No one suggested that it needs an intimate understanding of trucks. It needs to be able to tell the difference between empty space and not empty space. It needs to be able to do that for the entire volume the vehicle will occupy not just some of it. Otherwise it will run into trees, wires strung up to decapitate motorcyclist[1], farm animals and other obstacles that may or may not extend all the way to the ground directly in front of the sensor.

[1] Unfortunately that is a thing. http://lanesplitter.jalopnik.com/police-hunting-sadistic-bas...