Civilization V CIVILOPEDIA Online
Game Concepts
 
Extending Trade Route Range

Summary:

You can boost the distance of your land-based trade routes by 50% through building Caravansaries in the origin cities. They also produce an additional 2 Gold when connecting to another civilization. Caravansaries are made available by researching Horseback Riding.

Additionally, Harbors boost the distance of your sea-based trade routes by 50%, and generate an additional 2 Gold when those routes connect to another civilization. They are unlocked by researching Compass.