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 Announces E-Bike Varia Radar with Battery-Free Design - Bikerumor
Garmin Varia eRTL615 eBike Rearview Battery-Free Radar with Wearable4U E-Bank Bundle : Electronics
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
Garmin Support, Varia™ eRTL615
IN THE BOX: 1x Garmin Varia eRTL615 (direct rack mount), 1x 2-bolt to friction flange mount adapter, 1x seat rail mounting kit, 1x Wearable4U E-Bank,
Garmin Varia eRTL615 eBike Rearview Battery-Free Radar with Wearable4U E-Bank Bundle
Garmin Varia eRTL615 eBike Rearview Battery-Free Radar with Wearable4U E-Bank Bundle : Electronics
Garmin Updates Varia System With E-Bike Connectivity