How does trading and traveling work?

I suggest something like this:

Between cities, where there are no barriers for trade (ie, entry and exit allowed), there will be natural roads, maybe by the soles of traders and travelers. (brown?)

Countries can, in fits of friendship, brethrenhood and good will, construct roads, á la the Roman Empire. (grey?)

You can't build sea-routes (well, except for canals and stuff), so those will come to existence like dirt-roads, because the ships use those routes.

What do you say?