Are all unmarked Bridges really minimum 16ft 6ins?

i see two issues here: 1. Midlift on many trucks automatically raise when its empty, and no way of lowering it. 2. When resurfacing, i believe they always take off a certain amount ( planing ) and replace it with the same amount, so for eg, 2" off…2" back on…i always believed also that motorway bridges are 17 ft…but some abroad are 4m. but clearly marked with a diversion in place if needed.