you’ve seen them used. and you too can use them, for free:

developed in the 70’s through a collaboration of the united states department of transportation (dot) and the american institute of graphic arts (aiga), these symbols are sometimes referred to as the dot pictograms or the aiga/dot symbols.

as work of the united states government, they are in the public domain, therefore can be used by anyone without any licensing issues.

download them at the aiga website in eps or gif format.