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.
"道" is bigger than "路" normally in Chinese. such as "康庄大道", "死路一条".
路: path
陆路(path on the ground)
水路(path on river)
海路(path on sea)
天路(path to heaven, special one)
死路(path to die)
活路(path to live)
生路(path to survive)
道: way
水道(waterway)
天道(nature law)
大道(boulevard, major/main way/road)
大路(clear path, main road)
There are a lot of names coming from Japan in the course of modernisation of China. Chinese characters are common treasure in the East Asia and it is very easy to share idea among East Asian states. Japan government picks the name 國道 for the national highway / expressway of Japan, and Taiwan (ROC) and China (PRC) follows. It is similar to numbered highways in the United States and Great Britain road numbering scheme. You might expected 國道 is funded by nation. Japan prefixes name with 國 to represent the mandatory or establishment of nation, for example, 國語 (national language) and 國立 (national establishment). China borrows this idea well.
公路 means public road and it is not necessary highway or expressway. But with the advance of technology it now become the synonym of modern highway and expressway. Chinese does not name it 公道 as 公道 has another meaning in Chinese.
You can also think of 道 as corridor, more of a mean to connect parts and ways, both methodically and physically. 国路 Would instead just be the national street, a physical artifact. As mentioned by @Omnibus, 公道 means fair way, and is not associated with movement at all.
