Which of the following is a requirement of the U.S. Manual on Uniform Traffic Control Devices?

The requirement that lettering on guide signs should use lower-case and initial upper-case letters is rooted in the goal of enhancing legibility and readability for road users. Research has shown that using lower-case letters makes guide signs more easily readable and understandable, especially at higher speeds where quick recognition is crucial for safe navigation. The U.S. Manual on Uniform Traffic Control Devices emphasizes this principle to improve communication with drivers, as it helps convey information more clearly and effectively.

The other options, while relevant to traffic control devices, do not meet the established requirements outlined in the Manual. For instance, while pedestrian warning signs may utilize specific colors for visibility, the exact specification of fluorescent yellow-green is not a blanket requirement across all signage. Similarly, while symbols on warning signs are generally recommended to conform to the Manual, the statement implies an absolute condition that can vary by context. Finally, the advisory speed plaque for curve warning signs, though important for conveying safety information, does not apply universally to all curve warning signs, as some might not include this feature. Hence, the focus on appropriate lettering style in guide signs reflects a specific, documented requirement in the Manual.