Transportation systems, the environment (especially air quality), and sustainable development are the research interests of Oliver Gao. He addresses various issues in urban/regional transportation planning, engineering, air-quality impact and conformity, and related transportation policies. Most of Gao’s research involves the intensive application of mathematical and probability theories, statistical/econometrical methods (estimation, inference, hypothesis testing, forecasting), scientific computing, and information technology (GIS, temporally and spatially indexed transportation/environment database).