The processes of transport and public health policy change: 20mph speed limits in Edinburgh and Belfast

•In 2016, large scale 20mph speed limit interventions were implemented in Edinburgh and Belfast.•Using a qualitative case study method, this paper describes how 20mph speed limit interventions became a reality in the two cities.•Both interventions are examples of radical incrementalism to achieve transport and public health policy change.•The gradualist approach proved successful in gaining support for the interventions and minimising political and public opposition.•These successes in policy change provide useful learning for other jurisdictions planning or thinking about similar interventions.