Pre-existing Modified Supplementary Safety Measure Law and Legal Definition

According to 49 CFR 222.9 [Title 49 – Transportation; Subtitle B -- Other Regulations Relating to Transportation; Chapter II -- Federal Railroad Administration, Department of Transportation; Part 222 -- Use of Locomotive Horns at Public Highway-Rail Grade Crossings; Subpart A – General], Pre-existing Modified Supplementary Safety Measure (Pre-existing Modified SSM) means “a safety system or procedure that is listed in appendix A to this Part, but is not fully compliant with the standards set forth therein, which was installed before December 18, 2003 by the appropriate traffic control or law enforcement authority responsible for safety at the highway-rail grade crossing.”