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The result for one rider going 1 mile (i.e., "passenger mile"): Passenger mile per passenger mile, Denver's light-rail cars pump 0.59 pounds of CO2 into the atmosphere. Passenger cars, such as sedans, average 0.54 pounds, and the Toyota Prius just 0.26 pounds.
That means for every million passenger-miles traveled in Denver, light rail puts 50,000 more pounds of CO2 into the atmosphere than passenger cars do.