Maximum tractor-trailer weight in the US is 80,000 lb, with typical weight closer to 44,000 lb. Dry weight is about 32,000 lb, for a maximum payload of 48,000 lb.
An articulated bus has a curb weight of about 35,000 lb and a capacity of up to 92 passengers. At 177 lb each, that's a gross weight of 51,284 lb. (net: 16,284 lb of passengers).
I'd presume a potato's density is close to that of water. As such, capacity is actually mass rather than volume-constrained --- you'd be unable to fill a truck to its volumetric carrying capacity before exceeding gross vehicle weight limits.
Many grocery products have much lower density: chips, bread, paper products, etc.
I still tend to agree it makes more sense to move goods than people, but the differential may not be quite as great as you might think.
Well, as this article is about the Netherlands, you can go and have a look there, at rush hour. Say, boarding a train from Almere to Amsterdam. And the answer is: a lot..
