Which of the following is NOT an effective method to warn approaching traffic about a work site?

The correct choice is construction equipment, as it does not serve as an effective warning method for approaching traffic about a work site. While construction equipment is often present at work sites, it is primarily used for tasks related to the project and does not inherently convey a warning to drivers.

In contrast, traffic cones, flags, and warning signs are specifically designed to alert drivers to the presence of work zones. Traffic cones create a physical barrier that directs vehicles away from hazards and provides a visible cue that work is occurring nearby. Flags serve as a dynamic warning mechanism, typically used by flaggers who can actively guide traffic and communicate the potential dangers or necessary detours. Warning signs are installed to inform motorists well in advance about upcoming work zones, thereby helping to facilitate safe navigation around the site. Each of these options plays a crucial role in traffic management and safety during construction activities.