路上與旅程上的單向
這一支就是箭頭畫在路面上的原始畫面，走在上面的是車，或是拿著票的人。
adjective · A2
A one-way street or road system is one where vehicles are allowed to travel in one direction only, so you cannot drive back along it the other way.
單行的（街道、路線）
核心畫面
One arrow painted on the road, and only one. Everything travels along it in that direction: cars, light, water, a ticket, a message, an effort. Nothing comes back towards you. The important half of the picture is the missing half — there is no return trip built in. Once you can see the single arrow, every use of this word is a matter of asking what is moving along it and what is not coming back.
可以想成路面上只畫了一個箭頭，而且只有一個。所有東西都順著那個方向走：車子、光線、水、一張票、一則訊息、一份心意。沒…更多收起
可以想成路面上只畫了一個箭頭，而且只有一個。所有東西都順著那個方向走：車子、光線、水、一張票、一則訊息、一份心意。沒有東西朝你回來。這張圖真正重要的是缺掉的那一半——裡面沒有回程。只要看見那個單一箭頭，這個字的每個用法就只剩下兩個問題：順著箭頭走的是什麼，以及什麼沒有回來。From the word’s history
Wiktionary gives one-way simply as one + way, and dates the uses: 1906 for tickets, 1914 for streets, 1940 for mirrors. The compound has always meant what its two parts say — a single direction.
Wiktionary 記載 one-way 就是 one 加 way 合成的，並標出各用法的年代：1906 年用在票、1914 年用在街道、1940 年用在單向玻璃。這個複合詞從頭到尾就是它兩個部分的字面意思——只有一個方向。
Wiktionary (kaikki:3f6f69a190036ae4:822396597) — "From one + way. First attested in 1906 in reference to one-way tickets; 1914 in reference to one-way streets; 1940 in reference to one-way mirrors."
Three branches
只通一邊的東西
同一個箭頭被畫在一片玻璃或一個閥門上：過得去，回不來。
adjective · B2
Letting light, air, liquid or information pass in one direction only, and not back the other way.
單向的（只通一邊）
只有一方付出的互動
把箭頭放到人與人之間：所有東西都從一邊出去，另一邊什麼也沒回來。
adjective · B2
A one-way relationship, process or conversation is one in which only one side makes an effort, gives something or sends anything, and nothing comes back the other way.
單向的；一頭熱的