Here is an interesting chunk from an NYT article that looks at Japan's attempt to boost its economy through infrastructure spending in the 1990s (bold is mine):
Dr. Ihori of the University of Tokyo did a survey of public works in the 1990s, concluding that the spending created almost no additional economic growth. Instead of spreading beneficial ripple effects across the economy, he found that the spending actually led to declines in business investment by driving out private investors. He also said job creation was too narrowly focused in the construction industry in rural areas to give much benefit to the overall economy.
He agreed with other critics that the 1990s stimulus failed because too much of it went to roads and bridges, overbuilding this already heavily developed nation. Critics also said decisions on how to spend the money were made behind closed doors by bureaucrats, politicians and the construction industry, and often reflected political considerations more than economic. Dr. Ihori said the United States appeared to be striking a better balance by investing in new energy and information-technology infrastructure as well as replacing aging infrastructure.
Me: That part about crowding out private investors is key. Government spending crowds out private spending. So resources are used less efficiently.

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greseeseseeat of AL 1:52PM April 27, 2009
Aniggicky of AL 7:56PM April 22, 2009
lissatrdon of AL 10:15AM April 21, 2009