United States. Bureau of Labor Statistics. "Transportation and Public Utilities," in United States. Bureau of Labor Statistics. "Employment, Hours, and Earnings, United States, 1981-93 : Bulletin of the United States Bureau of Labor Statistics, No. 2429," Employment and Earnings, United States (August 1993) : 331-353. https://fraser.stlouisfed.org/title/189/item/5436/toc/327806 , accessed on March 29, 2024.