1. Bureau of Transportation Statistics. Transportation statistics annual report (2016). Washington, DC: Bureau of Transportation Statistics, U.S. Department of Transportation, www.bts.gov/sites/bts.dot.gov/files/docs/TSAR_2016.pdf, (2016, Accessed 10 July 2019).
2. Federal Highway Administration (2017a) Highway statistics—2016. Washington, DC: Federal Highway Administration, U.S. Department of Transportation. https://www.fhwa.dot.gov/policyinformation/statistics/2016/pdf/hf10b.pdf
3. International Organization of Motor Vehicle Manufacturers—Motorization rate worldwide (2015) http://www.oica.net/world-vehicles-in-use-all-vehicles-2/
4. EPA (2000a) National air pollutant emission trends, 1900–1998. EPA-454/R-00–002. U.S. Energy Information Agency, Research Triangle Park, NC. https://www.epa.gov/air-emissions-inventories/air-pollutant-emissions-trends-data
5. Regulations for Emissions from Vehicles and Engines. EPA, Regulatory Information by Topic. https://www.epa.gov/regulations-emissions-vehicles-and-engines/regulations-smog-soot-and-other-air-pollution-commercial