re: How does a cop's radar on a moving motorcycle work?

The next time the St Gabriel rolling speed trap stops me for doing 20 over, I'll ask him and get back to you.

I figured something as small as a Harley wouldn't register.

Edit: I thought you meant the cop was in a car and you on the motorcycle. Your situation is new to me.

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wouldn't the path and angles of each vehicle come into play in this wild formula?

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Yes, but the same exact factor comes into play with stationary radar too.

Radar measures the relative speed between the radar unit and the target using Doppler effect. For stationary radar aimed at an object moving directly toward/away from the gun, the “radar speed” is 0 so the difference is the target speed.

Moving radar requires the radar unit to know how fast the police vehicle is moving (patrol speed). I believe most moving radar units nowadays calculate patrol speed by measuring against the pavement - i.e. a radar is pointed toward the pavement either in front or behind the police vehicle and that reading is used to determine patrol speed.

For forward-looking same-lane radar, target speed is the measured speed plus patrol speed. For forward-looking opposite-lane radar, target speed is measured speed minus patrol speed.

The relative angles make a difference but that difference is always in the driver’s favor. If the target vehicle is moving in any direction other than directly toward or directly away from the police vehicle, the target speed calculated by the radar will be slightly lower than the target’s actual speed.

If the target is moving perpendicular to the radar gun, calculated speed will be 0.

But the radar unit detects the speed of the vehicle its in, and the speed of vehicles headed towards or away from the radar unit. It adds or subtracts and can do same direction or opposing direction..