As the New York State Legislature considers whether to classify the Segway as a motorized vehicle (like a car) or a a "personal assistive mobility device" (like a wheelchair), environmentalists are taking the position that the Segway increases air pollution by troubling pedestrians, pressuring them to switch to using cars.
I switched from walking to work, which I did for fifteen years, to driving, in part because I got quite annoyed by people bicycling on the sidewalk. Bicyclists at least know they are wrong, but rationalize that they need to protect themselves from cars. But Segways supposedly belong on the sidewalk, yet on most sidewalks, if two people are walking together, there is not enough room for the Segway to pass them. The thing clearly seems capable of knocking you down. There don't seem to be enough of them right now to matter much. They simply haven't caught on. (Because you look like a dork riding one?) People are tolerant of carelessly driven motorized wheelchairs -- which I see a lot of -- but insensitively driven Segways are going to drive pedestrians crazy.
Which is an environmental problem.