New York State used to mark their bridges from the lowest point. That could be a curb and not actually on the road.
There was a bridge along the way from Batavia to Mount Morris that was marked too low for trucks but, trucks could easily pass under that bridge. I used to use that bridge with my trainees just to see if they were paying attention to the bridge height signs.
New York no longer measures their bridges from the lowest point. They measure them now from the lowest point on the road.