For those who have the dinero - rearview radar from Garmin [View all]

https://buy.garmin.com/en-US/US/prod518151.html
Alerts You to Vehicles Approaching from Behind

World's first cycling radar that warns of vehicles approaching from behind up to 153 yards (140 meters)
Works independently and wirelessly integrates with compatible Edge® cycling computers¹
Edge computer or Varia head unit can detect multiple vehicles and indicates the relative speed of approach and threat level
Tail light unit brightens and flashes to notify approaching traffic of a cyclist ahead

The world's first cycling radar, Varia helps create a safer cycling environment by warning cyclists of vehicles approaching from behind up to 153 yards (140 meters). The radar tail light also warns approaching vehicles of a cyclist ahead. Works independently or integrates seamlessly with compatible Edge cycling computers¹. Edge computer or Varia head unit can show multiple approaching vehicles and indicates the relative speed of approach and threat level.

"Edge computer or Varia head unit can show multiple approaching vehicles and indicates the relative speed of approach and threat level."
Getting Hit From Behind is Leading Cause of Cycling Fatalities in U.S.²

The threat is real. That’s why getting some extra help knowing what’s sneaking up behind you is an important breakthrough. You already use caution and check over your shoulder, especially before making turns, but Varia radar gives you an important tool by watching behind you even when you aren’t.