What action is taken when the controller loses 24 volt DC voltage?

Answer: B — Transfer to flash mode

When a traffic signal controller loses 24 volt DC voltage, the appropriate action typically taken is to transfer the system to flash mode. This is a critical safety measure designed to maintain some level of operational functionality in the event of a power loss. Flash mode ensures that the signal lights continue to be visible, thereby alerting drivers to the intersection's status and promoting safety. In practice, when the controller identifies the loss of power, it cannot operate the standard traffic signals as intended; therefore, switching to flash mode activates the amber lights to indicate caution to approaching vehicles. This helps in preventing accidents and maintaining traffic flow even when full signal functionality is compromised. Flash mode serves as an essential response to power disruptions, allowing the intersection to remain as safe as possible until power is restored or maintenance can be performed.