Bridge-tunnels

A bridge–tunnel is a persistent, unbroken road or rail connection across water that uses a combination of bridges and tunnels, and sometimes causeways, and does not involve intermittent connections such as drawbridges or ferries.

Bridge–tunnels are a common form of fixed link or fixed crossing which replaces ferry service. Fixed links are often, but not necessarily, intercontinental links between continents or transoceanic links to offshore islands.

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 crossing shipping channels, necessitating the use of a tunnel. Examples of such tunnels include the Downtown Tunnel and Midtown Tunnel under the Elizabeth River between Norfolk and Portsmouth, Virginia in the United States and the George Massey Tunnel in Greater Vancouver, Canada.

In other instances, when longer distances are involved, a bridge–tunnel may be less costly and easier to ventilate than a single, lengthy tunnel. This situation may occur when more economical drawbridges are not allowed for one reason or another. 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 km (23 mi) long 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.

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