Florida inmates convicted of non-violent drug crimes spent 194 percent more time behind bars in 2009 than they did in 1990, costing the state billions of dollars but providing little public safety benefit, a new study found.

The study, by the Pew Center on the States, examined trends in 35 states that provided data on incarceration for inmates convicted of violent crimes, property crimes and drug offenses. It found that nationally, state inmates across all categories of offenses served an average of nine additional months in custody, a rise of 36 percent since 1990. Florida led all states with an increase of 166 percent in time served for all prisoners.

The rise in time served costs states an estimated $10 billion per year, the study found.

The increase in time served for drug offenders, which was roughly the same as the rise for violent criminals, did not lead to safer streets, an analysis of post-release data showed.

"The idea behind longer prison terms is that they will cut crime and recidivism. But for a large number of lower-risk offenders, that just isn't the case. There's a high cost and little to no crime control benefit," Adam Gelb, the study director, said in a statement.

The study found wide disparities in incarceration trends among the states, with some lengthening stays and others significantly reducing them. All of the states that reduced average prison stays experienced overall drops in crime, suggesting that longer periods of incarceration had minimal public safety benefit.

Florida drug offenders now serve an average of 2.3 years in prison, compared to an average of about nine months in 1990, increasing prison costs by $1.4 billion annually.

Other states saw similar increases. In Arkansas and Oklahoma, time served for drug crimes rose by 122 percent, to an average of three years.

Several states reduced average prison terms for non-violent crimes. Illinois drug offenders served 1.2 years in prison in 2009, 25 percent less time than in 1990. Prison time for Nevada drug inmates fell by 16 percent, to 1.8 years. Overall crime rates fell sharply in both states during this period.

Tough-on-crime policies requiring harsh mandatory sentences and a tightening of parole standards have played pivotal roles in driving the increase in time served, the study found.

In Florida, the sharp increases in time behind bars came after a period of relatively lax sentencing for all inmates. In the early 1990s, Florida prisons suffered from extreme overcrowding, and prison authorities responded with aggressive parole programs that let prisoners go after serving an average of 30 percent of their sentences.

After several parolees were convicted in high-profile crimes, state legislators passed a law requiring all prisoners to serve at least 85 percent of their sentences, and the state embarked on a prison building spree. Many other states passed similar laws.

Lengthening prison stays contributed to a record increase in the U.S. prison population, which rose more than 700 percent between 1970 and 2005, the study found.