We examine to what extent Public Participation GIS (PPGIS) contributes to a successful participatory process and how context affects the tool’s role in both process and results. We tested a tool to visualize accessibility effects of transport projects, using it in participatory workshops related to a major bus corridor proposal in Santiago de Chile. We innovated in the tool itself, to visualize overcrowding effects as a way to include comfort issues. We found that the GIS encouraged a sincere and realistic conversation among participants. Perceptions of the project did not necessarily improve, but the tool did reduce polarization of opinions.