How do errors occur when developing speed correction factors for emission modeling?

Abstract

• Detailed explanation of errors in speed correction factors (SCFs) • Quantifying the impact of vehicle specific power (VSP) distribution on SCFs. • Quantitative analysis of relationship between emission rate and SCFs. Speed correction factors (SCFs) reflect the impact of traffic conditions on vehicle emissions. However, uncertainty often occurs when calculating SCFs in existing emission models. Investigating how uncertainty occurs is helpful in improving accuracy when calculating SCFs. This study calculates SCFs of carbon dioxide (CO 2 ) of non-hybrid light-duty vehicles on urban restricted access roadways, and analyzes two important explanatory variables when calculating SCFs: speed-specific vehicle specific power (VSP) distributions and emission rates. The characteristics of how SCFs change in response to changes in VSP distributions and emission rates are discussed in detail. The results show that distortions in VSP distributions and emission rates significantly affect SCFs. A 1% change of the fraction in a VSP bin or a 1% change in the average emission rate in a VSP bin can cause 1.3–3.2% or 0.08–0.13% of changes in SCFs, respectively. These findings are helpful in reducing uncertainty when calculating SCFs.