1. An inquiry into the nature and causes of the wealth of nations, 1776;Smith,2003
2. Office of Management and Budget, Executive Office of the President. Economic analysis of federal regulations under executive order 12866. Available at: www.whitehouse.gov/omb/inforeg/riaguide.html. Accessed July 31, 2003.
3. The costs of poor health habits;Manning,1991
4. Economics at the wheel;Porter,1999
5. Transportation statistics annual report, 2000,2000