Inmates sit in a discussion group at the Stanislaus County Jail in Modesto, Calif. (Rich Pedroncelli/AP)

The first of the convoys left Albuquerque in August 2013. A cruiser would take the lead, followed by a few buses and vans and another cruiser at the rear. For more than a year, they ferried inmates between an overcrowded jail on the city's outskirts and another facility with empty cells, more than 800 miles away.

The jail in Albuquerque, like many others around the country, didn't have enough room. Authorities were spending $35,000 per day to house inmates outside the county.

“This was something that had to be done,” said Capt. Ray Gonzales of the Metropolitan Detention Center in Albuquerque.

Then, things changed. The jail inmate numbers plummeted. The convoys stopped. Bernalillo County, which includes Albuquerque, now incarcerates 38 percent fewer people than it did in 2013.

Despite this transformation, jails such as Albuquerque's have gone almost unmentioned in the bipartisan discussion about the huge number of Americans behind bars, which has solely focused on prisons.

On Monday, President Obama commuted the sentences of 46 prisoners convicted of drug crimes, and he'll call on Congress to reform prison sentences Tuesday, according to a White House official. Obama will become the first sitting president in history to visit a federal prison later this week.

[Obama almost never commutes the sentences of inmates, but that could change]

The commutations followed a painstaking review by senior administration officials, and sentencing reform would take enormous legislative effort. The effect on the incarcerated population as a whole, though, will be small compared with what has already happened in some jails around the country and what could happen with many more.

Jails might look similar to prisons from the outside, but the differences are crucial to understanding the challenges of incarceration in America.

The federal government or states run prisons, while most jails are run by local governments. Prisons house convicted criminals who are serving their sentences, while about three in five jail inmates have yet to go to trial. A short stay in jail can be consequential: After just a few days inmates are more likely to commit crime when they leave, research shows.

Jail is what a man in North Charleston, S.C. feared when he fled arrest in April and was shot to death by a police officer. The New York City jails — on Rikers Island — are where a boy was held for three years without trial, beaten and held in solitary confinement, until the charges against him were dropped. He took his own life last month, two years after his release.

“Jails really are where the action is in terms of addressing mass incarceration,” said Cherise Fanno Burdeen. She is the executive director of the Pretrial Justice Institute, a research organization in Washington that advocates for jailing defendants awaiting trial only when necessary.

Burdeen and other criminal justice reform advocates say that jails, not prisons, are where reforms can both improve the lives of the most people and address some of the most difficult challenges in the criminal justice system. The scope of the jail system is much larger, and because the inmates in jails are less likely to be dangerous criminals, political progress may be more feasible.

That's what officials in New Orleans; Hampden County, Mass., and Mesa County, Colo. — all places that saw rapid reductions in jail populations — have discovered. “That type of decline could never happen in a state prison,” said Christian Henrichson, an analyst at the Vera Institute of Justice.“It's just for all purposes not mathematically possible.”

‘More punitive about everything’

The population in American prisons began to increase in the 1970s when Congress and state legislatures started passing harsher sentencing laws in response to rising rates of violent crimes. As criminals were convicted, they stayed in prison longer, and the total number of prisoners began to increase.

“The reality is, we got a lot more punitive about everything,” said Jesse Jannetta, a scholar at the Urban Institute.

Long after crime rates began to fall again, those who had been convicted at the peak of the crime wave weren't getting out. Only since 2007 has the share of Americans in prisons stabilized and begun to fall.

Taking state and federal prisoners together, just under half had been convicted of violent crimes, according to federal data from 2013. Such crimes are rare, but violent criminals serve longer sentences in prison and make up a larger share of the population as a result.

[The U.S. imprisonment rate has fallen for the fifth straight year. Here’s why.]

Meanwhile, the population in jail increased along with the population in prisons, more than doubling between 1983 and 2007. The Justice Department's annual report on jails, released last month, found that the number of Americans in jail held steady last year at about 745,000 on any given day, or about 302 inmates per 100,000 adults. All of this puts a financial burden on localities; a federal report found that local spending on corrections totaled $26.4 billion in 2011.

The reasons for the increase in jail populations are unclear. Whether and for how long someone stays in jail depends on a series of decisions by police, prosecutors and judges, and all of them may have adopted a less forgiving attitude toward suspects as a tough-on-crime culture took hold, criminal justice analysts say.

In contrast to those in prison, most people in jail haven't been convicted of anything. Many others are mentally ill, as a result of a shift away from housing the mentally ill in large hospitals.

Although jails house around 745,000 people at any given time, millions of Americans — there's no precise estimate — enter and exit the country's jails every year. The average inmate's stay in a jail is 23 days, according a recent report from the Vera Institute, compared with roughly two years in state prisons.

“You have a lot more people touched by the jail system than by the prison system,” Jannetta said.

Ending an era

Prominent politicians in both parties are now rethinking the past four decades of American criminal justice. They've suggested undoing some of the more punitive laws passed during the crime wave and reducing prison terms for minor offenses.

“We need common-sense criminal-justice reform. We need to give people the opportunity to earn a second chance,” Rep. Paul Ryan (R-Wis.) said last year. “There is no reason to lock someone up any longer than necessary.”

“It's time to end the era of mass incarceration,” Hillary Rodham Clinton declared in her first major speech as a presidential candidate, calling on Americans to rethink “how we approach punishment and prison.”

For the most part, these efforts would affect only a small number of prisoners. Many prisoners were convicted of more serious crimes, and they wouldn't be eligible for such relief. And while the debate over prison policy invites questions about whether dangerous criminals will be released and harm the public, that isn't much of an issue when it comes to jails.

In jails, in fact, evidence is mounting that reducing the number of inmates can make the public safer. Researchers have found that defendants who are detained in jail before trials become more likely to commit additional offenses. Even a few days in jail can be devastating. A student who misses a few classes because she's behind bars might have to drop out for a semester, and a busboy who misses just one shift might lose his job.

[How municipalities in St. Louis County, Mo., use jail to produce revenue]

"We probably underestimate intuitively the impacts of these very short stints," said John Pfaff, a law professor at Fordham University.

Those stints appear especially risky for the mentally ill. The suicide rate in prisons is only slightly higher than that in the general population, but the rate in jails is more than three times as high, according to federal statistics. Jail inmates are most at risk of suicide in the first two days of their confinement — whether because of the initial shock of being behind bars, or because guards might not be aware that a new inmate needs psychological care.

Gangs, addicts and government agencies

Bail is a component of the courts that dates at least to biblical times. Its simple purpose is to ensure that people accused of crimes appear before a court as scheduled by securing some kind of collateral from them.

And it's one of the aspects of the jail system that advocates and researchers say holds out some of the most promise for reform. These critics say that bail puts harmless defendants behind bars because they can't pay — and suffer the consequences of detentions — while allowing dangerous criminals with financial resources to go free until their trial. Initial research conducted in 1961 and 1962 by the Vera Institute showed that many defendants would return to court on time, even if they weren't asked to post bail.

[How bail punishes the poor for their poverty]

"If you think about really significant criminal organizations and individuals, they can meet many of the conditions that are placed on them," said former New Jersey attorney general Anne Milgram. The state's Republican governor, Chris Christie, signed into law a change in bail policy last year.

New Jersey's law establishes a system similar to one set up in 2011 in Kentucky, where judges use a formula to estimate the odds that defendants will flee or commit other crimes before their cases go to trial. The formula uses actuarial data, specifically a defendant's age and criminal history, and judges must file written explanations if they disagree with the results.

The data from Kentucky support the conclusions of Vera's researchers more than half a century ago. The state is releasing defendants with lower risk instead of those who can afford to post bail, and those who are released are committing fewer crimes.

[$500,000 bail for a Baltimore protester?]

Chicago, Milwaukee, Toledo and Pittsburgh, among other jurisdictions, also have plans to emulate Kentucky's system. And last week, New York Mayor Bill de Blasio said the courts would begin using a formula like Kentucky's to release a small group of defendants pending trial. The announcement came a month after the suicide of Kalief Browder, the young man who was held at Rikers for three years because he was unable to post a $3,000 bond.

In New York, though, state laws limit judges' authority to let offenders walk until their court date. The Manhattan district attorney, Cyrus Vance Jr., has called on politicians in Albany to change the rules.

Vance and others have encountered opposition from bail bondsmen, who rely on the existing system for their livelihoods.

Nick Wachinski, who until recently was the executive director of the American Bail Coalition, a trade group, argued that commercial bail is an effective and inexpensive way to ensure that defendants show up to court. Bail gives friends as well as family members as well as bondsmen a financial stake in the case.

"This is something that we've been doing as an industry now for decades. The record of performance is tried and true and proven," Wachinski said, while acknowledging some changes would be beneficial.

Other jurisdictions are taking different approaches to correctional reform. Hawaii has found that the threat of a very short stay — just a few days — is enough to encourage inveterate methamphetamine addicts to sober up. In Seattle, some officers have the option to take an offender to a meeting with a caseworker after making an arrest, bypassing the jail entirely.

[How Seattle is upending everything we think about how cops do their job]

These are all significant initiatives, but sometimes, reducing the jail population is as simple as getting officials from different agencies to compare notes. Coordination is a common problem in this country's fragmented, adversarial criminal justice system, said Milgram, the former New Jersey attorney general and now the vice president of criminal justice at the Arnold Foundation in Houston.

The warden doesn't decide who enters the jail or when they leave, and the people who make those decisions are used to seeing each other as rivals. Police and public defenders don't always trust each other. Nor do public defenders and prosecutors. The data necessary to understand how a jail works aren't all kept at one agency.

In Bernalillo County, the state funds and operates the courts while the local officials manage the jail. State and county officials hadn't been talking to each other.

"There was a lot of distrust, and animosity to some extent," recalled Lisa Simpson, a lawyer whom Bernalillo County appointed to help reduce the jail population. "The first step was to get everybody working together."

Once Simpson and her colleagues took a close look at jail and court records, they realized that probationers and suspects in minor crimes were filling up many of the cells.

In response, the county government paid more judges to hear more probation hearings in the state-run courts. And the prosecutor's office hired an additional assistant district attorney to handle minor offenses. With the extra manpower, the courts began to vacate many of the jail's cells. The guards have now closed a subsection for maintenance.

Debating prison policy invites controversy around moral issues such as punishment and second chances. By contrast, reducing the jail population is often just a mundane administrative task, as officials in Albuquerque discovered. It's a matter of finding ways to operate the criminal justice system more efficiently, so that suspects who don't need to be locked up before their trials aren't, and those who do need to be detained don't wait any longer than necessary.

That's rarely as easy as it sounds. Ending the era of mass incarceration was never going to be easy, though. And the jails, the evidence suggests, may well be the place to start.

Correction: An earlier version of this story misspelled the surname of an officer at the Bernalillo County, N.M., Metropolitan Detention Center. He is Capt. Ray Gonzales, not Gonzalez.

Also, an earlier version inaccurately stated that Kentucky uses information about defendants' employment status in its formula for determining the risk that they will not appear for their trial or commit another crime before it begins. The state stopped including employment as a variable in its formula in 2013, although some judges still consider whether a defendant has a job in making decisions about bail. 

This version has been corrected. We regret the errors.