Improvements to On-Road Emission Modeling of Freeways with High-Occupancy Vehicle Facilities

Because high-occupancy vehicle (HOV) facilities have become an integral part of the freeway systems in many parts of the United States, transportation practitioners must estimate emissions from these facilities accurately. This study examined the differences between HOV and mixed-flow (MF) lanes relative to their traffic dynamics as described by speed and acceleration profiles of the collected driving trajectory data, as well as to their vehicle fleet composition. It was found that the model year distributions of vehicles in the two lane types at selected locations were not significantly different. However, the differences in speed, acceleration, and road load power of their driving data samples were statistically significant at each of the six levels of congestion tested (i.e., Levels of Service A through F). Hence, lane correction factors were developed for adjusting freeway emission factors to result in emission factors specific for an HOV lane. These correction factors have higher impacts on HOV emission rates of carbon monoxide (CO) and carbon dioxide (CO2) than of hydrocarbon (HQ and oxides of nitrogen (NOx). For CO and CO2, the impact can be as much as 30% and 10%, respectively, at very low speed. The difference between NOx emission rates of HOV and MF lanes is less than 5%. HC emission rates for HOV lanes are different from those for MF lanes by merely +/- 2%. These lane correction factors can be used to improve the accuracy of emission analyses for freeways with HOV facilities.