There is a public awareness that prisons are expensive, but do people know how much they are paying for local jails?

No, according to a report from the Vera Institute of Justice and the MacArthur Foundation, which found that most people greatly underestimate how much it costs to run a local jail. When we talk about mass incarceration, though, the spotlight has mostly been on prisons -- where those convicted of a crime serve sentences. Local jails, however, are an important aspect of mass incarceration. They are found in many cities and require significant costs to operate.

What's the difference between jail and prison?

Ostensibly, jails are where people who have been arrested -- though not convicted -- are held as they await trial. Not all accused go to jail -- only those who cannot make bail or those deemed too dangerous to be offered bail, usually a person who is seen as a public safety risk or a "flight risk" (i.e., the fear that this person will flee the jurisdiction to avoid trial).

Bail is a payment made by a defendant awaiting trial that allows them to leave jail. It's generally paid with the caveat that the money will be returned when they come to trial, making it a kind of insurance policy against the person missing court. The problem is that people are often too poor to afford bail, and jails are increasingly housing poor people, disproportionately people of color, who cannot post bail while they wait to go through the court process. At any given time, local jails house one-third of the incarcerated population of the United States. According to Vera, 12 million are admitted to jail each year.

Jail populations have grown -- Vera reports that the number of people held in jails today is three times as many as in 1983. And jail costs have also soared. In 2011, it was estimated by Vera that jails cost local communities $22.2 billion. This number, however, is incomplete and doesn't include costs like medical care for inmates or employee benefits.

"For example," Vera reports, "in addition to the $1.1 billion spent by the City of New York Department of Correction in 2014, other city agencies spent an additional $1.3 billion for jail employee benefits, health care and education programs for incarcerated people, and administration, bringing the total cost to $2.4 billion."

The incomplete number means that taxpayers don't realize the true amount of money being invested in jails -- jails that are just the first step of mass incarceration.

The study surveyed 35 jail systems -- small, medium, and large --  in 18 states around the country. The survey sample represented nine percent of the total jail population. It found that out of six major jail expenditures, the largest chunk of money went to jail personnel.

"As government leaders and the public question whether jail is being used cost-effectively to meet a community’s safety and justice needs, they need to understand what the total costs are." the study's conclusion reads.

The solution?

The solution seems pretty obvious: hold fewer people in jail.

Kentucky has done it. The state managed to reduce its jail population and save millions by implementing a statewide pretrial release program in 2005 that evaluated defendants on an individual basis. The program "identifies appropriate candidates for release using an evidence-based risk assessment instrument and provides the court with a range of non-monetary release options," according to The Drug Policy Alliance. Jailing fewer people in Kentucky has not resulted in an increase in missed court dates, with the appearance rate still extremely high, the DPA reports.

Not everyone is thrilled with bail reform, though, particularly those who get rich off of it: commercial bail bondsmen, which are are private businesses that provide a loan -- with fees -- to a person looking to post bond. Broward County, Fla., offers a horror story about this industry's power. Like Kentucky, Broward County sought to reduce its jail population. It did so successfully in 2007 when it created a pretrial program that saved $20 million and allowed the county to close a section of its jail. But because fewer defendants were being required to post bail, the local bail bond industry was making less money and was not happy. So, according to Governing.org, they hired a lobbyist to roll back the reform -- a lobbyist who was already a representing the Broward County Board in a different capacity. In 2009, the county backed down and ended the pretrial program.

The good news is that the path to reform is clear. The bad news is that states and municipalities will no doubt face opposition from entrenched industries that depend on mass incarceration in our jails.