Traffic has become synonymous with air pollution, and many countries intend to ban the sale of new petrol and diesel cars in the next two decades.
2019年12月六级真题（第二套）阅读 Section A
柯林斯高阶英汉双解学习词典释义
ADJ
(与…)关系紧密的;等同(于)…的;互为依存的
If you say that one thing is synonymous with another, you mean that the two things are very closely associated with each other so that one suggests the other or one cannot exist without the other.