The severity of a collision follows from the laws of physics. At higher speeds, the kinetic energy released in a crash increases, as does the trauma experienced by those struck by, or occupying, the vehicles involved. The increase in crash risk is explained by the fact that when speed increases, the time to react to changes in the environment is shorter and manoeuvrability is reduced.

Lowering the default limit from 30 mph to 20 mph in built-up urban areas significantly decreases the risk of accidents. It may also save time, help make our air cleaner, and encourage more active lifestyles. People wrongly assume that lower limits delay journey times, yet average city speeds are generally well below 20 mph, owing to congestion and queues. Traffic flows more freely at 20 mph than 30 mph. Drivers make better use of road space by parking closer and junctions work more efficiently, and at a higher capacity, as it is easier to merge.

On air quality, mathematical modelling across a range of studies has demonstrated that improvements should result. Research by Imperial College London on speed restriction shows that where traffic flow was interrupted, there were higher emission rates. This study concluded that it would be incorrect to assume 20 mph would be detrimental to ambient local air quality, as the effects on vehicle emissions are mixed.

Lower limits reduce congestion by increasing flow rates and by smoothing traffic through urban environments where cars would usually be stopping and starting. NICE, the National Institute for Health and Care Excellence, recommends urban speed reduction for less pollution. Their guidance says 20-mph limits without physical measures in urban areas help avoid unnecessary acceleration and deceleration. Smoother driving with less wasteful braking and acceleration cut fuel use by 12 per cent in Germany after 30 km/h limits were implemented. Air quality also improves, since moving traffic emits less pollution than when standing still with engines on. Imperial College recommendations include reducing speed limits on urban roads and to incentivise cycling.

Llywydd, the introduction of this policy would also benefit public health in a number of ways in accordance with the future generations and well-being Act. In built-up areas, more people would feel comfortable to walk and cycle safely, and there would be a more pleasant environment for communities, encouraging community interaction and children's outdoor play.

Sustrans Cymru point to evidence that moving to slower speeds will lead to safer and healthier communities with greater levels of walking and cycling. In 2013, the charity surveyed residents in Wales, with six in 10 supporting 20-mph limits as the default speed for where they live. As a Welsh Government Minister, I was pleased to take forward the Active Travel (Wales) Act, passed in 2013. It places a legal duty on local authorities in Wales to audit existing routes for walking and cycling and then plan and deliver a comprehensive network of routes to work, school and local facilities. Twenty miles per hour limits will facilitate this.

Over the past few generations, unfortunately, there has been a dramatic reduction in the freedom given to children to get out and about without adult supervision. A comparative study by the Policy Studies Institute, spanning 40 years, shows that in 1971, 86 per cent of children of primary school age in the UK were allowed to travel home from school alone. By 2010, it was just 25 per cent. Traffic is a major factor in this change and one of the major barriers to children's freedom to play outside. Our residential streets have become hostile environments for children and teenagers, where informal street play has largely been displaced by the car.

The United Nations Convention on the Rights of the Child, which enshrines the right to play, states that consideration should be given to creating child-friendly urban and rural environments through road traffic measures, including speed limits. And Play Wales have provided important information on the role that 20-mph speed limits could have in improving children's ability to play—an activity that is central to their physical, mental, social and emotional health and well-being. The policy is also recommended by the Royal College of Paediatrics and Child Health Wales, which states that it might have a most wide-reaching and positive effect.

Llywydd, there is a pressing need to develop robust interventions that will have positive effects on the challenges facing public health today, on air pollution, obesity and road traffic injuries, which are all interrelated. A default 20-mph limit can play a major role and we do, I believe, need national consistency with local discretion to achieve the necessary change. These matters are as much about a public consensus as they are about traffic management. If we want consideration of the amenity and safety of residents of communities to be a national norm, then, at some stage, we need a national debate. We need to put communities first and redefine the spaces between our homes. I am a strong advocate of this change and I do believe that it will allow us to reclaim our roads and create community streets—community streets that become a better place to be.

So, over the last few months, I've met with Rod King, the founder of 20's Plenty for Us, an organisation that campaigns for this change, and I'd very much like to thank him for his expertise and advice. When the Assembly resumes in the autumn term, I will be hosting a round-table event on 3 October to discuss this topic further. I'm delighted that Rod King will be there, along with Public Health Wales, academics, Jeff Cuthbert, Sustrans Cymru and Welsh Government. 

Llywydd, this is a policy that will deliver important and significant benefits for our communities. I very much hope that we can continue to build and strengthen our campaign and that Welsh Government makes 20 plenty for Wales.