There are now a number of ebikes equipped with radar systems which warn riders of vehicles approaching from behind. If your bike isn't among those, you can now change that by slapping on Garmin's Varia eRTL615 radar/tail light module.
Garmin Varia™ eRTL615, Bike Radar
garmin varia
Garmin Varia eRTL615 eBike Tail Light - GPS Central
Garmin Varia™ eRTL615, Bike Radar
Garmin Varia™ eRTL615, Bike Radar
Garmin Adds a Camera to Its Luxurious Car-Tracking Bike Radar
garmin varia
Tracy Schumacher-Kanno on LinkedIn: Garmin Varia eRTL61 brings car-detecting radar tech to existing ebikes
Battery-free 24 GHz rearview radar and tail light for compatible eBikes (requires compatible Garmin eBike cable; sold separately) Radar detects
Garmin Varia eRTL615, Rearview Radar and Tail Light, Powered by Your Compatible eBike, Visibility up to 1 Mile Away