Iran's geography ensures that transport is a major problem for economic activity. Much of the country is sparsely settled; the major cities are hundreds of miles apart, with some roads running through inhospitable deserts. The population centers are separated from the oceans by high mountains. There are no rivers suitable for transportation over any distance. In short, transport is overwhelmingly by land through difficult terrain.
These natural barriers provide an explanation as to why Iran's economy consisted for many centuries of a series of local or regional markets, more than a national market. Producers and merchants did not consider their natural market to be Iran but rather their local region and, beyond that, the areas to which they could easily transport their goods, which could well mean nearby foreign regions rather than other parts of Iran.