A measure of the total wieght of mobile source pollutant emissions (carbon monodixe, oxides of nitrogen, and reactive organic gases) from transportation modes. Both the emission factors and the formula that enables the composite index to be calculated are provided by the California Air Resources Board (CARB). The emission factors are sensitive to the number, length, and speed of vehicle trips and take into account projected emission reductions due to such improvements as alternative fuels and electric vehicles.