The other problem is that, in my own opinion, I believe we are filling up prison space with non-violent offenders.
If the US would abandon the war on drugs and decide to treat substance abuse as a medical problem rather than a legal problem, we would free up lots and lots of space in prisons.
Right now, just over 1% of the US population is in jail or prison. That's higher than almost any other industrialised nation. We can't sustain that sort of spending on the prison system and take care of everything else that needs to be taken care of.
For instance, if anyone cares to google on "us bridges needing repair" they may never cross another bridge in the US without a few questions as to the structural safety going through their mind.
Between locking up more pot heads and fixing bridges? I'll always vote for fixing bridges.