Investigating the isolated and combined effects of congestion, roadway grade, passenger load, and alternative fuels on transit bus emissions

Abstract

This study investigates the isolated and combined effects of network congestion, roadway grade, passenger load, and fuel type on transit bus emissions of greenhouse gases (GHG) through a simulation of transit operations and emissions along a busy corridor. We also test the effect of changing random seed on overall corridor emissions. We observe that positive grades have strong effects on emissions. Grade also causes other variables to become important such as passenger load. While an increasing passenger load on the bus increases emissions, we observe that the addition of each passenger influences the per-passenger emissions differently depending on the bus occupancy. When the bus is less crowded each additional passenger can decrease per-passenger emissions by 5% whereas the reduction becomes 1.2% when the bus is crowded. Finally, we observe that the reduction potential of compressed natural gas (CNG) compared to conventional diesel could reach up to 40% depending on speed, grade, and passenger load. CNG benefits increase with increasing congestion, and decrease with increasing grade and passenger load. The results of this study are most relevant to transit planners in the evaluation of potential operational changes with emission reduction potential and in the allocation of alternative fuelled buses along selected transit corridors.