Road Narrows Sign
MUTCD W5-1
The road ahead becomes narrower on both sides, leaving less room; slow down and watch for oncoming traffic.
What it looks like
A yellow diamond warning sign whose black symbol shows the two edges of the road angling inward so the roadway gets narrower toward the top.
When you'll see it
Ahead of stretches where both edges of the pavement close in — older rural roads, approaches to bridges, or where a wide road drops to a narrower width.
What to do
Slow down, keep to the center of your lane, and watch for oncoming vehicles, since there is less room for two-way traffic through the narrow section.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.