Hong Kong spent USD$8.60 billion to build the high-speed rail (HSR) line linking with mainland China. (10 pages) They calculate a net present value (NPV) of up to USD$2,068.49 million for the calculated benefits. Conventional railway also has a positive NPV (about USD$177.60 million) due to its balance performance in all kinds of aspects. Roadway transport is the only alternative that has a negative NPV (-USD$15885.60 million). This is mainly due to its longest travel time which causes a large negative impact on the benefit of travel time savings.
All big projects have studies that support the financials of the projects. The assumptions supporting China's cost benefit analysis look more realistic than the studies for California's high speed rail.
Here is the cost benefit analysis for California high speed rail.
There is an OECD study from 2009 of high speed rail which looks at the benefits for french high speed rail after the fact.
There is a lengthy study on the economic benefits of high speed rail for Europe
Here is a study of the co-benefits of high speed rail.
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