This research effort examined 68 thousand miles of real-world operations data from 26 buses, collected from a local transit service provided by the Metropolitan Atlanta Rapid Transit Authority (MARTA), and express bus service provided by the Georgia Regional Transportation Authority (GRTA). Eco-driving strategies such as limiting transit vehicle acceleration rates and top speeds were evaluated to estimate impacts on fuel consumption and emissions.
The analysis utilized second-by-second operations data collected via global positioning system (GPS) devices from buses operated by each transit agency. Researchers simulated the implementation of transit eco-driving strategies, based on the modal emissions modeling framework employed by the MOtor Vehicle Emission Simulator (MOVES) designed to reduce engine load and emissions. This algorithm was designed to minimize fuel consumption by limiting instantaneous vehicle specific power (VSP) while maintaining average speed and conserving total distance. Fuel consumption and fuel-cycle emissions were compared across the monitored driving cycles and their modified eco-driving cycles. The savings from eco-driving were also compared against expected fuel and emissions reductions via conversion of the transit fleets to compressed natural gas (CNG) which is another popular fuel conservation strategy.
Eco-driving has significant potential to reduce fuel consumption and emissions from transit operations. For the entire MARTA fleet, eco-driving would reduce diesel fuel consumption by about 309,000 gallons per year. The MARTA fuel savings translates to an annual reduction of about 3,930 metric tons (5 percent) in fuel cycle carbon dioxide equivalent (CO2e) emissions. In terms of criteria air pollutants, eco-driving implementation in the MARTA fleet would reduce annual NOx emissions by 14 metric tons (4 percent), and annual PM2.5 emissions by 0.8 metric tons (7 percent).
For the GRTA fleet, annual fuel savings amounted to about 55,000 gallons of diesel fuel per year, a 7 percent reduction. Greenhouse gas CO2 emissions are also reduced by about 700 metric tons per year. NOx reductions would amount to 2 metric tons (5 percent), and the annual PM2.5 reductions would be 0.1 metric tons (7 percent) per year.
If MARTA replaced all 158 existing diesel buses with new compressed natural gas (CNG) buses, the fuel cycle NOx emissions reduced by 112 metric tons (30 percent) per year, and PM2.5 emissions reduced by 9 metric tons (85 percent) per year.
If GRTA replaced all of its 166 diesel buses with CNG buses, its annual fuel cycle NOx emissions would decrease by 29 metric tons (70 percent), and annual fuel cycle PM2.5 emissions would decrease by 2 metric tons (95 percent).
With a combined eco-driving and CNG fleet scenario, MARTA showed significant reductions in fuel cycle NOx and PM2.5 reduction, by 124 metric tons (34 percent) and 9 metric tons (87 percent), respectively. The GRTA fleet showed fuel cycle emission in CO2, NOx, and PM2.5 decreased by 17 percent, 73 percent and 96 percent, respectively.