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Within a few decades of the invention of the automobile, the horse-drawn carriage disappeared from city streets. Along with all the advantages offered by the automobile, this new means of transportation quickly became a significant source of air pollution. As larger cities became more crowded, the number of cars increased, making travel difficult and adding to the pollution problem as automobile traffic clogged the streets. The development of mass transit systems, including busses, trolleys, subways, and light rail, led to a reduction in traffic and, as a result, a reduction in air pollution.