u/BespokeDebtor, u/lenmea, and u/RedditUser91805 are having a proper slap fight over definitions of public goods and externalities over at r/ urbanplanning

Bespoke and lenmea are on the side of "standard Mankiw 101" but 91805 makes two points that may be interesting,

Since externalities are non-excludable does it make sense to consider them "public goods/bads" in the standard way we consider public goods?

I think not. I think that it makes sense to think of a good that is excludable and rival and has non-excludable impacts differently than we think about a good that is in and of itself not excludable and rival. Otherwise we get the kind of confusion shown here where if the government provides any good and it benefits people well then it is obviously a public good because it is external to the government.

If a good has negative externalities but has smaller negative externalities than its substitute, is it useful to think of that good as actually having positive externalies?

I think not. Travel by car and by bus both produce pollution which is a negative externality, while it is a much larger one by person mile traveled by car, that both should be fixed by a gas/carbon tax, but certainly not by government provision of those "public goods". But I think the main argument against conflating the two (smaller negative externality==positive externality) is that it obfuscates what is going on in the background especially in this context. The congestion externality, while larger for personal vehicles is still existent in transit, is caused by the government providing and "underpricing" the use of both transit and roadway space and in both situations leads to "too much" travel, especially during congested times.

What say you?