Can we use flags to control traffic?
Yes, but the use should be limited to emergency and unusual situations.
According to the MUTCD, the STOP/SLOW paddle should be the primary and preferred hand-signaling device because the STOP/SLOW paddle gives road users more positive guidance than red flags. Use of flags should be limited to emergency situations. Experience has shown that it is difficult to effectively control and direct traffic by waving a flag.
The concept of the emergency use is available so that if a crew responds to an unplanned problem, they can use a rolled up flag stuck behind the seat or in the storage compartment of their truck. As such, all highway trucks should have an emergency flag available for short term flagging and each worker should be trained in the proper use of the flag. As soon as possible, a more formal work zone set up using paddles should be put in place.
There are situations where a paddle may not be able to properly control traffic. An example is a complicated intersection or an entrance to the work zone between two flaggers where the slow on the back side of the paddle could send a mixed message. In this case, the use of the flag is allowed, but the reasons for using the flag must be documented.
Remember that at night, all signaling devices (flags and paddles) must be retroreflective and the flagger station shall be illuminated except in an emergency. A flashlight with a red glow cone may be used to supplement the paddle or flag operation. The flashlight is held in the free hand and helps the driver understand the signals from the flagger.
Manual of Uniform Traffic Control Devices, 2009. Chapter 6E: Flagger Control
Flagger’s Handbook Pocket Guide (pdf)
Work Zone Traffic Control for Local Roads (pdf)
CLRP workshop manual