Rapid urban population growth is straining transportation systems. A big data–centric approach to transportation management is already a reality in many developed economies, with transportation systems being fed a large quantity of sensor data. Developing countries, by contrast, rely heavily on infrequent and expensive surveys. With mobile phone use becoming ubiquitous, even in developing countries, there is potential to leverage data from citizens’ mobile phone use for transportation planning. Such data can allow planners to produce insights quickly, without waiting for the proliferation of sensors. Using mobile network big data (MNBD) from Sri Lanka, our article explores this potential, producing mobility-related insights for the capital city of Colombo. MNBD-based insights cannot produce all the insights needed, but the high frequency and spatial resolution of the insights that they do provide can complement existing infrequent surveys. For resource-constrained developing economies, even an incremental advance in their ability to produce timely and actionable knowledge can improve existing transportation and urban planning. However, more research will be required before such techniques can be mainstreamed.