I work in a company that makes safety signage, and in my experience people don’t put a sign up with the expectation that anyone is going to pay attention to it, the main reason for putting a sign up is so that it gives you the right to say “That’s your fault, we put up a sign!”

I have an idea for an impossible invention where you put sensors and a hydraulic ram hidden behind a sign, so when it knows it’s being ignored the ram triggers and the sign is rammed into their face like a cartoon boxing glove on a spring.