There is someone already on board the truck, why not arm him? I think the value of trailer loads is approaching that of Brinks trucks. A $40-50,000 a year driver is not going to sacrifice his life (nor should he) for a load of consumer goods.
He has to eat, he has to crap, he has to take a break every so often. They need a guard to watch the truck whil he does these things. Actually he only needs that guard while he is not moving. The truckstops could stop this crap by putting gates in and not allowing unauthorised people inside those gates and checking that the driver that moves the truck out is the same one who brought it in.