http://theconversation.com/public-goods-made-america-great-and-can-do-so-again-74421

The formal definition of a public good is that it’s something that is nonexcludable and nonrivalrous. That’s a fancy way of saying that everyone can take advantage of it and that one person’s use doesn’t reduce its availability to others.

Setting aside for a moment natural public goods, the ones provided by the government have been on the decline. U.S. public capital investment, net of depreciation, fell to just 0.4 percent of GDP in 2014 from 1.7 percent in 2007 and about 3 percent in the 1960s.