Jails across the country have become vast warehouses made up primarily of people too poor to post bail or too ill with mental health or drug problems to adequately care for themselves, according to a report issued Wednesday.

The study, “Incarceration’s Front Door: The Misuse of Jails in America,” found that the majority of those incarcerated in local and county jails are there for minor violations, including driving with suspended licenses, shoplifting or evading subway fares, and have been jailed for longer periods of time over the past 30 years because they are unable to pay court-imposed costs.

The report, by the Vera Institute of Justice, comes at a time of increased attention to mass incarceration policies that have swelled prison and jail populations around the country. This week in Missouri, where the fatal shooting of an unarmed black man by a white police officer stirred months of racial tension last year in the town of Ferguson, 15 people sued that city and another suburb, Jennings, alleging that the cities created an unconstitutional modern-day debtors’ prison, putting impoverished people behind bars in overcrowded, unlawful and unsanitary conditions.

While most reform efforts, including early releases and the elimination of some minimum mandatory sentences, have been focused on state and federal prisons, the report found that the disparate rules that apply to jails is also in need of reform.