What is the superstition about an ambulance?

Jules Essex answer: Seeing an ambulance is very unlucky unless you hold your breath or pinch your nose until you see a black or brown dog Touch your toes

Touch your nose

Never go in one of those

Until you see a dog

Agreed, it's unlucky. However, we used to touch our collars and say 'touch your collar never swallow (East end accent pronounced 'swaller'), never happen again' which kept us safe from whatever had happened to the person in the ambulance.