Earlier quoted context omitted.
The Bombardier Q400 can seat 80 people and weighs 40,000 lbs empty.
Which is about half of the weight of a loaded 18 wheeler.
That's with a high performance truck. A more average truck, or a truck with its speed limited by a governor (common in many commercial fleets) would have about 10% of the kinetic energy of the plane.
And remember, this is assuming the truck can get to its target at top speed. In reality a lot of targets are not reachable by a path that would allow a truck at speed. If the truck can only get to 50 mph (80 kph), it would only deliver about 6% of the plane's kinetic energy.
The plane has a much much larger set of targets that it can deliver full kinetic energy to.
The above is assuming the pilot decides to fly into the target on a fairly level flight path and obeying the manufacturer's safety limits. If the pilot is willing to fly unsafely before the crash, he could probably got 2x to 4x the kinetic energy.
In short, a stolen fully loaded truck is a much smaller threat than a stolen empty Q400, unless what the truck is loaded with is also a threat.