Imagine getting this letter in your mailbox:

Dear John,

Your vehicle has been spotted in an area where some less-than-stellar activities have been reported. Just wanted to give you the heads up.

Sincerely,

Your local police department

Some drivers in Oakland have received similar Dear John letters (though notably more formal) in the mail recently as part of a campaign by the Oakland Police Department to stop, or at least slow down, prostitution in the city.

The letter is specifically written in such a way as to not accuse the driver of participating in any of these illegal activities—just noting that the car, registered in their name, has been spotted there. The letters go on to talk about the dangers of prostitution and end by asking for the person’s help in keeping the city safe by reporting suspicious activity.

The idea is spreading; nearly 50 cities across the county have used the Dear John letter writing campaign in an effort to combat prostitution. The City of Los Angeles is currently considering the tactic as well.

Does it work? That’s the million dollar question. Oakland PD officers say they are making a good number of arrests, but it’s not clear if those arrests are directly tied to the letters. The letters are informational only.

Read more about the Dear John letters here.