equivalence theory of Nida would be used as theoretical basis and be discussed in  details.
   However the "equivalent" here means to point toward the source language message while "natural" means to point toward the receptor's language. The "closest"
means to bind the two above-said orientations together on the basis of the highest degree of approximation. (Nida&Taber, 2004:12) Thus one can never emphasis too much on and over-value only one party of both the source and target reader.
   Nida has proposed two criteria of functional equivalence: the maximal one and minimal one. To maximum, the readers of a translated text should be able to understand and appreciate it in essentially the same manner as the original readers did.
And to minimal, the readers of a translated text should be able to comprehend it to the
point that they can conceive of how the original readers of the text must have understood and appreciated it (Nida, 2001).
2.2 The academic research of public signs in China
   The academic study and research about the translation of public signs in China has a very short history. It was not until the early 1980s when Duan Liancheng published his book named How to Help Foreigners Know China that people started to realize the importance of China's international image in the publicity-oriented C-E translation.
   In his book, Mr. Duan mentioned that the cross-cultural communication skills and techniques are indispensable and significant in communicating or even cooperating with the other countries. In the preface of his book, he pointed out that the adoption of reform and opening-up policy ever since 1979 has opened a new era for China's cross-cultural communication. People with different skin colors, different faith of religions, different nationalities, have established good relationship with China ever since. Under such circumstances, it is urgent and necessary to try to help foreign people get to know China, and that is what people usually called publicity.
   After Duan Liancheng, many other scholars and researchers have been engaged in this translation field.
   Lv Hefa (2004) has concluded the characteristics of four applied functions as: directing, prompting, restriction and compelling. He focused on explaining "dynamics" and "statics" of public signs, and summarized the unique language style of public signs. He has advocated that we should choose standard English words or phrases instead of the original Chinese words when translating public signs. These versions of translations can be applied practically after the confirmation of not causing ambiguity and misunderstanding.
   Wang Yinquan (2002) and Yangfang (2006) have analyzed the errors existing in the translation of public signs in China. They offered some feasible strategies and suggestions for further improvement and discussed the guidelines and regulations on translation of public signs. Li Changjiang (2003) presented some modes for the translation of public signs based on the practical point by analyzing the characteristics of public signs. 功能对等理论指导下的南京市公示语翻译研究(3):http://www.youerw.com/yingyu/lunwen_19978.html