Brill, thanks for the Bushey example. I see there, though, you've also separated neighbouring areas along residential roads, not just primary-tertiary. Eg:
https://www.openstreetmap.org/way/1299092272,
https://www.openstreetmap.org/way/1299092269, & https://www.openstreetmap.org/way/1272132126.
How do you decide what object is significant enough to separate them, and also to create a corridor space where the road passes? 
I'm guessing this has all been discussed somewhere, I could head there to read up on the background arguments?