What document must be kept on file at each traffic system regarding traffic control?

The correct answer is that the document that must be kept on file at each traffic system regarding traffic control is part 6 of the Manual on Uniform Traffic Control Devices (MUTCD). This part specifically covers the standards and guidelines for temporary traffic control, which are essential for ensuring safety for both workers and the traveling public when roadwork or similar activities are being conducted.

MUTCD part 6 provides detailed information on the design, layout, and implementation of signage, barricades, and other devices needed in work zones. By adhering to the standards outlined in this document, traffic control personnel can make informed decisions that align with national safety practices, ultimately promoting compliance and reducing accidents.

While DOT regulations and state traffic guidelines may also be important, they often rely on the MUTCD as a primary reference for traffic control practices. Federal safety codes might address broader safety concerns, but they do not specifically focus on the traffic control measures needed for work zones, making the MUTCD part 6 the most relevant document for the specific context of traffic control.