I agree with Tara. The word as it appears in this sentence should be "curtails" because the act of using bicycles is grammatically singular.

It should be curtails...I actually had to read that sentence twice to understand what it was trying to say because it sounds incomplete, like something should follow it. (For example, "Using bicycles to travel shorter distances to curtail air pollution is good for the environment.") You can also say, "Using bicycles to travel shorter distances is a good way to curtail air pollution," or, "can curtail air pollution." But the sentence in the form you gave should use "curtails". Although you could add, "people" to the beginning of the sentence as I think Robert was trying to allude to in order to make it complete: "People using bicycles to travel short distances curtail air pollution."

It is understood that people are using bicycles, so people curtail pollution by using bicycles