According to ROSPA (The Royal Society for the Prevention of Accidents), 21,287 cyclists were injured in reported road accidents (in 2014)*. The report goes on to say "The most common vehicle involved in collisions with cyclists is a car or taxi, with the rider usually being hit by the front of the vehicle. In a quarter of fatal cyclist accidents, the front of the vehicle hit the rear of the bicycle."
Having a radar on the bike is a way that may assist riders and motorist avoid each other. There is at least one commercial product available, the Garmin Varia (https://buy.garmin.com/en-US/US/into-sports/cycling/varia-rearview-radar/prod518151.html
This project is an attempt to hack a bike radar together using a doppler Radar HB100 and a Raspberry Pi (zero/3) or CHIP or Arduino.