Yes, safety is primary, but not standing alone, but then too, most sinage is an adjunct to safety..
Although most signs have graphics included, some do not. in my own village, in fact on my own street, which leads to the village park, there is a sign which says CHILDREN AT PLAY.
I must admit, that when I see some clown careening down the street at abot 50-60 MPH..it vexes me.
Then too, a driver may come upon a scene following a flood, where somebody has put up a temporary sign, which says.. BLIND GRADE BRIDGE OUT.
Just two examples, but ability to read in the common language would be most helpful..