I've used mirrors often on our country back roads. I both listen and look for automobiles. On these roads I might encounter a vehicle every 30 minuets or not for hours. On these roads it is safer to just get off the road when a vehicle comes along. I ride wide tire bikes for just this reason.
I'm going to try one of these.
https://www.rivbike.com/products/ger...r&_ss=e&_v=1.0