Quote Originally Posted by vonsch
Good questions all.

But I don't have any answers. I wonder if land trade works something like sea trade, though, in that upgrades "enable" more trading partners. Maybe the market line is doing this too? It's hard to test.

Hmm, might be able to by saving, then destroying an advanced market and seeing what drops on the trade chart. The problem is getting a static test bed then altering the state of the one region while everything else remains constant.
Oh,it definitely works somewhat the same on land in that more advanced markets get you more land trade routes. I don't think you get land trade beyond your neighbors (someone with the game open could pop open some trade scrolls to verify). Building markets on islands gets you no additional trade.

Part of the reason why it's tough to gauge how much a trade building is getting you is that there are network effects. Some of the benefit may be showing up in another province. Some of the benefit may not be realized until you are not at war with a trading partner.

It would be interesting to start with a game where you own the whole map and see what various buildings cause to happen.