Let me give you an Example for my State(Michigan).
We're allowed(Intrastate) to haul 160,000 Pounds of Logs(This includes I-75 but all roads in my area are 2 lane but I could, if there was a reason, to haul a load Downstate on I-75.
If we were to stay strictly with Federal Rules and Regs we couldn't do that.
That is the reason I suggested you contact your state(Alabama) because all states are different on Intra-state Regs.
I suppose that's clear as mud eh? :P