The new criminal justice law Governor Baker signed last week could prevent hundreds of small-time defendants from spending time in Massachusetts prisons just because they can’t scrape together a few hundred dollars for bail.

Bail is designed to be an economic incentive to show up in court – you pay your money to go free and get the money back when you when show up for your court date. But if you can’t afford the bail, you have to wait in prison.

An analysis by the state courts system two years ago found that more than 1,500 people at any one time are in jail awaiting trial because they couldn’t pay bail amounts less than $500.

Muriel Kramer, of Hopkinton, is one many local volunteers challenging the system, by taking small withdrawals from a cash fund to bail out poor offenders.

Kramer is not the most likely activist. She and her husband are Air Force veterans and live in a 4-bedroom house in Hopkinton where they raised six kids.

But around dinner time on a rain-swept Tuesday evening last winter — more suited to cozying up on the couch — Kramer was getting ready to drive to a prison to post bail for two women she’s never met. And the plan was to drive one of them home, an hour south in Brockton.

“It’ll be a 2-cup of coffee night, at least, for me,” she said, laughing.

Kramer volunteers for the Massachusetts Bail Fund, a nonprofit based in Cambridge that has bailed out just over a thousand men and women held in local jails on bail amounts of $500 or less.

“One of the women that we may post bail for tonight has a $200 bail,” she said. “Now if you or I had a $200 bail for whatever reason, it is unlikely that we would be staying over a night.”

But poor arrestees who can’t pay bail can sit in jail for days, weeks, months and even years waiting for the wheels of justice to turn.

The state Supreme Court unanimously ruled last August that judges should consider a defendant’s finances before setting bail and explain the reasoning if the amount is clearly unaffordable. That’s been codified in the state’s new criminal justice law.

State Sen. William Brownsberger (D-Belmont), one of the co-authors of that law, said bail doesn’t always make sense.

“Sometimes, it’s just not worth it,” he said. “We want judges to think carefully about whether in any given case holding the person is really in the commonwealth’s interest.”

The state can end up spending hundreds or thousands of dollars housing prisoners just because they can’t pay a few hundred dollars in bail.

The woman Muriel Kramer was posting bail for had been jailed for three weeks on $200 bail, already costing taxpayers about $3,200, according to recent state data on correctional spending.

The Massachusetts Bail Fund is making its own dent in that public spending. With money from private donations, the group bails out about 40 people a month, all of them referred by public defenders. Half the defendants helped by the fund see their cases dismissed. And 94 percent show up to court and get the bail money back to the fund, said Atara Rich-Shea, a former public defender who now runs the Bail Fund.

“Bail is important because imagine what would happen in your life if you were not allowed to go home,” said Rich-Shea. “Your car would get towed, you couldn’t pick up your kids from school, you wouldn’t be able to go to work tomorrow. People lose their homes, their kids, they lose their jobs.”

Those are all outcomes that could be solved by a few hundred dollars, along with the patience and stamina of volunteers like Kramer.

After nearly three hours of sitting and waiting at the Framingham women’s prison, Kramer was called down the hall into a commissary and laid out a wad of cash on a table – $200 for bail and $40 to cover the fee for the bail commissioner.

The formalities took just minutes. But the wait dragged on for another two hours before the woman Kramer posted bail for was actually released.

Lorraine — not the woman’s real name — climbed into the front seat of the car next to Kramer.

“Thank you all so much,” she said. “God is good. This is a blessing.”

The 42-year-old woman freshly bailed out from prison told Kramer that she is mentally ill. Three weeks earlier, she was charged with assault, disorderly conduct and threatening to kill a hospital staffer. She couldn’t pay the $200 bail and was sent to Framingham prison.

“I'm out of jail. It can’t get no better than that,” she said as the car rolled closer to her home and familiar sights. “I just want to relax and be home, that's all I really want to do.”

By the time Lorraine walked onto the porch of her Brockton home, it was midnight and the rain was still falling. Kramer felt satisfied with her mission, with what she calls “evening the playing field.”

“Being able to directly help a person in a very concrete way access their freedom, is an amazing gift to my spirit,” she said.

Chris Burrell is a reporter for the New England Center for Investigative Reporting, a nonprofit news partner of WGBH News. Chris can be reached at burrellc@bu.edu. For more, go to www.necir.org.