Abstract
The emission modeling approach has evolved from using the average speed associated with the driving cycle to using the parameter vehicle-specific power (VSP). With this evolution, a new research need focuses on developing ways to characterize traffic conditions by using the VSP distribution. For urban restricted-access roadways, an applicable mathematical model has been developed in existing studies to describe the VSP distributions based on the average travel speed for traffic with a speed higher than 20 km/h. For the average travel speed lower than 20 km/h, the characteristics of VSP distribution are still unclear. For this reason, this study uses large samples of second-by-second floating car data, collected from the expressways in Beijing, to associate the VSP distributions with the average travel speed from 0 to 20 km/h. After VSP distributions of 1,195 pieces of 60-s speed segments are examined, regular patterns are found between the VSP distribution and the average travel speed. Specifically, the highest fraction always appears in the VSP bin of 0, which decreases monotonically with the increase of the average travel speed. A mathematical model of VSP distribution is proposed on the basis of the separate analysis of VSP fractions in negative, zero, and positive VSP bins. A comparative analysis between the estimated and actual fuel consumption demonstrates that the proposed VSP distribution model is reliable and accurate for the estimation of fuel consumption. The findings of this study may help in monitoring the dynamic traffic fuel consumption or emissions when the real-time speed data are available.
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