So in your model, road wear is priced into the costs of all goods, no? That's one way to do it, but the problem is that then a single potato logistics company has no incentive to use a (presumably more expensive) transport method that causes less road wear, because the benefits are spread evenly across everyone, and costs are borne entirely by the switching company, so nobody would switch. (unless you made some rule forcing everyone to switch, which is another way to do it)

If road taxes are directly tied to the things causing wear, then every company causing that wear has an incentive to find a transport method that causes less wear (assuming that the additional cost of that transport method is lower than the cost of the road wear - that's why some people argue that using markets is better than using rules; if you've properly priced the market, the market will balance between road and rail based on cost.)

That's why it's important to make sure that all externalities are directly tied into the cost of whatever is creating those externalities.