Why is China National Highway called "国道", instead of "国路"? I know both 道 and 路 mean "road, highway", but I am always confused which one to use in a certain condition. 

For example, since "highway" in Chinese language is 公路. Is it more logical to call "National Highway" "国路"? Maybe I am missing something here.
The common term is 国道 for national highway, which would be across the provinces or cities. another term is 高速 short for 高速公路， which is also highway across the provinces. The difference between 国道 and 高速 is the speed limit, speed limit on 高速 is typical 120km/hour, and 国道 typical 80 or 100km/hour. 国道 has traffic lights, while 高速 doesn't have it. You have to pay money for driving on 高速, while you won't pay money for 国道.

As for ‘why not 国路’, I don't have a good reason for it. It's just a convention I think. 

Within the city, we call it "高架" or "高架桥"， for example, 沪闵高架 in Shanghai.