You should be able to get a reducable weight permit for the flat just like an open top or dump trailer. Tell the permit office you want a reducable load permit my as well spend the extra and get I90 eligible
These permit systems aggravate me. State residents can buy them or in some cases apparently get them for free. For an out of state it isn't allowed. Permitted intrastate trucks can put added stress on the highways that I help pay for and yet I can't do the same.
