Reinforcing New York City’s improved policing strategies in the 1990s were tougher sentencing laws and a significant expansion of the city and state correctional systems. Would-be criminals came to realize that they were not only more likely to get caught, but more likely to end up serving hard time.

The results have been nothing less than spectacular: By one key measure, serious crime in the city has dropped 70 percent over the past 15 years. And now that success is yielding another, less widely noticed, dividend: With felony arrests dropping as a result of the falling crime rate, New York’s once-swollen city jails and state prisons are becoming less crowded. This has begun to generate hundreds of millions of dollars in annual savings for taxpayers.

The number of state prison inmates rose steeply in the late 1980s, with major (“index”) crimes peaking in 1990. The prison population continued to rise for a few years even after the drop in crime began to accelerate rapidly in 1993. But in 1997, the prison population (roughly two-thirds from New York City) dipped slightly for the first time in 25 years. After slight rises in the next two years, it began a five-year decline in 2000.

The December 2004 population of just over 64,000 inmates was 10 percent below the 1999 peak of 71,431.

Nearly three-quarters of this decrease has consisted of nonviolent offenders, more than half of whom were serving drug-related sentences. Even before the recent changes to the drug laws, the state was channeling more nonviolent offenders into treatment programs, work release or “shock incarceration” (a form of behavior-modifying boot camp). As a result, a growing share of the remaining inmates are violent offenders. And in the wake of Pataki-era sentencing reforms, violent felons have been serving more time (26 months more, on average).

Recidivism — the rate at which ex-cons return to prison within three years of their release — decreased by about 16 percent during the 1990s.

New York City’s jail population peaked earlier, in 1992, and has dropped even further in recent years. By the end of 2004, the daily count of city inmates was below 14,000, about a third below the 1992 peak of 21,000. This was, in part, a ripple effect of the state’s prison expansion — the construction of more cells upstate meant that fewer convicted “state-ready” felons had to be held at Rikers Island.

The decreasing jail population has enabled the city to close its detention facilities in Brooklyn, Queens and The Bronx, to close some wings in other facilities and to cut the headcount of jail employees, saving at least $185 million a year in staff costs alone. The city corrections budget is down nearly 8 percent in the last three years.

At the state level, the number of corrections officers has dropped some 1,500 since 2000, generating a wage-and-benefit savings of roughly $120 million a year. But the Legislature has blocked greater operational savings by refusing to accept Pataki’s plans to close several prisons, which poorer upstate communities view as a jobs program. Given the state’s huge budget gap and the continuing prison-population drop, this fight should intensify in the year ahead.

Nationally, the number of criminals behind bars continues to grow. New York is one of only two states to report fewer inmates in 2003 than in 1995.

For years, the Left has warned that tougher sentencing would create a syndrome of “mass incarceration” and the creation of a “prison-industrial complex.” But as the New York experience shows, the crackdown has worked on more than one level. Good policing and tough sentencing have pushed New York to a tipping point, deterring some potential malefactors from crime. Now, if crime goes down and stays down, the prison population should keep dropping as well.