Trade flows can be predicted based on the proximity of two places and size of their respective economies. While not very detailed, it has shown to be very successful in it’s predictions that it is still used today, more than 50 years after it was introduced.
Distance makes some intuitive sense, if trade costs anything, merchants would aim to reduce that cost. But, what underlies that is also potentially cultural similarities and language for doing business.
See also:
- A-Star algorithm which computes the shortest path between two points based on a cost function. (Could this be used along with the gravity equation as a predictor of trade routes?)