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:
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
This westbound highway in
southwestern Virginia simultaneously carries Interstates
77 and
81 in opposite directions. The "wrong-way concurrency" is also reflected in
U.S. Route 52 and
U.S. Route 11, which are concurrent with I-77 and I-81, respectively.
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.
