I guess the other sign wasn't doing the trick.

There's a really entertaining channel on youtube where the guy just takes the video from their security camera. It's almost nothing but cars being towed out of their parking lot. They have something like 6 signs indicating that your car will be towed unless you have their parking pass and people still park there. People read the signs after they've parked there and still leave their cars. There was even one where they used to intercom to warn people not to leave their car cause it would be towed. https://www.youtube.com/watch?v=j7BPvNotqEc