Which type of sign is posted on the left-hand side of two-way roads to warn drivers not to pass?

The correct answer is that pennant-shaped traffic signs are posted on the left-hand side of two-way roads to warn drivers not to pass. This specific shape is designed to effectively convey the no-passing message, allowing drivers to quickly recognize the warning without confusion. The distinctive triangular shape points to the right, indicating that passing is prohibited in the area ahead.

Understanding the function of different traffic sign shapes is crucial for safe driving. While square signs typically provide regulatory information, circular signs often indicate railroads or other circular traffic situations, and diamond-shaped signs generally serve to alert drivers to potential hazards or changes in road conditions. In contrast, the unique shape of pennant signs is specifically intended to inform drivers about the passing regulations on the road. This clear visual distinction helps maintain safe driving practices.