Question, since I've always been curious about this: How difficult would it be to make your own preemption device? Are such things available on online stores?
I'm assuming it's illegal to use one obviously, but I wonder what precautions are used to prevent unauthorized usage, if that can even be disclosed to the public. What's stopping the average joe from getting one from Alibaba?
Honestly, pretty little, other than the risk of being observed. There may be systems that allow for authentication, but I've not seen them (though I'm not an expert).
There is an encoding standard for prioritization that some larger systems use, that mass transit may utilize, but can be overrode by Fire/EMS/LE.
I'm assuming it's illegal to use one obviously, but I wonder what precautions are used to prevent unauthorized usage, if that can even be disclosed to the public. What's stopping the average joe from getting one from Alibaba?