[xoxoxoBruce]

14 foot for interstate highways only. That was for missile carriers clearance. Then they raised it to 16 feet but backed off when they found what it would cost to retrofit the whole system.

There was never any intention to make everything 14 feet. That would bankrupt the country. On the low bridges commonly hit they have tried warning boards hanging low, flashing lights, and signs up the wazoo, with little effect. You can't prevent stupid.