I think you’ve got it backwards . . .
What Kay meant by “external costs” are things we do pay for in money, but not directly and obviously. E.g., a lot of our income tax, sales tax, and property tax payments go for auto-related things such as road building and maintenance. Some of those taxes also go to cover even less obvious things like the social costs of accidents, environmental cleanup, etc. Of course, individually, even persons who own no automobiles pay for those “external costs” to the degree they are personally liable for particular kinds of taxes. Which would suggest it’s actually better to be a car owner, since you get the benefits that you probably would be paying for anyway – just as, with a public school system, a parent gets the benefits even non-parents are paying for, if those non-parents also happen to be owners of taxable real property (property tax being what funds most public school systems).
Of course, it’s not that simple. There are enormous direct, personal, financial costs to being a parent, and there are enormous direct, personal, financial costs to being a car owner. If I lived in a dense urban area with good mass transit and very expensive parking costs, it might be a better economic decision for me not to own one, even if I am paying taxes to support the automotive system.
But the point I was trying to make when I brought up this whole subject is this: Many posters on this thread have argued that if a mass-transit system can’t pay for itself out of its farebox revenues, that is proof that it is uneconomical and, therefore, not worth creating, or not worth expanding. But it that’s wrong. It is mistaken to think of the automobile as “private” transportation and the subway as “public” transportation, just because individuals can own and operate cars but not subways. Either way, society as a whole is paying for the system.