TOPSHOT - People use monowheel as they enjoy a vehicle-free day in front of the Arc de Triomphe (Triumphal arch) in Paris, on September 16, 2018. - Europe should hold an annual car-free day in a bid to ease air pollution, the mayors of Paris and Brussels said on September 15, 2018 on the eve of a vehicle-free day in their cities. (Photo by FRANCOIS GUILLOT / AFP)