This reminds me of a speed limit sign in Oklahoma City I saw once that read "Speed Limit 52". One of my best friends lives in OKC and his dad is a cop. I asked about those speed limit signs and he said it was so you'd pay more attention to it. It makes sense, just think if it said 20 or 25, you wouldn't pay half as much attention to it as you do at 23. "23mph!? What the heck?"...then you'll be talking about it all day and you'll remember it next time.