They started doing it in Aylesbury when I lived near there a few years ago.
But I took "main roads" to refer to trunk routes and motorways. They don't need to be lit all the time, but I haven't been on many in the wee hours recently to know what the situation is.
That was my meaning, but unless there is a real safety issue, most could be switched off, plus tens of thousands of office lights etc.
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