In 1996, 27% of solid waste in the United States was recycled. Products that are recycled in large quantities include paper and paperboard, ferrous metals, aluminum and other nonferrous metals, glass, plastics, and yard wastes. Although many local communities have instituted comprehensive recycling programs, these remain expensive. Because the quality of recycled items is often inferior (often due to the mixture or age of the materials in the items being recycled) and not suitable for their original purpose, the price for many recycled materials remains low and makes recycling economically nonviable in some instances. In an attempt to solve this problem, new uses have been created for recovered waste material. Crushed glass, for instance, can be substituted for gravel or sand in road surfacing and other construction applications; the resulting product is called "glassphalt." Scientists and entrepreneurs are also working on ways to turn the world's growing piles of discarded automobile tires into new products or to use them to generate safe energy.