Caymen
Like I said, the Hazmat regulations are the most complex of the CDL rules. Particullarly those that apply to radioactive materials. Generally, if the load requires a placard (those are determined by the rule in the code of federal regulations) then the operator of the vehicle is required to have a CDL with a Hazmat endorsment. You can get a CDL with a Hazmat endorsment, and still not be licenced to drive vehicles over 26,000 lbs. Many pesticide operators, and people who use small quantities of some radioactive materials, as well as many blasting operators have to pass the CDL test and the Hazmat test, and are still only licensed to operate small trucks. To make maters more complex, there is nothing in the Hazmat regulations that prevent an entity from making regulations more strict that the Federal regs, so the operator of a tollway, bridge, tunnel etc, can prohibit materials in addition to those restricted by other laws.
While I am no longer in the business of transporting hazardouds material, I have shipped many hundreds of thousands tons of hazardous materials in a variety of modes both domestically, and internationally, and I learned a couple of things. First, the rules are always changing, and even the regulators don't always know the latest rule, and it is always better to try to adhere to any posted regulation, as the you are more likely to avoid trouble than if you try to educate the person with the athority to fine you.
I simply think it is easier for some one with a rental vehicle to go through open weigh stations, than to try to prove later that they were not required to. I am sure Uhaul will not pay any fines if they get one. I also know that here in So Cal, all the weigh stations have signs posted that state that all rental trucks must enter any open weigh station.
Zeke