Where is ITD not permitted to be used?

The correct choice pertains to the fact that Integrated Traffic Detection (ITD) systems have specific operational limitations to ensure the safety of roadway workers. In particular, ITD systems are not permitted to be used inside interlocking or control points with switches because these areas demand precise control and management of train movements. The presence of switches, interlocking systems, and control points means that the dynamics of train traffic can be more complex and hazardous, requiring highly reliable and tested methods of communication and control that ITD systems may not sufficiently provide.

In contrast, using ITD on tracks designated for freight transport, straight tracks with no crossings, or in areas where guardrails are present may not inherently conflict with safety protocols, as these environments might not carry the same level of operational complexity or risk as interlocking points. Therefore, the limitations of ITD usage in high-stakes areas like control points reflect a keen awareness of the intricacies and risks involved in train operations.