A cell at the federal prison in Coleman, Fla. (Nikki Kahn/The Washington Post)

THE FEDERAL Bureau of Prisons will release 6,000 inmates locked up for nonviolent drug crimes at the end of the month. If a bipartisan group of senators gets its way, that will be just the beginning. On Thursday, the group pushed a criminal justice reform bill through the Judiciary Committee that backers say would reduce the federal prison population by tens of thousands.

All of this is progress. But even if the bill passes, the number of people in prison in the United States would still be astoundingly high.

The Sentencing Reform and Corrections Act would adjust mandatory minimum sentences, with a focus on reducing penalties for nonviolent drug crimes. Only “serious” drug felonies and violent crimes would trigger enhanced mandatory sentences of 25, 20 or 15 years. Judges would have more leeway to excuse offenders from mandatory sentences, but not if they had prior violent or drug trafficking convictions. Judges would also be able to offer leniency if they found that “prior offenses substantially overstate the defendant’s criminal history and danger of recidivism .” The bill retroactively applies the Fair Sentencing Act, which removed the difference between sentences for crack and powder cocaine. That would help people in prison under old rules that are now widely regarded as unfair.

The bill demands that federal prisons assess each prisoner for his or her risk of reoffending and continually update these assessments. Prison programs would aim to reduce recidivism and would offer rewards for successful completion. Prisoners who complete the programs and are judged to present a limited reoffending risk would be eligible to serve parts of their sentences in pre-release custody at home or in a halfway house.

These are good ideas that should reduce the severity and expense of federal incarceration without sacrificing public safety. The ideologically diverse group of senators who negotiated the package — Cory Booker (D-N.J.), John Cornyn (R-Tex.), Richard J. Durbin (D-Ill.), Lindsey O. Graham (R-S.C.), Charles E. Grassley (R-Iowa), Patrick J. Leahy (D-Vt.), Mike Lee (R-Utah), Charles E. Schumer (D-N.Y.) and Sheldon Whitehouse (D-R.I.) — deserves credit for striking a bargain that has a good shot at passing Congress.

Even if this bill took full effect, however, the U.S. prison population would remain far higher than that of most other nations. About half of federal offenders are incarcerated on drug crimes, but the vast majority of prisoners are in state, not federal, prisons, where the proportion of drug offenders is significantly lower. So even if states adopted similar reforms focusing on nonviolent drug offenders, the effect would be limited. FiveThirtyEight’s Oliver Roeder calculates that if the United States released all drug offenders from federal and state prisons, the country would still have the highest per capita incarceration rate in the world — by a significant margin. Reducing that is not as simple as letting a few pot smokers out of jail.