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Wikipedia:
A detour or diversion route is a route around a planned area of prohibited or reduced access, such as a construction site. Standard operating procedure for many road departments is to route any detour over roads within the same jurisdiction as the road with the obstructed area.
On multi-lane highways (e.g. freeways, expressways, city streets, etc.), usually traffic shifts can be utilized in lieu of a detour as turn lanes can often be congested with detours.
Permanent detours
Various areas have systems of permanent detours as part of incident management.
Canada
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Ontario
United Kingdom
The United Kingdom uses yellow signs with a series of symbols for its diversion routes.
United States
Connecticut
Connecticut has two examples of permanent detours.
Michigan
Michigan has a system of "special route" Emergency routes which parallel major highways.
Ohio
Ohio has a permanent detour of Interstate 70 in place because of frequent flooding.
Pennsylvania
Pennsylvania has a system of "color detour signs" in which any detour follows signs with a specific color. The detours are meant to be used if a limited-access highway has to be closed for an emergency situation such as a flood or car accident, directing motorists along local roads near the limited-access highway to the next exit. They are generally color-coded according to direction, with blue meaning north, red meaning south, green meaning east, and orange meaning west. However, other colors, such as black and brown, are sometimes used when detour routes overlap with one another.
Wisconsin
Wisconsin has "alternate route designations" in various areas of the state.