Fixed link

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

Bridge-tunnels

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 CBBT system in Delmarva region, 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. This situation may occur when more economical drawbridges are not allowed for one reason or another. Examples 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 miles) 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 and could not afford to be blocked off by a bridge collapse in the event of disaster or war.

Another example is the Oresund Connection, connecting Sweden and Denmark. It has a 7.8 km (4.8 miles) bridge, an artificial island in the middle of the Oresund strait, and a 4 km (2.5 miles) tunnel nearest to Denmark. A bridge could not be built there because it is too close to the Copenhagen International Airport.

The Tokyo Bay Aqua-Line is a bridge-tunnel combination across Tokyo Bay in Japan. It connects the city of Kawasaki in Kanagawa Prefecture with the city of Kisarazu in Chiba Prefecture. With an overall length of 14 km, it includes a 4.4 km bridge and 9.6 km tunnel underneath the bay - which is the longest underwater tunnel for cars in the world.