A Google traffic app that allows users to tag the location of police has come under fire from US law enforcement for allegedly endangering officers’ lives. But supporters of the app say police are simply uncomfortable being monitored by the public.

With over 50 million users worldwide, Waze is the world’s largest community-based traffic app, using GPS and social networking to alert drivers to traffic jams, accidents and even potholes. In 2013 it was bought by Google for $966 million.

For many drivers in the US, it’s considered essential for getting around as quickly and conveniently as possible: something which can involve knowing where police are.

Users drop a pin on the Waze map to show where they’ve cited law enforcement, supposedly prompting users to drive more carefully. Supporters of the scheme say it encourages safer driving.

But for the past month, police have been campaigning for Google to disable it.

In an open letter to Google CEO Larry Page late December, Los Angeles Police Department Chief Charlie Beck warned that the app poses a danger to the lives of police officers.

“I am concerned about the safety of law enforcement officers and the community, and the potential for your Waze product to be misused by those with criminal intent to endanger police officers and the community,” Beck wrote.

He cited the deaths of New York patrol officers Wenjian Liu and Rafael Ramos, whose assassin, Ismaaiyl Brinsley, allegedly used the app to “track the location of police” before shooting the partners dead on a busy Brooklyn street on December 20.

For weeks before the shooting, Brinsley had posted anti-police messages on his Instagram account, threatening to avenge the 2014 police deaths of Michael Brown and Eric Garner by killing police officers