What it looks like
A yellow diamond warning sign with a black symbol of a road pinching inward to show a bridge narrower than the road ahead.
When you'll see it
You encounter it before a bridge whose width is less than the approaching roadway, common on older rural crossings.
What to do
Slow down, keep to your lane, and proceed with caution, watching for oncoming vehicles on the narrow bridge.
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.
