The Signpost project is a modular city-scale sensing platform that is designed to be installed on existing signposts. It is powered through solar energy harvesting, and provides six slots for generic sensing tasks. Modules have access to a set of shared platform resources including power, communication, gps-based location and time, storage, and higher-performance computation, and they use a Signpost-specific software API that enables not only use of these resources, but also supports the development of inter-module applications. The project is driven by several core applications, but also strives to be an upgradeable and adaptable platform that supports new applications for scientists and cities. Modules can be added and removed from the platform after deployment without disassembling the installed Signpost, reprogramming the Signpost, or interrupting the other functions of the Signpost. Additionally, by providing APIs to modules that support common operations, developing and deploying a sensing application in a city is significantly streamlined. A focus of this project is ensuring that domain scientists and researchers interested in city-scale applications can effectively leverage this platform to accelerate their projects.