What Travels Far
Before cheap transport, value per unit weight was the ruthless filter for trade. If cargo could not pay for thousands of kilometres of transport, it stayed local.
| Cargo | Value density | Range |
|---|---|---|
| Silk, spices, gold | Enormous | Anywhere |
| Wine, oil | Modest | Coastal far; inland short |
| Grain | Low | Sea only; overland it eats itself |
Grain is the instructive case. Move grain too far overland and the oxen consume the cargo itself. Pre-modern cities needed water access to import bread, because a trade route is a value-density filter.
Two corrections define the Silk Road: it was a shifting web, and almost nobody travelled its length. Goods moved through relays.
A bolt of silk reached Rome traded twenty times. This relay structure explains markups, mutual ignorance, and why the network's most consequential cargo was disease and ideas.
The Container Shift
For history, transport costs shaped the map. Then in 1956, Malcom McLean introduced the standardized steel container, and transport costs collapsed toward irrelevance.
This structural shift transformed global geography:
- Loading time collapsed: ports went from piece-by-piece dock labour to single crane movements.
- Value density stopped mattering: shipping became so cheap that bottled water and low-value bulk goods cross oceans.
- Location de-anchored: manufacturing fled expensive markets for cheap labour, and old ports died while automated stack-terminals rose.
The lesson: Pre-modern laws of economic geography—that luxuries travel and bulk does not—were simply statements about the high price of moving things. Change the price list and the rules evaporate.