Like any highway system, the federal airways have their main trunk line.

同任何公路系统一样，联邦航空线路也有其主航线。

youdao

Then divides the highway network into four levels: main trunk line, sub-trunk, feeder and connecting line, and also positions functions of the four levels.

进而按照通达程度将公路网划分为四个等级：主干线、次主干线、支线和连接线，并分别对四个层级进行了功能定位；

youdao