"Over the last 25 years, the number of state facilities increased from just fewer than 600 to over 1,000 in the year 2000, an increase of about 70 percent. In other words, more than 40 percent of state prisons in operation today opened in the last 25 years."

Source

Lawrence, Sarah and Jeremy Travis, The New Landscape of Imprisonment: Mapping America's Prison Expansion, Washington, DC: Urban Institute, April 2004.