High gasoline prices in recent years helped give public-transit agencies across the U.S. their best ridership numbers in half a century. Now, falling prices pose an obstacle to further growth.

The swift drop in gas prices, to less than $2 a gallon in much of the nation, already appears to be encouraging Americans to drive more. Miles driven increased 3% in October from a year earlier,the largest year-over-year gain since 2006,and advanced 1.1% in November, according to the U.S. Federal Highway Administration.

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