Old version of File:MUTCD S4-5 METRIC.svg before the MUTCD required all school zone signs to use a fluorescent green backing.
This file was derived from:

 MUTCD S4-5.svg: 
MUTCD S5-1 METRIC.svg:
MUTCD_S4-5.svg:
MUTCD_S5-1_METRIC.svg: Federal Highway Administration - MUTCD
derivative work: Fry1989