Road signs

[polinsteve]

Many years ago when I was bus instructing, I always made it clear to my trainees that they were free to ask any driving related questions. On one occasion I was asked a question but didn't know the answer, so I confidently said I don't know, I'll get back to you tomorrow.

Pride came before a fall and I couldn't find the answer, even after phoning Sussex, and Surrey traffic police. The question was, "On triangular bridge height warning and round prohibition signs, above and below the stated height there is a black inward facing triangle. Why do they sometimes have a large triangle top or bottom and the other triangle smaller?

Any guesses or definitive answers?