Street Signs: No Pee Zone

I've been meaning to do a series like this for a long time...

I think the iconography in signs is fascinating. As I travel, I run into signs that try to communicate something in iconic form -- something that will make sense to everyone regardless of the language you speak. Sometimes they are quite successful, other times they can be interpreted in a number of interesting ways. As a new feature on Duffergeek, I will be posting some of the pictures I run across and, when appropriate, trying to decipher their intention.

This time, it is pretty obvious.


Street Signs: No Pee Zone

I was in Amsterdam a couple of years ago and saw this sign. I only saw one in the entire city but it seemed to work because I had to walk a whole block away to find a guy actually peeing on the wall.

So I'm pretty sure this sign means you're not supposed to pee here but it could have some other meanings...
1) Don't drop stones
2) Stand up straight when peeing (no swayback urination)
3) No peeing on ants

Other thoughts on what this could mean?