The tales of two cities: Use of evidence for introducing 20mph speed limits in Edinburgh and Belfast (UK)

Background
In 2016, large scale 20mph speed limits were introduced in the UK cities of Edinburgh and Belfast. This paper investigates the role that scientific evidence played in the policy decisions to implement lower speed limits in the two cities.

Methods
Using a qualitative case study design, we undertook content analysis of a range of documents to explore and describe the evolution of the two schemes and the ways in which evidence informed decision-making. In total, we identified 16 documents for Edinburgh, published between 2006 and 2016, and 19 documents for Belfast, published between 2002 and 2016.

Findings
In both cities, evidence on speed, collisions and casualties was important for initiating discussions on large scale 20mph policies. However, the narrative shifted over time to the idea that 20mph would contribute to a wider range of aspirations, none of which were firmly grounded in evidence, but may have helped to neutralise opposing discourses.

Discussion and conclusions
The relationship between evidence and decision-making in Edinburgh and Belfast was neither simple nor linear. Widening of the narrative appears to have helped to frame the idea in such a way that it had broad acceptability, without which there would have been no implementation, and probably a lot more push back from vested interests and communities than there was.