Intermodal comparison of tailpipe emission rates between transit buses and private vehicles for on-road passenger transport

Modal shift from private vehicles (PVs) to transit buses has the potential to reduce energy use and emissions from on-road passenger transport. Comparisons between these modes may be sensitive to key factors, such as vehicle size, fuel and powertrains, passenger load, and travel routes. The objectives are to evaluate the sensitivity of emission rates to route alignment, and compare emission rates between PVs and buses accounting for variability in key factors. Real-world bus speed trajectories were measured on actual bus routes for four origin-destination pairs (ODPs). To evaluate the sensitivity of emission rates to route alignment, hypothetical alternative bus routes were posited based on shortest distance, shortest travel time, or observed PV routes for each ODP. Trajectories and emission rates for PVs were quantified based on prior measurements of two routes per ODP using portable emission measurement systems. Trip-based tailpipe CO2, CO, total hydrocarbons (THC), NOx, and particulate matter (PM) emission rates were estimated for each ODP for gasoline and gasoline-hybrid PVs based on a Vehicle Specific Power modal model and for compressed natural gas, diesel, and diesel-hybrid buses based on the Transit Bus Emissions Model. Break-even passenger load (BEPL) was quantified to assess the minimum bus passenger load needed to achieve lower per passenger-trip emissions compared to PVs. Bus emission rates per bus-trip on actual bus routes are generally higher than those on hypothetical routes. As a bounding analysis, compared to single-occupancy PVs, fully occupied buses are estimated to have 82%-94% lower CO2, 99% lower to 308% higher CO, 99% lower to 145% higher THC, 67% lower to 62% higher NOx, and 94%-99% lower PM emission rates per passenger-trip depending on vehicle size, fuel and powertrain, passenger load, and route. BEPL varies depending on vehicle size, fuel and powertrain, route, and pollutant. The relative importance of key factors affecting intermodal comparisons differs by pollutants. The intermodal comparison is also affected by interactions among key factors, such as passenger load and route alignment, which reinforces the need for joint consideration of key factors.

Automated summary

The study found that modal shift from private vehicles to transit buses has the potential to reduce energy use and emissions from on-road passenger transport. Comparisons between these modes may be sensitive to factors such as vehicle size, fuel and powertrains, passenger load, and travel routes. Generally, bus emission rates on actual routes are higher than on hypothetical routes, while fully occupied buses are estimated to have significantly lower emissions per passenger-trip compared to single-occupancy private vehicles.