The impact of 20mph speed limits on safety and public health is to be examined in a new study.
Researchers will evaluate how the policy of reducing the limit from 30mph has worked in parts of Edinburgh and Belfast.
Rates of traffic accidents and injuries before and after the change will be measured to look at whether road safety has been enhanced.
Any change in transport use will also be scrutinised by examining the take-up of cycling and walking, while local residents will also be asked their opinions of the move.
The research team, led by the University of Edinburgh and the Scottish Collaboration for Public Health Research and Policy, hopes the project will inform other cities planning to introduce lower speed limits.
The study is the largest of its kind in the UK and will run until 2020.
Dr Ruth Jepson, from the university'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."
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.
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."
Green MSP Mark Ruskell has launched a consultation on a Members' Bill to change the default speed limit in built-up areas across Scotland from 30mph to 20mph.
He said: "This is a very timely study and I'm confident the report's findings will help the case we're making in the Scottish Parliament to introduce 20mph speed limits in built-up areas across Scotland.
"By bringing forward legislation I want to create safer streets so we reduce the risk for pedestrians and cyclists, especially children and the elderly.
"By cutting speeds in the streets where we live, shop and go to schools or day centres we can reduce deaths and injuries and boost public health with cleaner air."