An interesting article on the FHWA's website explains the history of tolls and our nation's Interstate highways.
Today, it says, the 46,730-mile Interstate System includes approximately 2,900 miles of turnpikes.
Interestingly, in a 1939 report to Congress, the U.S. Bureau of Public Roads rejected the toll option for financing Interstate construction. The reasoning was that people would avoid tolls, so most Interstate corridors wouldn't generate enough toll revenue to retire the bonds that would be issued to finance them. Then the first segment of the Pennsylvania Turnpike opened on Oct. 1, 1940, and was an instant financial success.
(And by the way, according to Wikipedia, the term turnpike originated from pikes, which were long sticks that blocked passage until the fare was paid and the pike turned at a toll house, which today we call a toll booth.)
After World War II, the turnpike's continued success prompted other states to use the same financing method. Turnpikes appeared or were planned in Connecticut, Florida, Illinois, Indiana, Kansas, Kentucky, Maine, New Hampshire, New Jersey, New York, Oklahoma, Virginia and other states, often in corridors that had been designated as part of the Interstate System in 1947. These roads were built without any federal funding.
In 1956, as Congress debated how to pay for the new Interstate System, it decided to authorize the BPR to incorporate those existing toll roads into the Interstate System to ensure connectivity. So essentially those toll roads were grandfathered into the system.
Under current law, states generally cannot put tolls on previously toll-free interstate stretches built with federal money. But as our infrastructure continues to deterioriate, paid for by a fuel tax that hasn't been raised since 1993, many are pushing the idea of adding tolls to existing interstates. So far it's just limited to a pilot program, but with major opposition to the idea of any tax increases, I wouldn't be surprised to see more.