The road signs in the US are _well established_ in the US.
Of course they are! I am not trying to say that US signage is unclear. I am trying to illustrate that it does not automatically make things clearer if you use text instead of pictograms. Using text might make things clearer, but "SLIDES" is only clear if you already are familiar with it. Agree? :) (The same thing goes for the text "PED XING", but this example is less important since, as you say, it is usually accompanied by a pictogram)
In my experience, road signs in the US rely much more on text than road signs in Europe. This is why I brought this up as an illustrative example that text is not a magic bullet :)