Public transport, or mass transit, is believed to bring several benefits, including energy efficiency, air quality and an economy of scale with respect to total public transport costs. An important factor in providing the public with cost-efficient and timely service, is an ability to do resource planning and optimization based on up-to-date data on passenger counts and flows. In this work we give a novel approach to counting the number of passengers on public bus transport based on WiFi signatures from any mobile devices carried onto the transport vehicle. This provides traffic planners with an up-to-date and relatively cost-effective data source.