Why do we need to write a line of pinyin letters under the Chinese characters on road signs?

In fact, it is a matter of "transliteration". Nowadays, many Chinese phrases are literally translated from English, such as: coffee, T-shirt, Coca-Cola, chocolate, etc. As for the street sign you are referring to, there are corresponding pinyin letters under the Chinese characters. The same principle applies. It means directly transliterating the Chinese into letters that foreigners can understand. Just like Beijing mentioned above, translated into English is "Beijing", which is its Chinese pinyin.

As for whether you think foreigners can’t understand Pinyin, that’s not the case. Foreigners can roughly understand the meaning of Chinese characters through Pinyin, and this literal translation is the easiest and will not make mistakes, let alone “Gubuli”. A joke that translates to "a dog is not interested" in English.