Fig. 2.

Location and spread of cities covered in this study. We use an OECD classification (OECD, 2012), and complement it by adding a ‘megacity’ category. Accordingly, small urban areas host less than 200,000 people; medium-sized cities host between 200,000 and 500,000 people; metropolitan areas, between 500,000 and 1.5 million; large metropolitan areas host between 1.5 million and 5 million people; and megacities host more than 5 million people. In all cases, we have used estimations of population in the corresponding metropolitan areas – as population within administrative boundaries does not reflect entirely the size of a city.