The persistent haze over many of our cities is a reminder of the polluted air that we breathe. Over 80% of the world's urban population is breathing air that fails to meet World Health Organisation guidelines, and an estimated 4.5 million people died __26__ from outdoor air pollution in 2015.
Globally, urban populations are expected to double in the next 40 years, and an extra 2 billion people will need new places to live, as well as services and ways to move around their cities. What is more important, the decisions that we make now about the design of our cities will __27__ the everyday lives and health of the coming generations. So what would a smug-free, or at least low-pollution, city be like?
Traffic has become __28__ with air pollution, and many countries intend to ban the sale of new petrol and diesel cars in the next two decades. But simply __29__ to electric can will not mean pollution-free cities. The level of emissions they cause will depend on how the electricity to run them is __30__, while brakes, tyres and toads all create tiny airborne __31__ as they wear out.
Across the developed world, ear use is in decline as more people move to city centers, while young people especially are __32__ for other means of travel. Researchers are already asking if motor vehicle use has reached its __33__ and will decline, but transport planners have yet to catch up with this __34__, instead of laying new roads to tackle traffic jams. As users of London's orbital M25 motorway will know, new roads rapidly fill with more traffic. In the US, studies have shown that doubling the size of a road can __35__ double the traffic, taking us back to the starting point.