Keeping the transport infrastructure in good shape is paramount for good trade. But throughout the centuries few innovations have had a more beneficial effect on trade than establishment of organized wagon trains - large groups of cargo carts accompanied by replacement animals, guards and supply loads, which are able to stop at pre-established post stations to rest and resupply on-the-go. As a result, not only were trade caravans able to trade more effectively, but they also damaged the roads much less.
This is a great opening Policy for this Tree, which substantially reduces Gold costs for large empires (which generally have a larger road network). But besides the immediate increase in available cash, you will be able to benefit better from all your land trade routes. There are few cons to adopting this as your first Commerce Policy, unless you're in a war and could really use an interesting new unit such as the Landsknecht.
In various ages, as roads became better and less prone to banditry, wagon trains replaced caravans in overland trading ventures. Able to carry more goods of more types, to provide shelter, and to travel further, wagon trains were used for commerce, war and migration. Used heavily throughout history, wagon train usage has largely faded with the arrival of the industrial age.