe in The Thumb region of the Lower Peninsula. The highway runs through mostly rural and agricultural areas, connecting several small communities.
M-83 is a north–south state trunkline highway in the Lower Peninsula of the US state of Michigan primarily serving as a link between Interstate 75/US Highway 23 (I-75/US 23) in Birch Run, including a short east–west section with M-54, and the Bavarian-themed town of Frankenmuth. M-83 is primarily a north–south trunkline that passes by such landmarks as Bronner's Christmas Wonderland, Zehnder's and the Bavarian Inn before leaving town. The landscape in the remainder of the area is composed of farm fields between Frankenmuth and the northern terminus at M-15 near Richville.
M-90 is a state trunkline highway in The Thumb region of the US state of Michigan. It runs from near North Branch eastward to Lexington situated on Lake Huron. The highway is a lightly traveled roadway that runs through rural farmlands. The trunkline runs mostly east–west with two short north–south segments where it turns to run concurrently with other state highways. Along the routing, there are two river crossings and one railroad crossing.
M-121 is a state trunkline highway in West Michigan. The highway follows Chicago Drive, a local roadway, from Zeeland to Grandville. Chicago Drive itself runs past the M-121 segment on either side from Holland to Wyoming. The roadway passes through rural farmland on a route that runs parallel to Interstate 196 (I-196). M-121 forms the main street through the center of Hudsonville as it runs southwest–northeast. It forms a major street through the unincorporated community of Jenison before M-121 terminates at I-196 in Grandville.
Interstate 69 (I-69) is a part of the Interstate Highway System that will eventually run from the Mexican border in Texas to the Canadian border at Port Huron, Michigan. In Michigan, it is a state trunkline highway that enters the state south of Coldwater and passes the cities of Lansing and Flint in the Lower Peninsula. A north–south freeway from the Indiana–Michigan border to the Lansing area, it changes direction to east–west after running concurrently with I-96. The freeway continues to Port Huron before terminating in the middle of the twin-span Blue Water Bridge while running concurrently with I-94 at the border. There are four related business loops for I-69 in the state, connecting the freeway to adjacent cities.
There have been nine business routes for Interstate 75 in the US state of Michigan. Numbered either Business Loop Interstate 75 or Business Spur Interstate 75 depending if they are a full business loop or a business spur, these highways are former routings of I-75's predecessor highways in the state. They were designated as I-75 was completed through the various areas of Michigan. The business loop in Pontiac runs through that city's downtown along a section of Woodward Avenue and a segment of roadway formerly used by M-24. The former Saginaw business loop was once a part of US Highway 23 (US 23), as was most of the original Bay City business loop. The roadways that make up the business loops in West Branch and Roscommon were previously part of M-76, I-75's predecessor through that part of the state. In Northern Michigan, the Grayling and Gaylord BL I-75s were part of US 27, and the two business routes in St. Ignace and Sault Ste. Marie in the Upper Peninsula of Michigan were part of US 2. A tenth business route, a loop through Indian River has been proposed. Each of the business loops connects to I-75 on both ends and runs through their respective cities' downtown areas. The two business spurs only connect to I-75 on one end and run into the appropriate downtown.
US Highway 10 (US 10) is a part of the United States Numbered Highway System that runs from West Fargo, North Dakota, to the Lower Peninsula of the US state of Michigan. The highway enters Michigan on the SS Badger, which crosses Lake Michigan between Manitowoc, Wisconsin, and Ludington. As the highway crosses the state, it is a two-lane undivided highway between Ludington and Farwell and a freeway from Farwell east to the highway's terminus in Bay City. US 10 runs concurrently with US 127 in the Clare area along a section of freeway that includes a welcome center in the median. Outside of the Clare and Midland areas, US 10 runs through rural areas of Western and Central Michigan in a section of the Manistee National Forest as well as farm fields.
The State Trunkline Highway System consists of all the state highways in Michigan, including those designated as Interstate, United States Numbered, or State Trunkline highways. In their abbreviated format, these classifications are applied to highway numbers with an I-, US, or M- prefix, respectively. The system is maintained by the Michigan Department of Transportation (MDOT) and comprises 9,669 miles (15,561 km) of trunklines in all 83 counties of the state on both the Upper and Lower peninsulas, which are linked by the Mackinac Bridge. Components of the system range in scale from 10-lane urban freeways with local-express lanes to two-lane rural undivided highways to a non-motorized highway on Mackinac Island where cars are forbidden. The longest highway is nearly 400 miles (640 km) long, while the shortest is about three-quarters of a mile. Some roads are unsigned highways, lacking signage to indicate their maintenance by MDOT; these may be remnants of highways that are still under state control whose designations were decommissioned or roadway segments left over from realignment projects.
There have been 10 business routes of US Highway 127 in the state of Michigan. The business routes are all sections of state trunkline highway that run through the central business districts of their respective towns connecting them to the mainline highway outside of those downtown areas. These various business routes were formerly part of the routing of US Highway 127 (US 127) or its predecessor in Central Michigan, US 27, before the construction of highway bypasses. The southern two, in Jackson and Mason were previously parts of US 127, while seven of the northern eight were originally part of US 27, a highway which was replaced on its northern end by US 127 in 2002. The business loop through Alma was once numbered US 27A.
common sections ... 2 freeways share a single right-of-way
Coincident with Rte 299
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