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Submitted on: 09 September 2008.
Revised on: 16 December 2008.
Accepted on: 31 January 2009.
___________________Economics of Transportation---ELSEVIER
Submitted on: 16 January 1999.
Revised on: 18 February 1999.
Accepted on: 20 March 1999.
___________________Economics of Transportation---ELSEVIER
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