A big folded sheet opened on a table, covered in roads. Thick lines for the main , thin ones for the small ones, towns marked along the way. You put a finger on where you are, another on where you want to be, and the sheet answers one question: what do I pass through to get from here to there? Every other use keeps that answer and drops the paper.
可以想成一張攤在桌上的大紙，上面全是路。主要幹道畫得粗，小路畫得細，沿路標著城鎮。你一隻手指按住現在的位置，另一隻按住想去的地方，這張紙就回答一個問題：從這裡到那裡，中間要經過哪些點？後面那些用法都留著這個答案，只是把紙拿掉了——變成一份計畫、一份指南，照樣告訴你先到哪、再到哪。
From the word’s history
The source records this compound simply as 'From road + map' — a map of roads. The later uses keep the shape of that object (a route with points along it) while the roads themselves drop away.
來源記的字源很單純：road（道路）加 map（地圖），就是一張畫著道路的地圖。後來的用法留下了這個東西的形狀——一條有沿途節點的路線——而真正的馬路不見了。
Wiktionary, "road map"
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