The Bureau of Transportation Statistics collects and supplies information about transportation in the United States. Its Statistical Products and Data page may be most helpful.

From the BTS About page:

“As the independent statistical agency within the Department of Transportation (DOT), the Bureau of Transportation Statistics (BTS) is a politically objective supplier of trusted and statistically sound baseline, contextual, and trend information used to shape transportation policy, investments, and research across the U.S. and abroad. 

BTS is the preeminent source of statistics on commercial aviation, multimodal freight, and transportation economics.”