Potentially dumb suggestion: what if one just declares anything that has a significant chunk of non-bridge or non-tunnel street on it land? It's not a universal solution but perhaps sufficient for the application?

I recently made a map (using Generic Mapping Tools) of one of the Aleutian Islands in Alaska, for one of my dad’s grad students to use in a paper. That would constitute a good example of where you couldn’t just tell my # of built structures (since at certain zoom levels, there are none!)

Sure, but are you likely to have users want to A/B test traffic light configurations there? My suggestion was very specific to the application in question.