From Maryland State Highway Administration (SHA) Frequently Asked Work Zone Questions web site:
"A flagger receives a flagger card on the successful completion of the SHA‐ or ATSSA‐provided flagger’s course. A flagger must successfully complete the SHA‐ or ATSSA‐provided flagger’s course in order to be deployed in Maryland work zones. Exceptions for approved flaggers can be made during emergency situations, where a non‐approved flagger may be deployed for up to 15 minutes."
From Maryland SHA Standard Specifications for Construction and Materials:
"Flaggers shall have completed an Administration approved flagger training course within the last four years. The failure of any flagger to perform the required duties will be grounds for replacement."
Information about flagger training programs are available on the web site below.