More trade routes are gotten by civics (Free Market) wonders (ToA and GL to name a few) techs (corporation) and some buildings. Other buildings like harbors and custom houses increase the amount of gold gotten from a trade route. The distance of the trade route makes it more profitable, and if a trade route is intercontinental is is much more valuable. That's why in a continent or hemispheres map astro is a very important civic.
That's why in a continent or hemispheres map astro is a very important civic.
astronomy is not a civic.
Distance does nada for trade route value as far as I know.