The impact of 20mph speed limits is to be evaluated in a major study in two British cities.
Casualties and traffic accident rates will be measured to determine whether 20mph limits improve road safety, say researchers.
Rates of cycling and walking will be measured to assess the impact on transport use.
Experts say the project -- the largest of its type in the UK -- will also look at wider effects on residents and the local area and will inform other cities planning to introduce lower speed limits.
Speed limits have been dropped from 30mph to 20mph in parts of Edinburgh and Belfast with the aim of improving safety, but the move remains controversial. The new study -- which will run until 2020 -- will shed light on its widespread effects, say scientists.
The impact on road safety will be measured by establishing rates of traffic collisions and fatalities before -- and after -- the speed limit was lowered.
The study team -- led by the University of Edinburgh and the Scottish Collaboration for Public Health Research and Policy - will also assess how the change has affected the number of people walking and cycling.
Local residents' attitudes towards the lower limit -- and how it has impacted on their quality of life - will be measured.
The study is funded by the National Institute for Health Research and involves partnership with other UK universities, NHS Health Scotland, and the charity Sustrans.
Dr Ruth Jepson from the University of Edinburgh's Usher Institute of Population Health Sciences and Informatics, said: "We are excited to launch this major project, which we hope will provide very important insights into the public health effects of such initiatives.
"We anticipate that our broad focus will generate a wealth of evidence and learning that will be invaluable for informing future roll-outs of similar schemes in the UK and around the world."
Andy Cope, Director of Insight, Research and Monitoring at Sustrans, said: "This study will provide vital intelligence on the impact of 20mph speed limits on safety and levels of physical activity. We await its findings with keen interest."