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.
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."
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