After a tour of the New Belconnen Bus Stations in Canberra I was reading the Wikipedia entry on "Bus rapid transit". The idea is simple: use buses for a rapid transit system, by providing stations, and dedicated roadways where possible. This is cheaper and easier than putting in rail lines and is almost as efficient.
One limitation is running buses in tunnels. Unless complex and expensive hybrid or electric buses are used, the there will be emissions from the bus engine, requiring ventilation. But it occurred to me that newer buses will have less emissions and so will require less ventilation in tunnels (I edited the Wikipedia entry to point this out).
Another point is that road tunnels are designed mostly for cars, which will emit much more pollution than buses, per person carried. Also engineers designing the tunnels have to assume that the cars are old and not well serviced and so will be emitting more pollution than the latest regulations allow. In contrast a tunnel just for buses can be designed specifically for the latest emission standard and the bus fleet monitored to check they meet the standard. So less ventilation will be required, and for short tunnels, not mechanical ventilation at all.
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