In the United States, what does a pennant-shaped road sign indicate?

In the United States, a pennant-shaped road sign indicates a no-passing zone.

The yellow pennant sign is mounted on the left side of a two-way road, making it visually distinct from the diamond-shaped warning signs used for many other hazards. It marks the beginning of a section where drivers traveling in that direction must not pass other vehicles.

The sign is especially useful because it supplements road markings: a solid yellow line, or a double solid yellow line, commonly reinforces the restriction. Drivers must still obey any applicable pavement markings and other posted signs. The pennant shape is associated specifically with no-passing warnings in the U.S. Manual on Uniform Traffic Control Devices.

A common mix-up is confusing this sign with a general warning sign. Its shape does not identify a physical hazard such as a curve or intersection; it communicates a passing restriction. The restriction can end later where markings or signs indicate that passing is permitted again.