A study I just found, Behind Bars, has this to say about the connection between drug use and violence:

Contrary to conventional wisdom and popular myth, alcohol is more tightly linked with more violent crimes than crack, cocaine, heroin or any other illegal drug. In state prisons, 21 percent of inmates in prison for violent crimes were under the influence of alcohol–and no other substance–when they committed their crime; in contrast, at the time of their crimes, only three percent of violent offenders were under the influence of cocaine or crack alone, only one percent under the influence of heroin alone.

It looks to me like the way to solve this problem is obvious. Since drug prohibition is so effective in combating violence caused by drug use we need to bring back alcohol prohibition. And one other point. I don’t know why they left out all the violent crimes committed by pot heads.