When federal Interstate weight standards were adopted, states that already allowed heavier operation were not made to come down. Three separate grandfather clauses in the statute preserve those allowances.
"When considered together, the successive grandfather provisions provided by Congress in 1956, 1974, and 1991 and other exceptions results in 37 States and the District of Columbia having allowances to exceed Federal weight limits on their Interstate highways (in many States these exceptions are very limited)."
Compilation of Existing State Truck Size and Weight Limit Laws, pp. 15–16 — FHWA, May 2015. Public domain.
The trap sits right next to it: in at least eight states the weight limits printed in statute are higher than the federal Interstate limits without a grandfather right behind them, and a federal compliance clause pulls them back to the federal figures on the Interstate itself. Taking one of those statutory numbers onto an Interstate lane misprices the move.
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