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. Confirm placement and supplemental plaques against the applicable CA MUTCD standard.
View this sign on the Federal MUTCD (FHWA)California-specific application notes and adoptions may differ. Review California requirements where applicable.
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.
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.
Learn more about Detour (Left Arrow) sign requirementsPlaced at left-turn decision points along a California detour, keeping detoured traffic on the approved alternate route.
In Los Angeles, this sign must conform to CA MUTCD 2026. Work in City of LA right-of-way is reviewed by StreetsLA and the Bureau of Engineering (BOE); state highways in LA County are permitted and reviewed by Caltrans District 7; county roads are permitted by LA County DPW. Sizes, retroreflectivity, and placement must meet CA MUTCD minimums — local agencies may require larger signs based on roadway class, speed, pedestrian volume, and proximity to schools or transit.
Verify the arrow orientation matches the turn at every decision point. Public Ready reviews detour turn-by-turn guidance.
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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