Editing Tunnel (section)

=== Choice of tunnels versus bridges ===
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{{Anchor|Choice of tunnels vs bridges}}
[[File:2007 09 19 - 895tunnel - WB 3.JPG|right|thumb|The [[Harbor Tunnel (Baltimore)|Harbor Tunnel]] in [[Baltimore]], USA, which carries [[Interstate 895 (Maryland)|I-895]], serves as an example of a water-crossing tunnel built instead of a bridge.]]
For water crossings, a tunnel is generally more costly to construct than a bridge.<ref>{{Cite web |title=Tunnels - an environmentally attractive option? |url=https://www.worldhighways.com/wh10/feature/tunnels-environmentally-attractive-option |access-date=2024-10-08 |website=World Highways |language=en}}</ref> However, both navigational and traffic considerations may limit the use of high bridges or [[drawbridge]]s intersecting with shipping channels, necessitating a tunnel.

Bridges usually require a larger footprint on each shore than tunnels. In areas with expensive real estate, such as [[Manhattan]] and urban [[Hong Kong]], this is a strong factor in favor of a tunnel. Boston's [[Big Dig]] project replaced elevated roadways with a tunnel system to increase traffic capacity, hide traffic, reclaim land, redecorate, and reunite the city with the waterfront.<ref>{{Cite web |title=FHWA - Center for Innovative Finance Support - Project Profiles |url=https://www.fhwa.dot.gov/ipd/project_profiles/ma_boston_central_artery.aspx |access-date=2024-11-08 |website=www.fhwa.dot.gov}}</ref>

The 1934 [[Queensway Tunnel]] under the [[River Mersey]] at [[Liverpool]] was chosen over a massively high bridge partly for defence reasons; it was feared that aircraft could destroy a bridge in times of war, not merely impairing road traffic but blocking the river to navigation.<ref>{{Cite web |title=Queensway Tunnel which links Liverpool to Birkenhead marks 90th anniversary |url=https://www.bbc.com/news/articles/c7283vplp56o |access-date=2024-10-08 |website=www.bbc.com |date=20 July 2024 |language=en-GB}}</ref> Maintenance costs of a massive bridge to allow the world's largest ships to navigate under were considered higher than for a tunnel. Similar conclusions were reached for the 1971 [[Kingsway Tunnel]] under the Mersey. In [[Hampton Roads, Virginia]], tunnels were chosen over bridges for strategic considerations; in the event of damage, bridges might prevent [[US Navy]] vessels from leaving [[Naval Station Norfolk]].

Water-crossing tunnels built instead of bridges include the [[Seikan Tunnel]] in Japan; the [[Holland Tunnel]] and [[Lincoln Tunnel]] between [[New Jersey]] and Manhattan in [[New York City]]; the [[Queens-Midtown Tunnel]] between Manhattan and the [[Borough (New York City)|borough]] of [[Queens]] on [[Long Island]]; the [[Detroit-Windsor Tunnel]] between [[Michigan]] and [[Ontario, Canada|Ontario]]; and the [[Elizabeth River (Virginia)|Elizabeth River]] tunnels between [[Norfolk, Virginia|Norfolk]] and [[Portsmouth, Virginia]]; the 1934 [[River Mersey]] road [[Queensway Tunnel]]; the [[Western Scheldt Tunnel]], Zeeland, Netherlands; and the [[North Shore Connector]] tunnel in [[Pittsburgh, Pennsylvania]]. The [[Sydney Harbour Tunnel]] was constructed to provide a second harbour crossing and to alleviate traffic congestion on the [[Sydney Harbour Bridge]], without spoiling the iconic view.

Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather, and shipping during construction (as in the {{convert|51.5|km|mi|adj=on|disp=or}} [[Channel Tunnel]]), aesthetic reasons (preserving the above-ground view, landscape, and scenery), and also for weight capacity reasons (it may be more feasible to build a tunnel than a sufficiently strong bridge).

Some water crossings are a mixture of bridges and tunnels, such as the [[Oresund Bridge|Denmark to Sweden link]] and the [[Chesapeake Bay Bridge-Tunnel]] in [[Virginia]].

There are particular hazards with tunnels, especially from vehicle fires when combustion gases can [[asphyxia]]te users, as happened at the [[Gotthard Road Tunnel#2001 collision and fire|Gotthard Road Tunnel]] in [[Switzerland]] in 2001. One of the worst railway disasters ever, the [[Balvano train disaster]], was caused by a train stalling in the Armi tunnel in [[Italy]] in 1944, killing 426 passengers. Designers try to reduce these risks by installing emergency ventilation systems or isolated emergency escape tunnels parallel to the main passage.