"Free-swinging" signal heads are located on which of the following support systems?

Free-swinging signal heads are designed to be mounted on support systems that allow them to sway freely in response to environmental factors such as wind and vibrations. This design helps to prevent damage to the signal heads during high winds or storms. Span wires are particularly suited for this purpose as they hold the signal heads at a distance and allow for some degree of movement, thus accommodating the dynamic stresses that could affect the signal integrity.

In contrast, the other support systems mentioned—traffic poles, concrete mast arms, and gantries—provide more rigid and fixed mounting solutions. These structures do not permit the same level of movement as span wires, which is why they are not categorized as free-swinging. Traffic poles provide sturdy, upright support for signals, concrete mast arms are solidly anchored to provide strength and stability, and gantries are large structures that also offer stability rather than flexibility. Therefore, the nature of span wires makes them the correct choice for supporting free-swinging signal heads.