As a rule bridges in Upstate New York most towns outside of the NY City & Long Island area.
As a rule are marked a foot higher than the actual height.
I have lived in Upstate all of my nearly 70 years,& retired after being a city driver for 45 years.
But never really knew the reason why the signage is marked higher?
Only explanation I have ever heard was from other truckers that in the winter the snow builds up on the roads,or that the ground surface raises up from the cold.
However I think those two reasons are just speculations from other truckers who deliver in this area.
Best to not count on any GPS,or even signage.
I drove long enough that I could eye ball the height between my trailers & overpasses,also utility lines.
That is why I find it so odd that so many bridge hits are posted here on this accident forum.
For the most part all drivers need to use their own judgment, based on their driving experience, to know where they can,& can't make it under.