For some less serious Chicago bulletins, there was SquirrelTruth[0]. A kickstarter was created to post signs in the CTA about the danger of squirrels. I am not sure how many variations there were, but the only one I ever saw was, "Statistically speaking, at least one 'person' on this train is actually 7 squirrels wearing a human suit. Don't be a victim."
[0] https://www.squirreltruth.com/