I'm sure its not easy, but the potential is enourmous. I don't even need a game anymore, this could be Google Earth 3d....err, I mean Outerra Earth :-)
For roads, well I think you should just go by hierarchy, and when they meet have the lower one of the two disconnected.
Please just make sure that you do this generically, so its quite easy for useres (modders) to add support for different types.
So say i can just add a definition of how "Power" tags should be handled, the user should simple by able to point at a, let's call it VectorLayout definition file. Which then could point at a 3d model (for poles) and define that this should be placed every...50m..., and connection with a 3d model of the power lines themselves.
Similarly, if someone wants to add support for "amenity-fuel", the definition file could just point to a ground texture, a few 3dmodels (of which the engine will choose the biggest that fits into the area or something like that).
The definition should also specify different types based on a hierarchy of locations, so let's say continent-country-state-city, so more specialist, eg. Fuel station can appear.
My Point is, think User Generated content. You don't have to support all this stuff, users will. The Flightsim community especially, once interested, will jump onto this. Hell, the openstreetmaps stuff makes it easy to have their corner appear properly.
I'm so excited by this possibility right now.