Transport initiatives such as 20-mph (≈30-km/h) speed limits are anticipated to result in fewer road casualties and improve perceptions of safety, leading to increases in active travel.  Lower speeds may also lead to more pleasant environments in which to live, work and play. The main objective of this study was to evaluate and understand the processes and effects of developing and implementing 20-mph speed limits in Edinburgh and Belfast.  The focus was on health-related outcomes (casualties and active travel) that may lead to public health improvements.  An additional objective was to investigate the political and policy factors (conditions) that led to the decision to introduce the new speed limits.  The study was set in Edinburgh, Scotland, and Belfast, Northern Ireland, from 2000 to 2018. (Full report)