Which of the following types of conduit is NOT typically used for traffic signals?

The choice of flexible conduit as the type that is not typically used for traffic signals is based on its application characteristics. Flexible conduit is generally less rigid and lacks the structural integrity required for the protection of wiring in the often demanding environments of traffic signal installations. Traffic signals must endure exposure to various environmental factors, including weather conditions, and the physical stresses associated with traffic.

Rigid conduit types like PVC Schedule 40, rigid steel conduit, and high-density polyethylene (HDPE) offer the necessary durability and protection against impacts, moisture, and abrasion that flexible conduit may not provide. These rigid conduits ensure that the wiring remains safe and functional over long periods, which is critical for the reliability of traffic signal systems. Therefore, flexible conduit is typically avoided in favor of more robust and protective conduit options.