The environmental hazards of motorized vehicle traffic are no secret. According to the University of North Carolina Highway Safety Research Center, transportation emits large amounts of pollutants into the atmosphere. The university's research found that 80 percent of the carbon monoxide in the atmosphere came from motorized vehicles that operate on gas and diesel. These sources also contribute 55 percent of the nitrogen oxide pollution in the air. Riding a bike, however, contributes zero pollutants, a statistic that is definitely a pro for the environment.