This study compares the operational carbon dioxide (CO2) emissions of three main travel modes (rail, air, and road – both passenger
vehicle and bus) for four real-world trips between different city pairs across the U.S. For each city pair and mode, operational emissions
were estimated by calculating the roundtrip per-passenger-mile fuel efficiency and emissions. Operational emissions were bounded by
emissions from the movement of passengers only (i.e., a pump-to-wheels analysis). First- and last-mile emissions for traveling to and
from transit centers were calculated for five different inter-city modes. For the four scenarios analyzed, travel by car (i.e., single
occupancy vehicle or SOV) or air were the most carbon intensive modes. Traveling by rail or bus was found to have substantially lower
operational CO2 emissions than either SOV or air. The electric train had the lowest CO2 emissions of the modes compared.