Which of the following is NOT one of the areas where flaggers should never flag?

The statement that flaggers should never flag at a traffic signal is correct because traffic signals are specifically designed to control the flow of traffic independently. Flaggers are typically used in situations where there are no signal controls, such as in temporary work zones or areas where traffic is disrupted and needs manual intervention. Flaggers can effectively manage traffic in conditions without established controls; however, positioning a flagger at a traffic signal can create confusion and may lead to accidents, as motorists are already accustomed to following the signals.

The other options, such as flagging on or under a bridge or overpass, and in a vehicle, are unsafe or impractical for flagging operations. These locations can present hazards due to restricted visibility, potential for falling objects, and the inability for flaggers to effectively communicate with drivers. Thus, the choice regarding flagging at a traffic signal stands out as the practice that goes against standard flagging safety protocols.