This thread has got me looking into things. There is not a National Height Limit. The federal Government leaves it up the the states. If you look you will find for the most part states have set the maximum height to either 14' or 13'6". Folks use the 13'6", the lesser of the 2, as the maximum because is is simpler to remember. I you live in a state that has a 14' limit you are legal at 13'9. If you only visit states with a 14' maximum you are legal. but in all states there are structures that are less than their maximum stated vehicle height, and we must make sure we do not test their limits.