The existing Terrain Brush is only suitable for corner-based terrain transitions. In the upcoming Tiled 1.1, the added Wang Brush may be of use in your case, but it still has some way to go before it could conveniently edit a combination of both corner-based transitions and road connections. Please try it and provide feedback on what you'd expect from the tool (best on the forum or on GitHub), so we can continue to improve it.
Ah, sorry I've already come pretty far in writing my own editor. It supports the 9 cells per tile with full zoom, pan, variable materials/terrain. I just need to add the UI elements for add/remove material and the XML persistence code, and it will cover the same immediate needs I had of Tiled, with the benefit of being very easy for me to evolve alongside my game. It also gives me the opportunity to throw in game-specific metadata and other features, which will be important for procedural generation. Really, I should have gone this way from the start.
I appreciate the response, and I think it is great you are providing a tool like this for people who, perhaps, cannot make one or would prefer not to spend the time doing so. And out of those I demo'd, yours was--by far--the most polished and pleasing. Cheers!
Sure, sometimes it's better to roll your own solution (or sometimes people fork Tiled and add whatever they need to it, since it's free software after all). Your use-case is still something I'd like to support in Tiled as well.