Mees's work was principally concerned with the planning of public transport in cities. He was a strong advocate for public transport, but less keen on urban bicycles as a realistic mass transport solution. His work provided the basis for the European Union's 2005 HiTrans project on improving public transport in medium-sized cities and towns. He was also a member of the international advisory council for Paris's New Mobility Agenda Project. His most recent research was on planning decision support tools for multimodal urban transport systems, and improvements to urban public transport planning in Australia.