ShotSpotter is an acoustic gunshot-detection system marketed by the public-safety technology firm SoundThinking and used by well over 100 cities in the U. S. In some ways, it sounds like a law-enforcement dream come true. Before ShotSpotter, a citizen who reported hearing gunshots could report them, but usually had no idea where the sound came from. In an area covered by ShotSpotter, police can now often pinpoint the source of the gunshot with an accuracy in the range of 2 to 8 meters (about 6 to 26 feet). What's not to like about ShotSpotter?
A lot, it turns out, at least if you're the mayor of Chicago. Back in February, the office of Mayor Brandon Johnson, who won his first election campaign promising to end the use of ShotSpotter, announced that the city would not renew its ShotSpotter contract. Understandably, having spent millions of dollars on the system, Chicago Police Superintendent Larry Snelling defends ShotSpotter. He was quoted in an Associated Press report as saying, "If we're not utilizing technology, then we're falling behind in crime fighting."
Mayor Johnson and other critics have three main charges against the way ShotSpotter is used. They say it's "inaccurate." That is of course a relative term. In a technical paper published on the ShotSpotter website, the location accuracy was tabulated in a simulated test in a typical environment, with the results cited above (2 to 8 meters, typically). With regard to false positives (saying there was a gunshot when there was actually some more benign sound such as a car backfiring) and false negatives (missing a true gunshot), the company claims that typically 96% of gunshots are detected correctly. So although no system is perfect, ShotSpotter engineers seem to have achieved a remarkable success rate in a highly challenging acoustic environment by using advanced signal-processing techniques to enhance the accuracy of what is basically a time-of-flight location system.
Another accusation leveled against the system as typically deployed in an urban setting is that it is racially biased. A survey by Wired Magazine based on a leaked document giving the secret physical locations of about 25,000 ShotSpotter sensors backs up this accusation. Wired found that about three-fourths of the neighborhoods where at least one ShotSpotter sensor was deployed were non-white, with an average income of about $50,000 a year.
When confronted with these results, SoundThinking senior vice president of forensic services Tom Chittum said that given a limited number of sensors, the company chooses to deploy them in areas that are "likely disproportionately impacted by gun violence." In other words, they place sensors where they are most likely to pick up gunshot sounds. For reasons that have nothing to do with ShotSpotter but are deeply rooted in historical and cultural factors, these neighborhoods tend to be poorer and where minority groups live.
The third accusation is harder to refute: that law-enforcement personnel "misuse" the data provided by ShotSpotter. Critics cite cases in which police officers are deployed to a Shotspotter-indicated location and find bystanders who they then arrest and charge with violations unrelated to gun use. Sometimes this leads to cases such as a Chicago grandfather arrested after a ShotSpotter location led officers to him, in which the accused was later released after a judge found insufficient evidence to convict him.
Undoubtedly, ShotSpotter has also assisted in the capture and conviction of real criminals. Otherwise it seems hard to believe that police forces all over the country would spend hundreds of millions of dollars on it, unless they are all playing keep-up-with-the-Jonesville-police-department and making sure they have the latest technology just because it's there.
In the case of Mayor Johnson dumping Chicago's ShotSpotter system, there has been no love lost between the mayor and the police force in general. In April, Chicago's Fraternal Order of Police—their union—endorsed a drive to recall Mayor Johnson. And in numerous ways, Mayor Johnson has made it no secret that he is highly critical of how the police do their job.
Not everyone in a neighborhood plagued by drug abuse, crime, and violence dislikes the police. Lots of ordinary citizens would like to see more of the police than they do, and they are probably in favor of anything which helps the police do their job of fighting crime, including ShotSpotter. After all, most of the sensors are on private property, and the owner's permission has to be granted for the sensor to be installed. If there was a groundswell of opposition to ShotSpotter, you would think the company would have problems installing their sensors.
One of the most basic functions of a city's government is law enforcement. But over the past few years, especially during the George Floyd riots of 2020, we have been treated to the spectacle of supposedly responsible officials proposing to defund entire police departments. In the cities where movements in that direction actually gained headway, the results were in keeping with what common sense would predict: soaring rates of crime and an exodus of both residents and businesses.
District attorneys who make blanket announcements that certain types of crime will no longer be prosecuted find that those exact kinds of crimes proliferate. How this policy is supposed to benefit society (unless you consider "society" to be restricted to the class of petty criminals) is not clear.
As a technology, ShotSpotter works about as well as the state of the art permits, given the challenging environment it works in. But technology is always about more than technology. The social environment and the way ShotSpotter results are used has led to a perception in some circles that it is just another way to beat down black people and other persecuted minorities. Many police personnel are of the same race and culture of the people they are sworn to defend. It is profoundly demoralizing to be told that a dangerous, tedious job which you do to the best of your ability is not only not appreciated by the highest official of the city, but actively criticized. With regard to law enforcement and the use of ShotSpotter, Chicago is clearly a house divided. And we know what happens to a house divided sooner or later: it cannot stand.
Sources: The Wired report on ShotSpotter sensor locations was published on Feb. 22, 2024 at https://www.wired.com/story/shotspotter-secret-sensor-locations-leak/. I also referred to the Associated Press article "Chicago to stop using controversial gunshot detection technology this year" at https://apnews.com/article/shotspotter-chicago-gunshot-technology-mayor-f9a1b24d97a1f1efb80296dbe9aff1ed. I referred to ShotSpotter Inc.'s technical note TN-098, "Precision and accuracy of acoustic gunshot location in an urban environment," dated Jan. 2020.
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