Reasonable prison systems incarcerate much smaller percentages of their populations than the USA one does. Any reform worth contemplating will result in the release of many prisoners, and the closure of prisons in all but the most overcrowded states.
Already in the Prison Treatment Act of 1945, the view was expressed that the deprivation of freedom itself should be regarded as the penal element of a prison sentence and not the actual prison experience itself. Thus, the PTA of 1974
states explicitly that an inmate shall be treated with respect for his or her human dignity. [1]
As ever when I look for an country that I think will be an example of doing something well, I start with the nordics. And I see that Sweden has recently closed 4 prisons because they don't have enough prisoners to fill them any more. [2]
I would love to hear counter points if my cursory reading has missed something - but in general I was once again impressed by the attitudes I see there.
Counterpoint: the Norwegian government is considering hiring capacity in the Netherlands for 700 long term prisoners. Sweden is said to have considered hiring capacity as well but decided against because of the legal minefield surrounding this topic.