system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How license plate readers, both stationary and on police patrol cars. Help stop gang violence.

I was recently watching my local news station.  They had a pretty large segment on how license plate readers stop gang violence. The segment ended up having less information then I was hoping for. After watching it I still do not understand how the police would use the collected information to stop gang violence.
gpt
Often when a news source says something helps "stop" a particular kind of crime, they don't mean it helps pre-emptively stop a particular thing from happening.  They really mean it helps catch perpetrators faster, cut down on repeat behavior between the first offense and arrest, and therefore reduce that kind of crime over time. 

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Say a known gang member goes and shoots up someone's house, killing the residents. Police investigate, gang-member says he was at home playing poker with his homies all night, they corroborate that story. Without some other evidence, the police can't prove he wasn't there. NOW, police have license plate readers (both cruisers passively scanning the cars in front of them and stationary ones maybe at major intersections) that see this gang member's car at 3 different locations in a direct route between his house and the victim's on the night in question. The reader helps provide police with more evidence of where a certain suspect was in relation to a crime when they don't have witnesses at the scene and the suspect's neighbors all play dumb. This is particularly helpful in the area of gang violence, where gang members often live in close proximity to one another and are more likely to lie about where their fellow gang member was than a random non-affiliated neighbor.