Comparison of least-cost and least-pollution equipment fleet configurations using computer simulation

Construction vehicles and equipment, like all other fuel-powered vehicles, contribute to air pollutant emissions, such as nitrogen oxides (NOX) and particulate matter (PM). The U.S. Environmental Protection Agency has established standards for pollutant emissions for different tiers of equipment to attain a "least-pollution" equipment spread or fleet. Although using higher-tier vehicles and equipment can reduce pollutant emissions under otherwise identical conditions, policies and contractor operations can also significantly affect the total pollutant emitted for a given amount of work, such as cubic yards of earth moved or tons of surface course placed. Previous studies demonstrated the direct relationship between pollutant emissions and operational productivity and the quantitative effects of that relationship using real-world emissions data. This paper describes the results of a study conducted using computer-based simulation techniques to compare costs and pollutant emissions for a typical earth-moving project using a wheel loader and dump trucks to move a quantity of soil. Emissions were based on previously published field data. Simulation results are compared to classical, closed-form solutions from queuing theory. This study found that excess pollution - that is, the amount of pollution created without generating productive work - was lowest for the least-cost fleet and that the marginal increase in emissions was relatively small for changes in fleet configuration near the least-cost and least-pollution alternate.