You can bet your ass that local jurisdictions will be outfitting these things with radar, and mailing out speeding tickets!
Won't work, Doppler shift radars measure the "vector" speed of a target as it approaches the radar gun. The angle between the path of the vehicle and the axis of the radar beam, "Θ", is near zero when the target vehicle is at maximum range and the radar gun is at the same altitude as the target. The indicated speed is the actual speed multiplied by cosine (Θ), so at the point where the target is approaching nearly head on you get an actual reading close to correct. As the target approaches and passes the radar the reading falls to zero and then goes negative (Cosine (Θ) = 1 at 0°, goes to 0 at 90°, and -1 at 180°). The radar is measuring the component of target velocity relative to the radar beam. When the angle is near 90° the target is not approaching the radar (it's moving sideways) and the result is a zero reading. As the target passes it is moving away from the radar and the indicated speed goes negative as it should.
When you move the radar to a drone, you have to factor in the operational altitude as well as the speed and direction of the drone as it effects the vector solution. Far from holding a hand held radar gun at the same altitude as the target, a moving platform measuring relative velocity to a ground target also moving requires knowing exactly where the drone is (GPS??) in three dimensional space, a vector track of the target (not Doppler) allowing the calculation of a solution vector of the actual target velocity in three dimensional space. That means; more elaborate radar, GPS linkage, hardwired computational support, and a lock-on targeting laser (everything except the "mini gun").
Regards,
GtG