Not a traffic sign, but anyway.
I just love the artwork.
These signs are posted all over the network of streams and small rivers in this area - because all those waters are (or used to be) crammed into straight half-open pipes that are really hard to climb out of and also make the stream/river prone to surprisingly strong flooding in case of heavy (or even not-so-heavy) rainfalls.
(upper sign) Emscher Collective - Entering the site is dangerous and not allowed.
(lower sign) Entering forbidden - flood waves
(The actual stream is not pictured, it's left of the tress a couple of metres below)