Official MUTCD M4-9 Detour Signs and Arrows

Guide traffic safely around closures with Federal Detour Signs. We stock the complete M4-Series navigation system, including "Detour Ahead", "End Detour", and the critical M4-9 Detour Arrow markers. Available in Fluorescent Orange (for temporary construction) or Black on White (for long-term incidents), our Detour Signs feature High Intensity Prismatic reflectivity to ensure drivers never miss a turn, day or night.
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Detour Route Continuity & "Trailblazing" (MUTCD 6F.59)

A compliant detour requires a continuous, unbroken chain of communication known as "Trailblazing." If a driver misses a single sign, the detour fails.

  • 1. Advance Warning: Place a "DETOUR AHEAD" (W20-2) sign 500+ feet before the road closure to allow drivers to prepare for the diversion.
  • 2. The Turn: Install M4-9 Detour Arrow signs at every intersection where the route changes direction. These must be placed before the turn, clearly visible to approaching traffic.
  • 3. Reassurance: If the detour continues straight for more than 2 blocks, install M4-9 Straight Arrow signs to confirm to drivers they are still on the correct path.
  • 4. Termination: You must legally release drivers from the route. Install an "END DETOUR" (M4-8a) sign where the diversion rejoins the original roadway.

Federal Standards for Detour Signs and Management (MUTCD Part 6)

Establishing a Detour is one of the highest-liability activities in traffic control. If a driver misses a single Detour Sign in the chain, they can end up lost in a residential neighborhood or, worse, driving back into the construction zone. Under MUTCD Part 6, Detour Signs must form a continuous, unbroken "blaze" of information that guides the motorist from the point of closure back to the primary route.

The M4-9 Detour Arrow: The Anchor of the System

The most vital component of any diversion is the M4-9 Detour Sign.

  • The Design: A rectangular sign featuring the word "DETOUR" and a large black arrow.
  • Directionality: You must stock specific variants: M4-9L (Left), M4-9R (Right), and M4-9S (Straight). Using a generic arrow turned sideways is often non-compliant if the retroreflectivity is polarized.
  • Placement: An M4-9 Detour Sign must be installed before every intersection where a turn is required.

Orange vs. Black/White Detour Signs

Choosing the correct color code is mandatory for legal compliance:

  • Fluorescent Orange (Temporary): Used for Construction Work Zones (TTC). If the road is closed for paving or utility work, you must use Orange/Black signs.
  • Black on White (Permanent/Emergency): Used for long-term bridge closures or established emergency routes. These look like standard regulatory signs.
  • Pink (Incident Management): Used strictly by fire/police for unplanned crash diversions.

The "Reassurance" Assembly

A major error contractors make is only signing the turns.

  • The Rule: If a detour route continues straight for more than a few blocks, you must install "Reassurance Markers" (Straight Arrow Detour Signs) to confirm to the driver that they are still on the correct path.
  • End Detour (M4-8a): You must legally release the driver from the route. The "End Detour" sign is placed where the diversion rejoins the original roadway.

Reflectivity for Night Navigation

Detours are confusing enough in the daylight; at night, they are treacherous.

  • High Intensity Prismatic (Type IV): The standard for all Detour Signs. The prismatic optics ensure the arrow remains distinct from the background glare of streetlights and oncoming traffic.
  • Size: We recommend 30" x 24" for all arterial road detours to ensure legibility at 40 MPH.

Frequently Asked Questions

Common questions regarding MUTCD compliance, installation standards, and material specifications for regulatory signage.
How many Detour Signs do I need for a project?

You need one sign for every single decision point. A standard "U-Shape" detour around a block typically requires:

  1. "Detour Ahead" (W20-2) in advance.
  2. Detour Arrow at the first turn.
  3. Detour Arrow at the second turn.
  4. Detour Arrow at the third turn (rejoining the road).
  5. "End Detour" sign.
    Total: Minimum 5 signs per direction.
Can I put Detour Signs on existing street poles?

In many municipalities, Yes, provided you use non-destructive mounting hardware (like banding straps) and do not obscure the existing stop signs or street names. However, for highway projects, Detour Signs are typically mounted on their own driven U-Channel posts or portable skids to ensure they are at the correct regulatory height (7 feet).
What is a "Detour with Street Name" plaque?

For complex urban detours, the MUTCD recommends adding a Street Name Plaque above the Detour Arrow (e.g., "DETOUR [Main St]"). This confirms to the driver which specific route is being detoured, reducing confusion if multiple roads are under construction nearby.

Need specific code assistance?

If your project requires a specific State DOT certification or a specialized variation not listed here, please contact our Compliance Support Team for a custom specification sheet.