I agree that on or in public transport is fair for advertising- but I strongly dislike billboards that block interesting views of buildings, poke their brightly coloured cheeks into the skyline or glare brightly into lofts and apartments. One of the differences is that advertisers on public transport are often highly censored- where as the car-yard or similar can go for the cheapest artist, horrid paint, off-colour humour. Some advertising- like some tv, is actually quite good, as a form of art. For examples of this, - and some general roasting of the advertising industry, check out episodes of "the gruen transfer" by Australian national broadcaster ABC.