Well, I think, it's a rather strange way to stop the air pollution or just any other ecological problem just because it sounds too simple to be really effective. We produce so many exhaust fumes and just a day without using vehicles which function on fuel doesn't help. We need more deep actions, more organized and mature ones such as reducing the use of plastic or fuel, start to use reusable materials and technologies, do more activities in the field of waste sorting, etc. Also, as to the no-cars day, I suppose it will be even too inconvenient for humans. It may really stop the normal function of our society. Every day lots of people should move somewhere with the help of cars, buses, planes, trains and such a day can ruin their everyday plans. Even if someone needed help, the ambulance wouldn't come immediately