What aspect is used to indicate a temporary speed restriction?

The temporary speed restriction on railroads is indicated by a yellow flag. The use of a yellow flag serves as a visual signal to train crews that they must reduce their speed to a specified limit due to certain conditions ahead, such as track work or other temporary hazards. This is important for maintaining safety on the railway as it communicates the need for caution without the need for verbal or written communication.

The other flags serve different purposes; for example, a red flag typically indicates a stop or danger, a green flag often signals the presence of an approaching train or that it is safe to proceed, and a blue flag is used to signify that workmen are on or near the tracks, indicating a more permanent situation rather than a temporary speed restriction. Therefore, understanding the specific meaning behind these flags is critical for safe railroad operations.