Human nature is such that carpooling does not work unless you make it 'very' financially attractive. We've had decades to figure out a way to make it work, and it hasn't happened, nor will it. The only workable schemes of carpooling i can think of involve driverless cars.
The average users per car matters less than the total usage per car. The bulk of the environmental cost of a car is production. A car once made must be used instead of parked. Dual-purpose vehicles that move people during the commute and goods outside of it will boost usage a great deal, cutting down on the number of vehicles needed.
Oh, it's not human nature so much as complexity. Carpooling is most popular for regular commutes at set times, for exactly this reason.
When I decide to visit the hardware store on Saturday, there is probably someone I have never met that I could carpool with. But how do I contact them and arrange a carpool in minutes? If I have to post a listing a week in advance, it just isn't worth it.
The average users per car matters less than the total usage per car. The bulk of the environmental cost of a car is production. A car once made must be used instead of parked. Dual-purpose vehicles that move people during the commute and goods outside of it will boost usage a great deal, cutting down on the number of vehicles needed.