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Any offset between a stationary radar gun and a moving target creates a "cosine effect" that reduces the return frequency by the cosine of the angle between the gun and the vehicle, thus lowering the speed reading. Therefore, police try to park close to the road and detect vehicles far downfield. In a patrol car that is cruising with traffic, however, the moving gun determines relative speed between the car and a target, and any angle may artificially add to the speed reading. Officers are trained to check the gun's displayed patrol speed against their own car's speedometer.