This applies to all drivers, but trucks more so -
Along with having to pay attention to what is in front of, behind, to the left of, and to the right of - the driver has to pay attention to what is under him (road conditions, potholes, water, etc.) and what is over him - overpasses, wires, etc.
Chicago has several bridges that are LOW - more than one driver has peeled back the top of a trailer - probably only once!