My question is whether anyone is aware of a standard for how State roads, US roads and interstates should be displayed on a map? Not the label, but standards for the color and width of the symbology used to display the road. Any pointers are appreciated.
I am not aware of any standards, look at the difference that exists between Google, Bing, Yahoo, DeLorme...etc.
I have two suggestions for you:
If you have access the ESRI Data & Maps media, there are layer files for the ESRI style street maps. If you like their appearance, it shouldn't be too hard to hack at those layer files to make them fit your data structure.
Look at the other common street maps out there (Google, Bing, even OSM) and emulate what you like best about them. This gets a little tedious when you take different scales into consideration, but its still not that bad of a task.
Hope this helps.