I tried using Garmin's rear view radar for bicycles 'Varia R TL515' that can detect the approach of a car with a millimeter wave radar and can also be a tail light -
GARMIN RCT716 Radar Bike Tail Light Owner's Manual
Garmin GMR18 MARINE RADAR TRANSMITTER User Manual USERS MANUAL
Garmin Marine Radar User Manuals Download | ManualsLib