In this paper we present the MapMover project as a case study into the use and design of an interactive system for collective expression. Informed by analysis and reflection we advance the concept of constructed publics: publics that are established, shaped, and maintained through the actions and influence of others. We conclude by discussing the relevance of constructed publics as a theorectical frame for the analysis and evaluation of projects in the domains of urban computing and exploratory design in HCI. Author Keywords Constructed Publics, Collective Expression, Design ACM Classification Keywords H5.m. Information interfaces and presentation (e.g., HCI): Miscellaneous. H5.3 Group and Organization Interfaces ? collaborative computing, theory and models.