I would prefer that the government not sell ad space at all. (The reason a subway should be built is to transport people from one place to another, that's what you are paying taxes for). The main point I was trying to make is that we should consider other rights when defending free speech (or other rights, including not being exposed to things you don't want yourselves, or your children to be exposed to). Defending almost any right to an absolute extreme produces consequences that are harmful to individuals and society (as your example illustrates).