New York crime rates have fallen a lot in recent years, something some had attributed at least in part to a practice known as "stop-and-frisk." This involved police officers stopping large number of people on the street based on what they contended was reasonable suspicion. The idea was that this wouldn't just get illegal weapons off the street, it would discourage potential criminals from carrying them in the first place.
The practice had a lot of opponents as well as supporters and was constitutionally questionable, a possible violation of the Fourth Amendment's prohibition of unreasonable searches, though the police claimed that they were careful to stop someone only on the basis of legitimate suspicion that met constitutional requirements based on a 1968 Supreme Court decision. But in 2013 a federal judge held stop-and-frisk to be racially discriminatory in practice and hence a violation of the Fourth and Fourteenth Amendments. (For a summary of the legal details, see this article from FactCheck.org.)
When the Bill de Blasio became mayor in 2014 he kept one of his campaign promised by ordering a drastic cut in stop-and-frisk, in keeping with the federal judge's order and his own progressive views. There were widespread predictions that this would lead to a violent crime wave, but in reality crime continued to drop, quite a lot in fact. Last month conservative author Kyle Smith wrote in The National Review:
... Mayor Bill de Blasio, who made ending stop-and-frisk the centerpiece of his successful 2013 campaign for mayor, struck me as a man who was cynically willing to tolerate an increase in crime if he thought it to his political advantage to amplify leftist voters’ core belief that policing was out of control.
Today in New York City, use of stop-and-frisk, which the department justified via the 1968 Terry v. Ohio Supreme Court ruling, has crashed. Yet the statistics are clear: Crime is lower than ever. It’s possible that crime would be even lower had stop-and-frisk been retained, but that’s moving the goalposts. I and others argued that crime would rise. Instead, it fell. We were wrong.
Major crime in New York City has continued to decline almost across the board in the four years of the de Blasio administration, to the lowest rates since New York City began keeping extensive records on crime in the early 1960s. Crime is literally off the charts — the low end of the charts. To compare today’s crime rate to even that of ten years ago is to observe a breathtaking decline.
Former New York Times metro editor Joe Sexton wrote a brief article for ProPublica that's also worth reading about the history of stop-and-frisk and different views of its relationship to the drop in crime in New York.
Crime in the U.S. overall has declined a lot since about 1990. The national murder rate rose in 2015 and 2016, but was largely the result of an upsurge in a few large cities such as Chicago, and while the final data are not yet available, it's likely murders will turn out to have fallen again in 2017. Here are some posts on the subject from last year with links to additional data:by