KANSAS CITY, Mo. -Twelve feet means 12 feet. CRST Semi Stuck Under Bridge

A good bit of damage to that trailer,is a sign that the driver was fully trusting an object that is 13 feet 6 inches high would fit under a 12 foot opening.
At another forum, this drivers error would be known as a 'common mistake'. If this is a common mistake, take your CDL and return it to the cracker jack box you got it from.