Don't know if there's any paperwork involved but the ambulances round here have a blue 'running' light on the rear number plate that comes on when the blues are turned on.
I thought it was so the camera people could see the vehicle was on an emergency call, but are'nt gatso photos in black and white ?
My other theory was that the blue light somehow makes the number plate invisible to the camera because it's at a certain wavelength or something, and that it's all a big secret otherwise everyone would fit them !