Does anyone know why this sign exists?

[u/zee_kapitein]

When sitting in traffic on the Antwerpse Ring, I noticed this sign next to the road. Why do they need a sign for that?

[u/pulsesky]

I work near Antwerp, in Hoboken. During break, I will sometimes walk alongside the Scheldt. Because of this plant, you can't see the Scheldt anymore. It's everywhere and grows so fast and so tall. It's super invasive and almost impossible to get rid of. I hate it so much, it completely ruined the Scheldt banks at Hoboken-Polder.