The Justice Department's recent investigative report on Ferguson, Mo., has become required reading for police chiefs across the nation. It should be for judges and city and court administrators as well.

The report lays out a compelling case of police abuse, including evidence of intentional discrimination against African-American residents. The report's core finding, however, is not focused on racism but instead goes to the tension in many local governments around the role and importance of criminal fines and fees. Justice Department investigators concluded that "law enforcement practices are shaped by the City's focus on revenue rather than by public safety needs."

The DOJ report joins a growing number of investigative reports by the American Civil Liberties Union, the Brennan Center for Justice and even HBO's "Last Week Tonight" that question the application of fines and fees by local and state courts. They cite cases where fine and fee structures have resulted in a return to debtors' prisons -- where individuals who fail to pay a parking or traffic ticket can go to jail or accumulate debt that can never be repaid.

There is another side of the story, though. Fines are a legitimate means of punishing wrongdoing, both big and small. Yet, like other sanctions, criminal fines have the desired punitive or deterrent effect only if they are enforced and are collected.

The problem of non-collection of criminal fines is a real and growing one. In Iowa, for example, court debt has more than doubled over 10 years -- increasing to more than $630 million in 2013. In California, unpaid court debt reached $10.2 billion in fiscal year 2012.

Six principles should help guide state and local governments to reforms that make criminal fines and fees more fair, effective and efficient.

Justice and public safety concerns should be paramount. Judges and court officials often reject a role in fine collection, arguing that it is not their job to generate revenue. The policy goal for fines and fees should be to increase safety, not revenue.

Courts should enforce fines and fees as they would enforce any other sanction. A 2009 study by the National Center for State Courts argued that "enforcement of a fine is as important to the integrity of the court as the enforcement of any other sentence or judgment." Collection rates are a valid and important means of measuring the justice system's effectiveness.

Fines and fees need to be set at levels that are both fair and proportionate to the offense. Fines and fees that seem unreasonable undermine the court's integrity and ultimately lead to the problems of non-payment and the costs of enforcement.

Courts should expect that fines and fees are paid at sentencing. Most successful collection programs start with an expectation set by the court that offenders are expected to pay on the day of sentencing. When courts assume that most offenders cannot pay, it contributes to both abuse and inefficient and ineffective fine collection.

Fines should be set based on a combination of the gravity of the offense and the defendant's ability to pay. One size does not fit all. Fines and fees tailored to ability to pay will make payment at sentencing more likely -- ensuring that sanctions are swift and certain and that non-payment is reduced. The European day fine model, for example, recognizes that a fine of $100 has a different deterrent effect for someone making the minimum wage than for someone making $100,000 a year.

Jail should be an option only where fines are an alternative to incarceration and where failure to pay is truly willful. In Ferguson, the DOJ report suggests that warrant, arrest and jail were the principal means of enforcing uncollected fines and fees. But unless fines are meted out as a sanction in lieu of incarceration, there is no public-safety case for incarceration for non-payment. Moreover, jail costs frequently are higher than the outstanding court debt. For offenders who lack any means to pay, there should be a viable option of community service.

Generally, governments should explore every path to recoup outstanding fines short of incarceration -- liens, tax and other payment intercepts, garnishment, and active and frequent efforts at collection. The last thing America needs now is a return to debtors' prisons.

This column represents the views of the author only and not necessarily those of the National Resource Network.