Air draft

A bridge's "clearance below" is most often noted on charts as measured from the surface of the water to the underside of the bridge at the chart datum Mean High Water (MHW),[3][4] a less restrictive clearance than Mean Higher High Water (MHHW).

In 2014, the United States Coast Guard reported that 1.2% of the collisions that it investigated in the recent past were caused by vessels attempting to pass under structures with insufficient clearance resulting in bridge strikes.

The world's largest cruise ships, Oasis of the Seas, Allure of the Seas and the Harmony of the Seas will fit within the canal's new widened locks, but they are too tall to pass under the bridge, even at low tide (the two first ships are 72 m (236 ft), but do have lowerable funnels, enabling them to pass the 65-metre (213 ft) Great Belt Bridge in Denmark).

New vessels are rarely built not clearing 65 m (213 ft), a height which accommodates all but the largest cruise and container ships.

The Bayonne Bridge, an arch bridge connecting New Jersey with New York City, undertook a $1.7 billion modification to raise its roadbed to 66 m (217 ft).

The deck of the Allanburg Bridge on Canada's Welland Canal typically rests only a few metres above the water level. When a ship approaches, the deck is raised to provide sufficient air draft (or draught) for the vessel to pass through. This bridge was involved in a collision with a lake freighter in 2001 as a result of lowering the span before the ship fully cleared the bridge.
Bridge of the Americas