el número de vehículos en movimiento

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Note added at 5 mins (2008-07-05 19:41:48 GMT)
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Mary,
eso es lo que significa el término, sin embargo, la frase que has dado de contexto o está mal escrita o trunca, porque no le encuentro sentido

ver: IATE



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Note added at 14 mins (2008-07-05 19:50:47 GMT)
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http://en.wikipedia.org/wiki/Fleet_vehicle
Fleet vehicles are groups of automobiles with a single owner, typically a company or corporation, rather than an individual or family. Typical examples are the vehicles owned by a car rental company, the vehicles owned by a utility company, public buses, and the vehicles owned by a security or police department.



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Note added at 16 mins (2008-07-05 19:52:05 GMT)
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http://www.greencarcongress.com/2008/06/report-projects.html

Report Projects 4% Shrinkage of US Vehicle Fleet in Face of $7/Gallon Gas by 2012
26 June 2008

CIBC’s Rubin projects a decrease in the US vehicle fleet of 10 million by 2012 as scrappage overtakes new sales. Click to enlarge.
CIBC World Markets Managing Director/Chief Economist and Chief Strategist Jeff Rubin is projecting a shrinkage in the US vehicle fleet of 10 million vehicles in light of projected $7/gallon gasoline by 2012. A decline on that order would represent approximately a 4% reduction in the overall fleet—the largest such adjustment yet.