Bridge–tunnel

However, navigational considerations at some locations may limit the use of high bridges or drawbridge spans when crossing shipping channels, necessitating the use of a tunnel.

In other instances, when longer distances are involved, a bridge–tunnel may be less costly and easier to ventilate than a single, lengthy tunnel.

For example, in the U.S. state of Virginia, such crossings include the Hampton Roads Bridge–Tunnel and the Monitor–Merrimac Memorial Bridge–Tunnel, both of which cross the harbor at Hampton Roads, and the Chesapeake Bay Bridge–Tunnel, a 37-kilometer-long (23 mi) structure (including approach highways) that crosses the mouth of the Chesapeake Bay with a combination of bridges and tunnels across two widely separated shipping channels, using four artificial islands built in the bay as portals.

Tunnels had to be used instead of drawbridges because the waterways they cross are critical to military naval operations (Naval Station Norfolk is nearby, and Chesapeake Bay provides access to the Potomac River and thus to Washington, DC) and could not afford to be blocked off by a bridge collapse in the event of disaster or war.

A bridge could not be built there for two reasons: height restrictions imposed by adjacent Copenhagen International Airport (the route's path passes the ends of some of the runways; the minimum practical height for a bridge would have interfered with airplanes using those runways) and the perennial threat of ice (a bridge's columns would have encouraged ice dams which could block the strait).