This guide summarizes the MUTCD requirements and practical application guidance for the W20-4 ONE LANE ROAD AHEAD sign. Use this resource to understand when it is required, how to place it correctly, and how to maintain clear visibility for safe operations.
Alternating one-way traffic control through a work zone
Advance warning area, ahead of the work.
Mount at the standard height and setback for the roadway with clear sight lines.
High reflectivity and clear sight lines are essential day and night.
Warns that two-way traffic will be reduced to a single shared lane ahead, where opposing directions must take turns through the work area.
Warns that two-way traffic will be reduced to a single shared lane ahead, where opposing directions must take turns through the work area. In the field, W20-4 One Lane Road Ahead is typically positioned at the advance warning area, ahead of the work. Common deployments include alternating one-way traffic control through a work zone; paired with W20-7 Flagger and BE PREPARED TO STOP; common on rural two-lane roads and bridge or culvert work. Always confirm its size, retroreflective sheeting, spacing, and placement against the CA MUTCD 2026 and the reviewing agency before finalizing the traffic control plan.
Advance warning area, ahead of the work.
Typical minimum: 48" × 48" on conventional roads; 60" × 60" on expressways and freeways.
Used where a California work zone narrows the roadway to one lane for alternating one-way control, typically managed by flaggers, a pilot car, or temporary signals.
Go to California NotesEducational 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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