Chips! Behind you!
I have been cycling since 1947. In recent years, I have become increasingly nervous on our roads, and concerned at the aggressiveness of many motorists. Recently, I purchased an inexpensive handlebar mirror from Checkers Hyper and fitted it to my bike; I haven’t looked back since! I am now able to monitor what is approaching without relying on my hearing alone, and to take corrective action when necessary. I suspect cyclists who ride two abreast are often unaware of traffic behind them. Mirrors are easy to fit, and indeed, a suitable motorcycle or car unit from a scrapyard could be adapted to fit the average bike with a little thought.