[lnsru]

I see in Germany older folks often using a mirror on the left handlebar. Once I borrowed such bicycle, it was somehow weird, but I got used to that. Such device identifies cars and dangerous situation very well. Zero false positives, works pretty well in the night too. No batteries needed and costs between 5 and 15€.

[Luxonis-Brandon]

Yes, mirrors are a fantastic solution. And for getting the attention of the distracted driver (as sometimes the dynamics of the bike preclude maneuvering out of the way in time), LOUD bicycle is a great solution: https://www.kickstarter.com/projects/lansey/loud-bicycle-car...