RE: How long before an ethnic food becomes road food?
An interesting take on this topic is Waverly Rott's works, esp. "The Food of Italy" and "The Food of France". (I think his book "Eating in America" does not handle this as well).
Root's theory is that local foods are so delicious and blend so well in part because of the natural geography of the area (ie, climate, soil, etc.) and that truly good ethnic food cannot be replicated outside of the homeland. For example, Italian foods are grown in Italian soil, herbs, seasonings (even like butter/olive oil) used in these dishes are grown in or are from animals who have grazed on the same specific land (region, micro-region), wines that are served with the food also are from grapes that grow in the same soil, climate, etc.) So, for example, even if you make a Italian (French, Vietnamese, etc. etc.) dish in USA, you are not using Italian (French, Vienamese, etc.) vegetables, water, butter, etc.
In this way, much traditional "Roadfood" is also local: local BBQ being from local meet, local wood, etc.
So, sure, in a way ethnic food can "become" roadfood, but I think the distinction is important about geography, in addition to culture, being an essential part of "real" roadfood.