Cost: $1400 for $12 worth of parts obviously hacked together in a project box seems a bit much. While not for this specific application, it's a really common project for people starting out with Arduinos: Arduino+ultrasonic sensor+display and a beeper. Usually used as a parking distance alarm in a garage. In this case the police add their own GoPro for evidence purposes I assume. Hopefully it clearly shows the position of the bicycle as well, and that he was fully to the right of the lane at the time of the measurement.
Safety: Could the possibility of a citation for uncertain margins with a bicycle cause the same nervous reactions that red light cameras cause possibly increasing the number of accidents?
Prior art: My car has an ultrasonic backup alarm with display and beeper that serves this exact purpose (measuring distance before impact). That was done in 2002, so not at all a new idea.
Value as evidence: Isn't a common defense to speeding tickets the lack of recent calibration of the RADAR gun? This thing is probably subject to all manner of interference and inaccuracies.
Countermeasures: It could be easily jammed/confused by a transmitter designed for that purpose, and that should be legal since we're talking about audio frequencies, and inaudible ones at that. Ever see those ultrasonic deer transducers that go on the bumper? They don't work for deer, but they do emit sound in the ultrasonic range at certain speeds. I could see designing one specifically as a passive jammer mounted to the right-hand mirror.
My own experiences: I've biked thousands of kilometers in the US and been hit a few dozen times, nothing very serious. Some were my fault, most the driver's fault. None were malicious, just poor observations/judgement/conditions. I ride defensively and don't really care about these laws and behave as though they don't exist since it's not wise to count on the driver, and the driver shouldn't count on me either.