In 2022, transport accounted for around one third of UK territorial emissions. Many decarbonisation pathways include modal shift as a possible way of reducing passenger transport emissions. Yet, the mitigation potential of mode shift often depends on generic behavioural assumptions rather than the technical feasibility for using different transport modes. Here travel microdata from the UK is used to systematically assess which trips could use more efficient transport modes, establishing a theoretical maximum for how much modal shift could take place with no reduction in mobility. Compared to current passenger transport use, emissions could be reduced by about 30% by changing the transport modes used for personal travel. This is possible if car use is reduced by approximately 27% and mainly replaced by trains. Further emissions savings are possible by increasing bicycle and motorcycle availability, increasing capacity of coach and surface rail, and increasing the time typically spent travelling.