YP is an agile programming methodology that has evolved over the past 15 years. Many of its features are common to other agile methodologies; its novel features include using a highly visible, physical software status indicator (a real traffic light), and a well-defined nested set of development cycles. It is also an exceptionally open process, with the current status of the development process visible to the customers, as well as the code and documentation. We are using YP in developing the software for UrbanSim, a sophisticated simulation system for modeling urban land use, transportation, and environmental impacts over periods of 20 or more years under alternate possible scenarios. Our purpose in developing UrbanSim is to support public deliberation and debate on such issues as building a new light rail system or freeway, or changing zoning or economic incentives, as well as on broader issues such as sustainable, livable cities, economic vitality, social equity, and environmental preservation. The domain of use is thus politically charged, with different stakeholders bringing strongly held values to the table. Our goal is to not favor particular stakeholder values in the simulation or its output, but rather to let different stakeholders evaluate the results in light of what is important to them. There are several implications of this for the development process. First, having credible, reliable code is important-and further, both the code itself and the development process that produced it should be open and inspectable, not a black box. Second, to allow us to respond quickly to different stakeholder values and concerns, a flexible agile development process is required.