What is the max. grade allowed on a “normal” state highway? I love curvy roads like the one pictured, but clearly they’re not just building them like this for my enjoyment... Looks like 6% or 7% is max for U.S. interstates, apparently a street in SF has a 37% grade, but what’s the accepted limit for most states when it comes to their small highways? International folks: please chime in with the max. grades on similar roads in your area.