Transport has affected economic development from the beginning of human civilization. Economic development focused on the confluence of transport systems — early cities grew up on natural bays and ports, and on rivers and lakes where transport was available. Romans built roads to unify and provide access to their far-flung empire. Geographic characteristics such as proximity to oceans, seas, and waterways, plains, mountains and the location of oases defined early transport systems (e.g., the “Silk Road” went from oasis to oasis, and city to city, where there were no reliable water or road routes).