https://www.openbikesensor.org/ https://www.openbikesensor.org/ is pretty popular at least here in Germany. It's a bit bigger and works with ultrasonic distance

https://www.openbikesensor.org/ https://www.openbikesensor.org/ is pretty popular at least here in Germany. It's a bit bigger and works with ultrasonic distance measurement. Apart from that (and some other features) it's basically the same idea...