(a) Change in Emissions Knowing the emission rate per travel activity (from step 1.5) and the amount of travel activity (step 1.4), estimating the total emissions (also referred to as emissions inventory) is a straightforward task. The differences in various approaches for estimating total emissions stem largely from their respective definitions of the term travel activity. In approaches where MOBILE6 is used to establish emission rates (step 1.5), travel activity is defined in terms of vehicle-miles of travel. Total emissions are estimated as follows: