To estimate near-roadway air quality and human exposure, vehicle emissionestimates are needed with high spatial and temporal resolution. The U.S.Environmental Protection Agency (EPA)’s MOBILE6 emission factor model is based on test cycles with transient speeds and durations on the order of 10 minutes. MOBILE6 does not report emission factors for average speeds higher than 65 mph. However, for near-roadway studies, emission factors areneeded for short highway segments that represent only a few seconds of vehicle travel time at approximately constant speed and speeds greater than 65 mph. The constant and high speeds correction factors for nitrogen oxides (NOx), hydrocarbons (HC), carbon dioxide (CO2), and carbon monoxide (CO), were developed based on 59,286 seconds of EPA dynamometer emission datafor diesel vehicles from which speed-acceleration modal models were derived. The constant speed correction factor is the ratio of emission factor at constant speed versus that for transient cycles with an equivalent average speed. For example, the constant speed correction factor for CO2 increases from 0.71 to 1 as speed increases from 31 to 78 mph. High speed correction factors are based on the ratio of emission factors at a constant speed greater than 65 mph to that at a constant speed of 65 mph. For example, at 80 mph, the high speed correction factor is approximately 1.5 for CO2. Sensitivity analysis of emission estimates to these correction factors was conducted for speeds of 40 to 80 mph.
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