Key results are presented from an exhaust emission study conducted on more than 105 buses in idling condition and selected sample buses from seven fleets in real-world operating conditions in Toledo, Ohio, of the effects of varying vehicular operation parameters on public transport bus emissions. It was observed that for the same amount of time in operation, vehicles in idling mode produced higher average concentrations of carbon monoxide, nitric oxide, nitrogen dioxide, and sulfur dioxide than those in on-road operation. Operating the vehicles in normal engine idling mode and with higher engine temperatures was found to reduce vehicular emissions by up to 30% to 42% as compared with fast idling and lower or colder engine temperatures. Performing regular preventive maintenance reduced emission concentrations by 15% to 20%. The results also highlight that a combination of multiple influencing factors including engine technology, vehicle age, vehicular maintenance, fuel, and past and present operating conditions should be evaluated to better understand and predict vehicular emission behavior.