The number of thefts and robberies of smartphones, particularly iPhones, is on the fall in New York, London and San Francisco, according to data to be released Wednesday.

Law enforcement officials, who have been at the forefront of demands to include a “kill switch” in all smartphones, hailed the news as proof that the technology is working as a deterrent.

In San Francisco, overall robberies and thefts dropped 22 percent from 2013 to 2014, but those involving smartphones were down 27 percent. Thefts and robberies of iPhones fell 40 percent. In New York, smartphone theft dropped 16 percent overall with iPhone figures down 25 percent. And London saw smartphone thefts from persons drop 40 percent in a year.

“The huge drops in smartphone theft that have occurred since the kill switch has been on the market are evidence that our strategy is making people safer in our cities, and across the world,” said New York State Attorney General Eric Schneiderman in a statement.

The kill switch is a software lock that can be remotely activated when a phone is lost or stolen. It can wipe personal data from a phone and “brick it” so it can’t be reused or reprogrammed.

Law enforcement officials campaigned to make the technology standard in reaction to a growing numbers of thefts of robberies of smartphones on city streets across the U.S. and beyond. The assumption was that phones would be much less desirable targets if they could quickly be made useless.

Apple added a kill switch, called Activation Lock, to its iPhone in September 2013. Samsung followed in April 2014 with its Galaxy S5 and Google made it a standard feature of Android with the release of Lollipop.

Soon most smartphones sold will include a kill switch thanks to a new California law that mandates them in smartphones manufactured after July 1 this year and sold in the state. While the law only covers California, it’s leading to their introduction in phones sold worldwide.

San Francisco District Attorney George Gascon, who together with Schneiderman led the law enforcement effort on the kill switch, welcomed the news and said he expects to see further reductions in smartphone robberies as the kill switch makes its way into more phones.

The cellular industry at first resisted the efforts but later reversed its opposition. Major U.S. carriers are also being more proactive about sharing data on stolen phones so they cannot be activated on networks in the U.S. and abroad even if they don’t contain a kill switch.