Unless specifically defined below, words or phrases used in this chapter shall be interpreted so as to give them the same meaning as they have in common usage unless such meaning is clearly contrary to the intent of this chapter and so as to give this chapter its most reasonable application. For the purpose of this chapter, the words “must” and “shall” are mandatory and “may” is permissive. All distances, unless otherwise specified, shall be measured horizontally. For the purpose of this chapter, the terms in § 151.04 have the meanings given them.