This file and its description are from Wikimedia Commons.
Summary
MUTCD S5-1 School speed limit when flashing sign.
From: Manual on Uniform Traffic Control Devices for Streets and Highways - 2003 Edition. Category:Diagrams of road signs of the United States
Any traffic control device design or application provision contained in this Manual shall be considered to be in the public domain. Traffic control devices contained in this Manual shall not be protected by a patent, trademark, or copyright, except for the Interstate Shield and any other items owned by FHWA.