Detour Signs (W20-2 and M4 Series): What They Mean

DETOUR AHEAD (W20-2). Illustration drawn by Safe Pass Traffic; not to scale.

Detour signs guide you around a closed section of road and back again. The orange diamond DETOUR AHEAD sign (W20-2) warns that a detour is coming; orange rectangular M4 signs then mark the route itself, turn by turn.

What it means

Section 6H.04 of the California MUTCD 2026 Edition covers the DETOUR (W20-2) warning sign, used in advance of a detour over a different roadway or route. The distance legend may be XX FEET, XX MILES, or AHEAD. The manual refers to California Vehicle Code Section 21363 for detour signs.

Section 6I.02 covers the route-marking signs. Each detour shall be adequately marked with standard temporary route signs and destination signs.

M4-10 Detour Arrow: may be used where a detour route has been established; should normally be mounted just below the ROAD CLOSED (R11-2, R11-3a or R11-4) sign, with a horizontal arrow pointing left or right.
M4-9 DETOUR (with arrow): should be used for unnumbered highways, emergencies, short durations, or short distances where drivers are guided back without route signs. A street name sign should be placed above it, or the street name included on it.
M4-8P DETOUR plaque: may top a route sign assembly to mark a temporary route that bypasses the work zone and rejoins the highway.
END DETOUR (M4-8a) or the END plaque (M4-8bP) may mark where the detour ends.

What drivers often get wrong

Leaving the detour early. Following the arrows until END DETOUR or your original route is the only way to be sure you bypass the closure.
Treating DETOUR AHEAD as a closure. The W20-2 is only a warning; the closure is marked by ROAD CLOSED (R11-2) or local traffic only signs.
Being surprised by pink signs. For traffic incidents, detour signs may have a black legend on fluorescent pink.

Where it goes in a work zone

The W20-2 goes in advance of where drivers must leave the route, usually after ROAD WORK AHEAD, which Section 6H.02 says should be the first advance warning sign drivers see. At the closure, the M4-10 arrow sits under the ROAD CLOSED sign. Along the route, M4-9 signs or M4-8P plaques on route assemblies mark each turn.

Section 6I.02 also says the DETOUR (M4-8) sign should be placed on tangent sections at intervals not to exceed 0.25 miles and at major intersections, and in urban areas may be placed at every intersection.

What contractors need to know

A detour uses roads you may not own. Expect the permitting agency, and any other agency whose roads carry the detour, to review the route.
Show every decision point on the plan: advance warning, the closure with its arrow, each turn, and the end of the detour.
California adds its own DETOUR with Arrow sign (SC3(CA)) for unnumbered highways, emergencies, short durations and short distances; the agency may specify which version it wants.
Pedestrian and bicycle detours have their own signs (M4-9a, M4-9b, M4-9c) when those facilities are closed.
A traffic control plan with the detour drawn out is usually the fastest route to permit approval.

Related signs

Frequently asked questions

Why are detour signs orange?

The CA MUTCD says temporary warning signs and additional temporary guide signs in work zones use a black legend and border on orange. For incident management, detour signs may use fluorescent pink instead.

What is the difference between M4-9 and M4-10?

The M4-10 is just an arrow, normally mounted below a ROAD CLOSED sign at the closure. The M4-9 reads DETOUR with an arrow and guides drivers along the route.

Does every detour need a DETOUR AHEAD sign?

The manual calls for the W20-2 in advance of a detour over a different roadway or route. Your agency’s permit may add requirements.

Sources

Need this sign on a traffic control plan?

Safe Pass Traffic prepares PE & TE stamped traffic control plans, striping plans and typical plans for projects statewide in California, and helps with the permit submittal. We draw the plan that shows which signs go where; your contractor or traffic control provider sets them in the field. Call or text (714) 362-1348 or request a quote.