Are they standard between USA and Canada?

I believe so. Here's a quote from the guide that I linked to:

"The 2008 Emergency Response Guidebook (ERG2008) was developed jointly by Transport Canada (TC), the U.S. Department of Transportation (DOT), the Secretariat of Transport and Communications of Mexico (SCT) and with the collaboration of CIQUIME (Centro de Información Química para Emergencias) of Argentina, for use by fire fighters, police, and other emergency services personnel who may be the first to arrive at the scene of a transportation incident involving dangerous goods."

Based on that, I'm guessing that the placards have the same significance in those geographic regions and they might be international (for cargo transported on ships and airplanes).