W20-4 One Lane Road Ahead Sign
Warns that two-way traffic will be reduced to a single shared lane ahead, where opposing directions must take turns through the work area.
| Sign code | W20-4 |
|---|---|
| Federal code | W20-4 |
| Name | One Lane Road Ahead |
| Category | Warning Signs |
| Shape | Diamond |
| Color | Orange background, black legend |
| California status | Used in California per CA MUTCD |
| Typical CA minimum size | 48" × 48" on conventional roads; 60" × 60" on expressways and freeways |
What This Sign Means
Warns that two-way traffic will be reduced to a single shared lane ahead, where opposing directions must take turns through the work area.
California Temporary Traffic Control Use
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.
Common TCP Context
- 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
California Size Requirements
Typical minimum: 48" × 48" on conventional roads; 60" × 60" on expressways and freeways.
Sizes are CA MUTCD minimums and vary by roadway classification and speed. Verify against the CA MUTCD 2026 (CA MUTCD Part 6 (Temporary Traffic Control), Chapter 6F; warning-sign design in Part 2C), Caltrans sign specifications, and the reviewing agency.
Common Placement on a TCP
Where it sits: Advance warning area, ahead of the work.
CA MUTCD reference: CA MUTCD Part 6 (Temporary Traffic Control), Chapter 6F; warning-sign design in Part 2C.
Los Angeles Area Usage Notes
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.
Common Plan Review Comments
- Sign not shown at the correct advance distance for the posted speed.
- Advance warning sign missing or out of sequence on the TCP.
- Wrong size specified for the roadway classification (conventional vs. expressway).
- Sign sheeting / retroreflectivity class not noted on the plan.
Plan Review Notes
Verify the one-lane section has a defined control method and adequate sight distance for alternating traffic. Public Ready reviews flagging and pilot-car setups.
Signs Commonly Used Together
These signs are typically deployed alongside W20-4 on a California TCP.
Rent or Purchase This Sign
Setting up alternating one-way control? Public Ready can supply ONE LANE ROAD AHEAD (W20-4) and the supporting flagging signs.
Official References
- California MUTCD (Caltrans)
Free official resource — opens official government website
- Federal MUTCD (FHWA)
Free official resource — opens official government website
- Caltrans Sign Charts
Free official resource — opens official government website
- FHWA Standard Highway Signs
Free official resource — opens official government website
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.
Related Signs
Need signs for a California project?
Need to rent, purchase, or include traffic control signs on a California project? Public Ready can help with sign rental, purchase, sign packages, TCP sign legends, and permit coordination.