I used to believe that the "Wrong Way, Go Back" signs on a freeway were put there by 'Bunyips' - mythical Yeti type monsters which are said to reside in Australian billabongs.
A childhood belief about road signs

others believed things like this
- When I was little I thought rhinoceroses were mythical creatures.Anon
- i used to believe that swans were mythical creatures like unicorns or a phoenix.
— amber - I had never seen a dove in real life before so I figured they were just mythical beings from the bibleAnon
- Having seen enough red octagonal STOP signs in my time, I used to look out for green circular GO signs.— Gary
- I used to believe that a pigeon was a type of pigAnon
- I used to think giraffes were a type of horse— Crocotta1