In this paper, we present an end-to-end data-driven system, dubbed TransDec (short for Transportation Decision-Making), to enable decision-making queries in transportation systems with dynamic, real-time and historical data. With TransDec, we particularly address the challenges in visualization, monitoring, querying and analysis of dynamic and large-scale spatiotemporal transportation data. TransDec fuses a variety of transportation related real-world spatiotemporal datasets including massive traffic sensor data, trajectory data, transportation network data, and points-of-interest data to create an immersive and realistic virtual model of a transportation system. Atop such a virtual system, TransDec allows for processing a wide range of customized spatiotemporal queries efficiently and interactively. TransDec has a three-tier architecture, including a multimodal spatiotemporal database, an expressive and efficient query-engine and an interactive map-mashup presentation tier. With this paper, first we describe the components of the TransDec architecture. Subsequently, as proof-of-concept we present a number of decision-making queries enabled by TransDec.