Why do we need more jail space when the crime rate keeps falling?

There is both a general and a specific answer to this question.

Generally, the crime rate is an "often talked about, but seldom understood" statistic. The crime rate to which most people are referring is the FBI's Uniform Crime Rate, which has been collected nationally for more than 50 years. However, the State of Iowa also calculates crime rates. Unfortunately - they don't measure the same crimes.

The FBI's crime rate measures seven serious felony offenses. These offenses are most likely to influence the prison system, not jails. For example, the FBI's crime rate does not include drug offenses - which are one of the most frequent reasons why people are incarcerated. It also doesn't include any misdemeanor or traffic offenses, like Driving Under the Influence, Driving Under Suspension, Failure to Appear or Comply, etc, which are the most common reasons why people are incarcerated at the local jail.

The State of Iowa's Group A crime rate is more representative of all types of crimes, but is still weighted toward felony offenses. These offenses may influence the local jail on a pretrial basis, but once convicted, these individuals go to the state system. It is the Part B offenses, like DUI, that influence the jail more - because of both their volume and because when sentences are imposed, the local jail will be the place of confinement.

Specifically, generalizations about falling crime rates reflect what's going on as a whole. While crime rates may be going down nationally, they are stable or rising in some jurisdictions. Scott County crime rates, using Iowa's Group A definitions, have actually been increasing.

Mathematically, crime rates express the number of selected crimes in terms of the number of residents. Because both measures can be changing, it is possible to have falling crime rates, while the number of crimes are actually increasing - because of changes in the population as a whole. That also happened during some of the years that we studied - particularly in the City of Davenport.