Causal Analysis of an Acoustic Gunshot Detection System: Evidence From Detroit

<div> We provide the first evidence of the impact of ShotSpotter technology in Detroit, Michigan on the number of civilian-initiated gunshot-related 911 calls for service, law enforcement response times to 911 calls, and homicide and non-fatal shooting arrests. ShotSpotter geolocation information was combined with data on 911 calls reporting gunshots as an indicator of gunshot-related incidents from 2018 to 2022. We use a difference-in-difference (DiD) design that compares outcomes from Detroit Police Department (DPD) scout car areas (SCAs) where ShotSpotter was deployed to SCAs with no ShotSpotter deployment before and after its first implementation in 2021. </div> <div> <br> </div> <div> Per week, ShotSpotter is estimated to have caused a 42.1% decrease relative to the pre-ShotSpotter baseline mean value of 2.30 gunshot-related 911 calls. Viewing this as an impact assessment of AGDS technology, our findings indicate that the deployment of ShotSpotter deterred gunshot reporting but had no measurable impact on the average officer response time and rates of non-fatal shootings and homicide arrests. Furthermore, dynamic treatment effect analysis indicates that the difference between the number of gunshot-related 911 calls and ShotSpotter alerts decreased after a year of the system's deployment, suggesting a decrease in the utility of the system alerting to gunshot-incidents that civilians do not. Our findings raise questions on the long-term benefits of law enforcement agencies contracting ShotSpotter and provide an audit of the technology in Detroit that should be considered in future policy decisions.&nbsp; </div>