United States. Bureau of Labor Statistics. "Transportation and Public Utilities," in United States. Bureau of Labor Statistics. "Employment, Hours, and Earnings, United States, 1990-95 : Bulletin of the United States Bureau of Labor Statistics, No. 2465," Employment and Earnings, United States (September 1995) : 210-223. https://fraser.stlouisfed.org/title/189/item/5438/toc/240056 , accessed on May 1, 2024.