By taking the annual intake of prisoners and dividing by population, Keith Humphreys has made an incredible chart:
What's the story? Crime has been falling fast for over 20 years, for one. Second is the sheer size of the prison population, which is still so mind-bogglingly huge that growth was becoming very hard to sustain (and to be clear, America still has the highest incarceration rate in the world). A third reason, and one which isn't as appreciated, is a movement among conservatives to reform our prisons, in large part because of their sheer expense.
Whatever the reason, this is certainly great news.