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 Varia RTL515 Rearview Radar Rear Bike Light - Mantel
Garmin Rearview Radar Varia RTL515 with Tail-light – 99bikes.co.nz