In her Life Cycle Assessment of Transportation Options for Commuters, Shreya Dave at MIT analyzed the complete life cycles of various modes of transportation to compare their environmental impacts.
As you’d expect, human powered forms of transportation are much less polluting than those involving fossil fuel. One surprise in the report is that the differences between walking, conventional bicycling, and e-assist bicycling are negligible. From the conclusion:
The results reported in the previous section show the significant environmental as well as economic (in terms of energy purchased) benefit of using human-powered forms of transportation. According to this study, walking, conventional bicycling and electric bicycling are release exactly the same amount of greenhouse gas emissions. While an electric bicycle consumes more energy (both to produce and to operate) and releases the associated greenhouse gases, a conventional bicycle requires the operator to work harder and breathe more heavily. In effect, the impact of the electric bicycle is entirely negligible. All forms of personal transport are at least three times better than any other form of commuter transport.