1. U.S. Department of transportation. National transportation statistics. Table 1-49: U.S. Ton miles of freight.
2. U.S. environmental protection agency. Technical bulletin: diesel particulate filter general information. EPA-420-F-10-029, May 2010.
3. U.S. Department of Transportation, Federal Highway Administration, Office of Highway Policy information highway statistics series. Highway statistics 2012: truck and truck tractor registrations. 2012, Table MV-9.
4. Code of Federal Regulations, Title 40: Protection of environment, Parts 69, 80 and 86.
5. Frost, Rodney I., and Max R. Montierth. Diesel particulate filters for use with smaller diesel engines. U.S. Patent No.: US4415344. 1983.