This image is from the <a rel="nofollow" class="external text" href="http://www.aiga.org/symbol-signs/">AIGA symbol signs collection</a> which was produced through a collaboration between AIGA and the <a href="https://en.wikipedia.org/wiki/United_States_Department_of_Transportation" class="extiw" title="en:United States Department of Transportation">U.S. Department of Transportation</a> (DOT) in 1974 and 1979. All images in the collection are <a href="https://en.wikipedia.org/wiki/public_domain" class="extiw" title="w:public domain">public domain</a>.
<a href="//commons.wikimedia.org/wiki/User:Train2104" title="User:Train2104">Train2104</a>
AIGA