DETOUR (LEFT ARROW) Sign Requirements
This guide summarizes the MUTCD requirements and practical application guidance for the M4-9L DETOUR (LEFT ARROW) sign. Use this resource to understand when it is required, how to place it correctly, and how to maintain clear visibility for safe operations.
When It Is Used
Left turns at detour decision points
Where It Applies
Along the detour route at each decision point.
Placement Basics
Mount at the standard height and setback for the roadway with clear sight lines.
Visibility / Reflectivity
High reflectivity and clear sight lines are essential day and night.
1. When a DETOUR (LEFT ARROW) Sign Is Used
Combines the word DETOUR with a left-pointing arrow to direct drivers to turn left at this point of the detour route. Note: “M4-9L” is a directional label used on this site to identify the left-arrow version of the detour marker. FHWA does not issue M4-9L as a separate sign code — the official designation is M4-9 “Detour,” which is shown with either a left- or right-pointing arrow as the route requires.
- Left turns at detour decision points
- Used in sequence with M4-9 (right arrow) and M4-8 DETOUR markers
- Common on local-street detours around closures
2. How and Where It Applies
In the field, M4-9L Detour (Left Arrow) is typically positioned at the along the detour route at each decision point. Common deployments include left turns at detour decision points; used in sequence with M4-9 (right arrow) and M4-8 DETOUR markers; common on local-street detours around closures. Always confirm its size, retroreflective sheeting, spacing, and placement against the CA MUTCD 2026 and the reviewing agency before finalizing the traffic control plan.
3. Placement and Application Notes
Position M4-9L along the detour route at each decision point.
- Place the sign where drivers have a clear, unobstructed view on approach.
- Mount at the standard height and setback from the edge of the traveled way.
- Do not mount with other signs on the same post unless space constraints require it.
4. Standard Sizes and Visibility Considerations
Typical minimum: 24" × 12" (route marker / plaque).
- Choose size based on roadway speed, sight distance, and mounting location.
- Maintain high retroreflectivity for maximum night and adverse-weather visibility.
5. California Notes
Placed at left-turn decision points along a California detour, keeping detoured traffic on the approved alternate route.
Go to California Notes6. Common Field or Planning Mistakes
- Detour not signed continuously at every decision point.
- Arrow direction incorrect at a turn.
- Detour does not return traffic to the original route.
- Pedestrian detour not addressed where sidewalks are affected.
Related Resources
View All M-Series Signs
Educational reference only. This is not an official Caltrans, FHWA, or local agency publication and is not legal or engineering advice. Always verify sign selection, size, placement, spacing, and application against the current CA MUTCD 2026, Caltrans sign specifications, Standard Plans, project documents, and the reviewing agency’s requirements. Local jurisdictions may impose additional requirements, and final selection, placement, and dimensions may require engineering judgment or agency approval. Written against California MUTCD 2026 (effective January 18, 2026) and the Federal MUTCD 11th Edition. Official sources last verified June 2026.
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