Traffic causes negative externalities: pollution, wasted time, noise, etc. Public education causes positive externalities: educated populace, a third party to

Traffic causes negative externalities: pollution, wasted time, noise, etc. Public education causes positive externalities: educated populace, a third party to help ensure kids are well taken care of, encouraging people to have children who will grow up to continue society, etc. Even people who aren't making direct use of public education receive the benefit from it, so it makes sense to encourage it as broadly as possible, not to try to restrict people from it.