Bridge-tunnel

For water crossings, a tunnel is generally more costly to construct than a bridge. However, navigational considerations at some locations may limit the use of high bridges or drawbridge spans when intersecting with shipping channels, necessitating the use of a tunnel. Examples of such tunnels include the Holland Tunnel and Lincoln Tunnel between New Jersey, and Manhattan Island in New York City, and the Elizabeth River tunnels between Norfolk and Portsmouth, Virginia.

In other instances, when longer distances are involved, a combination of bridges and tunnels may be less costly and easier to ventilate than a single very long tunnel, resulting in a bridge-tunnel complex. Examples include the Hampton Roads Bridge-Tunnel, the Monitor-Merrimac Memorial Bridge-Tunnel, both of which across the harbor at Hampton Roads, and the Chesapeake Bay Bridge-Tunnel, a 23-mile long structure which crosses the mouth of the Chesapeake Bay with a combination of bridges and tunnels over two widely separated shipping channels, using four man-made islands built in the bay as portals.