From Herb Caens column in the San Francisco Chronicle:

A motorist was unknowingly caught in an automated speed trap that
measured his speed using radar and photographed his car. He later received
in the mail a ticket for $40, and a photo of his car. Instead of payment,
he sent the police department a photograph of $40. Several days later, he
received a letter from the police department that contained another
picture–of handcuffs.