Seems pretty straightforward to stop one of these with an impromptu roadblock. However they've also got an array of sensors with which to record the hijackers and alert the authorities with, precise location and all.
In a way, your point about authorities and highway patrol being alerted is actually a good thing. These trailers can go in lockdown and wait for authorities to arrive. Even if a human were on board, a silent alarm could be triggered without the human making any obvious moves for distress (perhaps a hidden panic button in case sensors don't trigger, assuming a human driver is required to be with the trailer). Also, trailers today aren't immune from hijacking/piracy if the driver isn't armed.
