In 1797, Mungo Park was surprised to come across a Toll Bridge in the Sahel. It illustrates that the internal economy of West Africa was far more sophisticated than most contemporaries in Europe could have anticipated. Interestingly, he drew himself into his picture in the bottom right hand corner.
"The chief of Manna, with a number of his people, accompanied us to the banks of the Bafing, or Black River (a principal branch of the Senegal), which we crossed upon a bridge of bamboos of a very singular construction. The river at this place is smooth and deep, and has very little current.
Two tall trees, when tied together by the tops, are sufficiently long to reach from one side to the other, the roots resting upon the rocks, and the tops floating in the water. When a few trees have been placed in this direction, they are covered with dry bamboos, so as to form a floating bridge, with a sloping gangway at each end, where the trees rest upon the rocks.
This bridge is carried away every year by the swelling of the river in the rainy season, and is constantly rebuilt by the inhabitants of Manna, who, on that account, expect a small tribute from every passenger."