No Passing Zone Sign
MUTCD W14-3
Passing is prohibited beginning at this point.
What it looks like
A yellow pennant, an isosceles triangle pointing right with the black legend NO PASSING ZONE, mounted on the left side of the road.
When you'll see it
On two-lane highways at the start of a stretch where sight distance is limited, such as hills and curves, facing oncoming-lane drivers.
What to do
Do not pass beginning at this point; stay in your lane until the road markings and signs show passing is again allowed.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.