I seem to remember that in RWT upgrading markets and / or ports increased the number of routes / places the settlement traded with, and therefore increased revenue.

On an island there are no neighbours to set up land based trading routes with, so does upgrading the market or roads have any effect?

Increasing number of goods to trade is useless if you have no one to trade with.

If you look at the roads on an island, there are carts going from the settlement to the port, but does this mean the market is trading or just the port?