AbstractÐTransportation improvement site selection exempli®es transportation decision making that is collaborative in nature and geographically based. Such decision-making is part of a broad societal trend toward shared and participatory discussions about public investment. Perspectives from three dierent transportation decision contexts in the Puget Sound Region of Washington State, a regional council, a county government and a public±private Coalition group, are combined with a literature review to develop a decision task model that summarizes the need for information technology support during transportation improvement site selection. The task model guides the development of a decision support system requirement speci®cation that outlines integrated information capabilities provided by geographic information system (GIS) and group support system (GSS) technologies. Together, GIS and GSS capabilities contribute to evolving group-based GIS. The kinds of capabilities a group-based GIS could oer in addressing the needs are identi®ed. A report on the use of a prototype, group-based GIS called Spatial Group Choice highlights the possibilities in an inter-organizational coalition decision context. The conclusions discuss needs for future technology developments and social±behavioral science studies on these developments.