Answer:
Interstate Highways are maintained by the Federal Government, and more specifically, by the Federal Highway Administration, which is part of the Department of Transportation.
The funds for their maintenance mainly come from fuel taxes collected by the Federal government, and also by state and local goverments. A smaller percentage of these funds come tolls located in different parts of the highway.
They traded things like information and spices, new diseases were brought over, and I imagine religious views were shared too!
