Source / Trigger for US Highway Symbols

ABRP shows that US interstate highway symbol on the highways around here, but not the symbol for the Washington state highways.

For US Interstate 5 the interstate symbol is shown:

But for Washington Highway 518 the Washington state highway symbol (a profile of George Washington) is not shown:

Is this because ABRP doesn’t know (yet) about US state highways? Or is there something in the way a highway is tagged in OSM that would cause the symbols to start appearing?