Before roads, rivers served as highways for travel and trade, and barges became flat-bottomed boats built mainly to move bulk goods on rivers and canals. Although some are self-propelled, barges are usually towed by tugboats or pushed by towboats with square bows, shallow draft, and bow-mounted knees, and the connected group of barges they push is called a tow. Depending on the waterway and cargo, a tow may include one barge or dozens, though on lock-and-dam rivers such as the Monongahela River it is limited to sixteen. Barge transport is more economical than rail or truck shipping, costing about 50 percent less than rail and 90 percent less than truck transport, while moving about $1 billion of goods each year, roughly 15 percent of all freight. On the Monongahela River, primary cargo includes coal, steel, coke, and iron, and an average cargo barge can carry 1,500 tons, 62,500 bushels, or 453,500 gallons, equal to 58 trucks or 15 rail cars, while a 15-barge tow can carry 22,500 tons, 767,500 bushels, or 6,804,000 gallons, equal to 1,050 trucks or two trains of more than 100 cars. Types of barges include open dry cargo barges for materials such as coal, steel, sand, gravel, ore, lumber, and garbage; covered dry cargo barges for goods that must be protected from the elements, including grain, soybeans, coffee, and paper; liquid cargo barges for petrochemicals, liquid fertilizer, refined petroleum products, and black oil products; split-hopper barges used in dredging waterways; coastal or ocean-going barges used chiefly in the United States for petroleum products and very large equipment; and car-float barges fitted with train tracks to move rail cars across water obstacles or to otherwise unreachable destinations.