Side Road Sign
MUTCD W2-2
A side road enters from the side ahead; watch for entering traffic.
What it looks like
A yellow diamond warning sign with a black symbol showing a smaller road joining the main road from the side.
When you'll see it
You see it before a point where a side road enters your road, common on highways passing driveways, farm lanes, or minor streets.
What to do
Slow down and watch for vehicles entering from the side road, being ready to adjust your speed or position.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.