Lockdown Terminology in K-12 Schools: Why It Is Okay to Use Codes and Which Codes Are Best [open pdf - 776KB]
From the Abstract: "In this paper, school leaders and their law enforcement partners will learn that using codes for lockdown terms does not violate the plain-language requirement of the National Incident Management System (NIMS). They will discover which lockdown codes are best at reducing confusion to help school students and staff members respond quickly and effectively during emergency situations. Finally, the reader will learn that the most flexible and intuitive lockdown terms are Code Yellow Lockdown and Code Red Lockdown."
Navy Postgraduate School Center for Homeland Defense and Security: http://www.chds.us/