If an overpass is listed at a certain height, is there a few inches of headway?

Treat it as what its marked as. Never ever assume theres a few inches of leeway because theres not.

 

Depends on the state. I know new york has a habit of posting bridges a foot lower than they actually are in some cases.

 

Some are close to what they are marked and some can be as much as a foot higher than marked.
When I’m in serious doubt in a unfamiliar area I throw my 4 ways on and jump out with my height stick and check.

 

Sometimes.......you may wanna measure your rig though

 

I ran the Windy for many years, home of the low bridge. Do what I did, I set my CB antennas on the mirrors at 13'7", and if the antennas cleared, I was good to go. I'd go by what was on the sign. Bosses have a hard time when you come home with an open top trailer.

 

The methodology for measuring bridges varies from state to state. Some post the lowest point over the traveled lanes, others include the shoulders. So there may or may not be leeway. My rule is I'm not going under one marked less than my height unless I've previously measured that bridge. There are a few that are visually obvious (i.e. I-94 WB in Jackson) why they're marked so low but are safe in one lane or the other.

But never, ever assume!