>One could tax single-occupant vehicles to encourage carpooling.

I'm not sure how the mechanics of that would work with ridesharing (e.g. if the driver has a passenger along at entry into the city, do they pay a tax?).

>Nit: most public transit involves vehicles. Walking, swimming, skipping, jogging, etc. aren't usually considered public transit.

Well, I had the NYC subway in mind, which while technically vehicular doesn't fall under the common usage of "vehicle" to mean "motor vehicle".