According to the "Manual on Uniform Traffic Control Devices," which of the following is not a specified shape for traffic signs?

The specified shapes for traffic signs are important as they help drivers quickly recognize the type of message being conveyed, which is crucial for safety on the roads. The "Manual on Uniform Traffic Control Devices" does categorize traffic signs into distinct shapes, each serving a specific purpose.

Round signs are typically used for railroads, square signs can indicate regulatory information, and rectangular signs generally convey informative messages. The crossbuck shape is specifically associated with railroad crossing signs and has a unique design that stands out for its particular function.

While square signs may exist in local jurisdictions or for specific applications, they are not part of the standardized shapes defined in the manual as common traffic signs. Therefore, square is not specified as a standard shape in this context, which makes it the correct choice for this question.