DETOUR (LEFT ARROW) Sign Requirements

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

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. 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

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.

6. 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.