Emission uncertainties were estimated using Approach 1 of the 2006 IPCC guidelines based upon error propagation (IPCC, 2006). Our quantitative uncertainty analysis dealt with random components of measurement errors and systematic errors associated with data representativeness. The uncertainties of emission factors, number of vehicles and VKT were combined by multiplication and finally aggregated for the computation of the overall uncertainty to the total inventory by species. This approach works well for relatively small uncertainties but as the uncertainty becomes larger, the error propagation approach systematically underestimate the uncertainty. For those cases for which the uncertainty exceeded 100% the correction factor proposed in the 2006 IPCC guidelines was employed.