post office
From BallotAccessNews.com: On September 16, the Ninth Circuit ruled 9-2 that cities may not criminalize the act of standing on a public sidewalk to solicit “employment, business, or contributions from an occupant of any motor vehicle.” The case is Comite de Jornaleros de Redondo Beach v City of Redondo Beach, 06-55750. Although this is not directly related to petitioning (because petitioners never expect anyone driving a car to pull over to the curb, get out of the car, and sign a petition), the decision illustrates the protection that the First Amendment gives to anyone who uses a public sidewalk for First Amendment activity.
In order to get a measure onto the ballot, supporters of a petition effort must get voters to sign their petition, the key to which is often getting access to where voters are. Those in initiative states are likely used to seeing people collecting signatures are fairs and festivals, as well as in front of various stores and businesses: places where lots of people are. One place where a lot of voters usually go is their local pose office, so the nations 32,471 post offices would seem like a natural place for voters to be engaged.