Incorporating Local Road Grades and Times-of-Day Traffic into Vehicle Specific Power Profiling for Urban Freeway Vehicle Emission Estimation

Vehicle Specific Power (VSP) is conventionally defined to represent the instantaneous vehicle engine power. It has been widely utilized that the impact of vehicle operating conditions on emission and energy consumption estimation is associated with vehicle speed, roadway grade and vehicle acceleration or deceleration on the basis of the second-by-second vehicle operation. VSP is hence incorporated as a key contributing factor into the vehicle emission models in MOVES. For practical application, however, it is always cumbersome to accurately profile VSP distribution by collecting and using localized grade and times-of-day traffic data. Therefore, it is necessary to clarify the impacts of these factors on highway vehicle emission estimation. This paper presents a study in which previous studies are extended by deeply investigating the characteristics of VSP distributions and their impacts due to varying freeway grades, as well as time-of-day traffic factors. Statistical distribution models with a scope of bins is identified through a goodness of fit testing approach by using the Global Positioning System (GPS) data collected from the interstate freeway I-75 segments in the Cincinnati area. The data was collected at a selected length of 30 km urban freeway for AM, PM and Mid-day periods. The datasets representing the vehicle operating conditions for the VSP calculation were then extracted from the GPS trajectory data. The results of distribution fitting show that the Wake by distribution is able to capture most distribution characteristics of VSP at all grade bins under a higher speed variation condition, and the generalized logistic distribution fits the sample data better at grade bins between -4% and 4%when the speed variation is lower. In addition, the speed variation lying behind the times-of-day differences is also identified to be a contributing factor of urban freeway VSP distribution. The enhanced understanding and modelling of VSP distribution by roadway grade provided by the study can facilitate the preparation of MOVES vehicle operating mode distribution inputs.