[AnthonyMouse]

The Downs-Thomson Paradox makes reasonable sense, but it doesn't prove the claim from the article. It explains how adding road capacity could make traffic congestion worse (if it causes the preexisting level of mass transit availability to become unsustainable), but not why it inevitably must. For example, if mass transit in a particular city is already useless and disused then that can't happen. It also points to a way out of the paradox: Expand road capacity and subsidize mass transit availability at the same time.

[tomarr]

That isn't a reality due to the massive cost of either options (basically, pick one). Successful mass transit using the roads (eg bus rapid transit) require taking capacity out of the highways and therefore aren't mutually compatible.