Illegal?
A commerical vehicle is any over 10,000lbs VGW or vehicle gross weight. These vehicles are subject to regulation by the Dept. of Transportation and operate effectively under the Interstate Commerce clause of the US Constitution.
(Some regs vary by state.)
The larger trucks, semis, with a VGW at 20,000 come under even more strict regulations and are subject to weighing, the scale house mentioned elesewhere, DOT hazmat regulations and Fed Hwy regulations.
These also entail multi state vehicle registration, reciprocity of fuel tax collection among the states etc.
Blood of Tyrants, funny you mentioned FDR. We have one of his fans with us today!
Smartaleck, the Interstate Commerce Clause does not override other parts of the Constitution. It was intened to keep states from levying tariffs against each other, that is of course unless you're a FDR/Al Gore style "living Constitution" liberal.
Even if the ICC gave Congress the authority to search commercial vehicles at will (it doesn't), the 4th Amendment amended the Constitution, and would have taken that power away.
Amendment IV:
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
I don't see exceptions for gross weight or commercial vehicles in there, do you?