Do Not Pass Sign
MUTCD R4-1
Passing other vehicles is prohibited along this stretch of road.
What it looks like
A vertical white rectangle with black legend reading DO NOT PASS in bold capital letters.
When you'll see it
Posted at the start of a no-passing zone, such as on hills, curves, or narrow stretches where sight distance is limited.
What to do
You must not pass other vehicles along this stretch; stay in your lane until a sign or road marking shows passing is again allowed.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.