Last week, Paul Brodeur, mayor of Melrose, Mass., apologized for a supposedly offensive traffic safety message and launched an investigation into it. He made a federal case about a police billboard reading “THE SAFETY OF ALL LIVES MATTER.” This simple statement expressing the importance of safety for everyone is purportedly offensive or racist or something because many respond to the “Black Lives Matter” movement with signs reading “All Lives Matter.” “I have just been made aware that the following traffic sign is being displayed on Main Street,” Brodeur tweeted with a picture of the sign. “I have ordered that it...