Congressional Budget Office. January 2012.
The U.S. has a network of over 4 million miles of public roads. That system has faced increasing demands over time. Almost all of those infrastructure projects were undertaken using a traditional approach in which a state or local government assumes most of the responsibility for carrying out a project and bears most of its risks, such as the possibility of cost overruns, delays in the construction schedule, and, in the case of toll roads, shortfalls in the road’s revenues. Some observers assert that an alternative approach, using a public-private partnership, could increase the money available for highway projects and complete the work more quickly or at a lower cost than is possible through the traditional method. Specifically, such a partnership could secure financing for a project through private sources that might require more accountability and could assign greater responsibility to private firms for carrying out the work.
https://www.cbo.gov/ftpdocs/126xx/doc12647/01-09-PublicPrivatePartnerships.pdf [PDF format, 44 pages].