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Bus rapid transit
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=== In tunnels or subterranean structures === {{more citations needed section|date=October 2014}} [[File:Breda dual-mode bus at University St station in Downtown Seattle Transit Tunnel 1994.jpg|thumb|The [[Downtown Seattle Transit Tunnel]] used [[Dual-mode bus|dual-mode buses]] from 1990 to 2004, which operated under electric power underground]] [[File:MBTA route SLW bus at Courthouse station, March 2017.JPG|thumb|right|An outbound [[Silver Line (MBTA)|Silver Line]] bus at Courthouse station in [[Boston]], [[Massachusetts]]]] A special issue arises in the use of buses in [[rapid transit|metro transit]] structures. Since the areas where the demand for an exclusive bus right-of-way are apt to be in dense downtown areas where an above-ground structure may be unacceptable on historic, logistic, or environmental grounds, use of BRT in tunnels may not be avoidable. Since buses are usually powered by [[internal combustion engine]]s, bus metros raise ventilation issues similar to those of motor vehicle tunnels. Powerful fans typically exchange air through [[ventilation shaft]]s to the surface; these are usually as remote as possible from occupied areas, to minimize the effects of noise and concentrated pollution. A straightforward way to reduce air quality problems is to use internal combustion engines with lower emissions. The 2008 Euro V [[European emission standards]] set a limit on [[carbon monoxide]] from heavy-duty [[diesel engine]]s of 1.5 g/kWh, one third of the 1992 Euro I standard. As a result, less forced ventilation will be required in tunnels to achieve the same air quality. Another alternative is to use electric propulsion, which [[Seattle]]'s [[Metro Bus Tunnel (Seattle)|Metro Bus Tunnel]] and [[Boston]]'s [[Silver Line (MBTA)|Silver Line]] Phase II implemented. In Seattle, [[Dual-mode bus|dual-mode]] (electric/diesel electric) buses manufactured by Breda were used until 2004, with the center axle driven by [[electric motor]]s obtaining power from [[Overhead lines|trolley wires]] through [[trolley pole]]s in the subway, and with the rear axle driven by a conventional diesel [[powertrain]] on freeways and streets. Boston is using a similar approach, after initially using trolleybuses pending delivery of the dual-mode vehicles that was completed in 2005.<ref>{{cite web|url=http://world.nycsubway.org/us/boston/silver.html|title=MBTA Silver Line|author=Duncan Allen |year=2005 |website=www.nycsubway.org |access-date=13 July 2010}}</ref> In 2004, Seattle replaced its "Transit Tunnel" fleet with diesel-electric hybrid buses, which operate similarly to [[hybrid car]]s outside the tunnel and in a low-noise, low-emissions "hush mode" (in which the diesel engine operates but does not exceed [[idle speed]]) when underground.<ref>{{cite web |url=http://metro.kingcounty.gov/am/vehicles/bustech.html |title=Downtown Seattle Transit Tunnel and Changing Bus Technology |author=Metro Online|date=14 December 2007|publisher=King County Metro|access-date=13 July 2010}}</ref> The need to provide electric power in underground environments brings the capital and maintenance costs of such routes closer to those of light rail, and raises the question of building or eventually converting to light rail. In Seattle, the downtown transit tunnel was retrofitted for conversion to a shared hybrid-bus and light-rail facility in preparation for Seattle's Central [[Link light rail|Link Light Rail]] line, which opened in July 2009. In March 2019, expansion of the light rail in the tunnel moved busses back to surface streets.<ref>{{cite web|url=https://www.kiro7.com/traffic/what-to-expect-when-830-daily-buses-move-into-downtown-seattle/933064842 |title=Last day for buses in Seattle's downtown transit tunnel |author=Graham Johnson |date=22 March 2019 |website=KIRO 7}}</ref> Bi-articulated battery electric buses cause no problems in tunnels anymore but provide BRT capacity.<ref>{{cite web | url=https://www.sustainable-bus.com/electric-bus/solaris-bi-articulated-e-buses-tide-bus/ | title=Solaris to deliver 14 bi-articulated e-buses in Denmark. They're powered by two motors and over 700 KWH battery | date=29 October 2021 }}</ref>
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