Earlier quoted context omitted.
Or the externalities are properly priced and the vast majority of the miles of transport are done by train, which would be cheaper if trucks had to pay for the road maintenance they cause. And cause less pollution (both in ton-miles and from reduced emissions during repaving). And the potatoes end up costing what they cost to get to you? I'm not sure the potato subsidy is worth distorting the transport economics.
Trucks do “pay.” They move goods that are sold and taxed. Until subways start shipping things other than people, they can’t be considered the same as roads. A subway can move people around; roads also move significant goods around. Every single thing in a store has arrived by road for at least some portion of the journey. Farmers aren’t shipping carrots via tram.
The problem is that non-truck solutions cost more relatively due to government intervention, messing up the market's ability to find more optimal solutions. This one is simple, it doesn't even require you to think about carbon taxes to do the math. It's direct costs, subsidies to a certain technology. Don't pick winners and losers, create an environment where capitalism can function and find the actual best solutions.
Charge trucks for damage they directly cause to roads. Simple.