Well, that depends. In general, there's no need for street lights to be on when there's no traffic. But I grew up traveling throughout my country from rural to main capital and there was always an area of the highway without street lights. That area was noticeably more dangerous because you simply see less, and have less reaction time. Especially during rain or snow or other extreme weather. It isn't just other cars. It is also deer who might pass the highway, for example. And that area was in a forest. If there were some kind of way to have them more intelligently work on/off (for example by seeing phone signals come closer) I'd wager we'd already have such. I actually dislike that premise and would like to agree with you (selfishly: I'd like my kids to grow up on a livable planet), so I hope you can prove me wrong.