人和車的頭頂空間
這一組是看得見、量得到的空隙，差別只在低頭的是人還是車。
noun · B2
The space between the top of your head and whatever is above you — the roof of a car, a low doorway, the ceiling of a tunnel. With plenty of headroom you can sit or stand without ducking.
頭上的空間
核心畫面
Stand under a low doorway. What headroom measures is not how tall the doorway is — it is the gap left between the top of your head and the wood. That is the whole word: current position below, hard surface above, and the space in between that you still have. Swap the head for the roof of a and you get the number on a low-bridge sign. Swap the wooden beam for a limit you cannot see, and you get how much louder a sound can go, or how much better a result could still be.
想像你站在一個很矮的門框下面。headroom 量的不是門框有多高，而是你的頭頂到門框之間還剩下的那一段空隙。整個字…更多收起
想像你站在一個很矮的門框下面。headroom 量的不是門框有多高，而是你的頭頂到門框之間還剩下的那一段空隙。整個字就是這個形狀：下面是現在的位置，上面是撞得到的東西，中間那段就是你還有的空間。把「頭」換成貨車的車頂，就是矮橋警告牌上的那個數字。再把門框換成一個看不見的上限，就變成聲音還能再大多少、成績還能再好多少。From the word’s history
The word is simply head + room, so the literal reading — room for your head — is also the picture the whole word runs on.
這個字就是 head（頭）加 room（空間），字面上是「給頭的空間」。難得的是它的字面意思正好就是整個字運作的畫面，其他用法都是把「頭」或「上面那個東西」換掉而已。
Wiktionary entry for headroom (kaikki:3f6f69a190036ae4:1024758323): "From head + room."
Two branches
離上限還剩多少
把上面那個東西換成看不見的上限，中間那段空隙就變成「還能再用多少」。
noun · C1
The amount of extra room a system still has before it reaches its limit — how much louder, faster or better it can go without running into trouble.
餘裕、成長空間