> buses and lightrail are significantly more efficient per person than personal vehicles

Assuming the transit is fully utilized and the car is mostly not. And maybe that's a good way to look at it. But in Portland the light rail is often well under capacity, and in that case a carpool likely wins on efficiency.

> the cost of cars also includes the cost of roads, the cost of land under those roads, the cost of parking

Partially. Those roads will have to exist even if we did not have personal cars.

Right, the reason it might be underutilized is if you're bad at designing cities for it. Which the US is, so it is.

We design cities for cars, which results in the cities spreading out further and further, which makes transit less desirable and more expensive. Other countries don't have this problem to this degree, because they don't design their cities exclusively for cars.

Also, I don't think most roads would need to exist if the amount of cars decreased. Because of the density problem noted above. Cars are sort of self-eating. The more cars you use, the more land-per-car you need as everything spreads further out to accommodate the cars.

> Assuming the transit is fully utilized and the car is mostly not.

The car is mostly not.

> But in Portland the light rail is often well under capacity,

Haven't looked deeply into it, but looking at how the US plans and designs its public transport, I'm surprised anyone was using it at all.