Of course it's meaningful. A Prius pays considerably less user fee than a Ford Ranger to cause the same amount of damage.
Actually, no. The Prius subsidizes the Ford Ranger, in fact.
* Curb weight of 2011 Ford Ranger = 3302 lb
* Curb weight of 2011 Toyota Prius = 3042 lb
Per the AASHO fourth-power law, the Ford Ranger racks up 38% more ESALs than the Prius.
* EPA highway MPG for 2011 Ford Ranger = 40 MPG
* EPA highway MPG for 2011 Toyota Prius = 48 MPG
So for one mile of operation, the Ford Ranger pays tax on 0.025 gallons of fuel, while the Toyota Prius pays tax on 0.0208 gallons of fuel; in other words, the Ford Ranger pays 20.19% more despite racking up 38% more ESALs. In other words, where damage to pavement caused by axle loading is the sole factor under consideration, the Toyota Prius is subsidizing the Ford Ranger. QED. Of course, the damage caused by both is far less than that caused by an eighteen-wheeler, where the permissible axle loads are several times the curb weights of a Ranger or Prius.
(This said, I don't actually agree that car drivers as a group are cross-subsidizing trucks on a large scale. It is certainly true that the fees truckers pay are nothing like the ESAL ratios between trucks and cars, but the added cost of designing and building pavements to accommodate trucks is far less than the ESAL ratios while cars tend to drive demand for capacity increases. FHWA actually commissioned a study into this issue several years ago, with the report still available somewhere on the FHWA website, and was not able to find any evidence of massive cross-subsidization in either direction.)
Even if the damage caused is negligible, the point still stands that one car is paying significantly more user fee than the other while causing just as much road wear. I chose vehicles with similar curb weights just so nobody would try to undercut the argument because of that difference. I'm not arguing that these vehicles cause anything like the damage trucks cause, just that one pays more than the other for what is supposed to be the same purpose.
Your argument definitely doesn't work in the limited case of the Ranger and Prius on the highway. It might work better in cities because the mileage disparity is more sharply in the Prius' favor (51 city MPG for a Prius versus 29 city MPG for a Ranger). But roads in cities tend to be specialized more for access and there are more sources of funding, e.g. property and sales taxes.
In any case, I think it is a mistake to focus on comparisons between one extremely fuel-efficient model and one gas hog, since that ignores what people actually own and drive. From the state DOT funding perspective, what really matters is what happens with the vehicle fleet in aggregate. Fleet fuel efficiency has stayed flat at around 20 MPG since 1980, as the graph here shows:http://earlywarn.blogspot.com/2009/12/us-new-car-fuel-economy-trends.html
For state DOTs the primary impacts are at the margin, i.e. in capital construction, since other costs like maintenance and administration are largely fixed and difficult to cut. I would say that the largest impacts on what state DOTs are able to do financially in terms of capital construction come from, in descending order, (1) price fluctuations in input materials like asphalt and concrete, (2) changes in VMT (the economic crisis, combined with high gas prices beginning in 2008, have led to noticeable drops in driving), and (3) changes in fleet fuel efficiency.
This is not to say that trends in fleet fuel efficiency should be ignored. Small changes in state DOT revenue can have large changes in the capital construction budget--if the entirety of a 10% revenue drop is applied to capital construction rather than state DOT activities across the board, that could mean a 30% cut in construction, equivalent to cancelling four monthly lettings. But it is emphatically not true that hybrids are a short- or even medium-term threat to the fuel tax funding model. That will come much later when fully electric vehicles become commercially viable. In the meantime, the policy option of increasing the gas tax remains open (if unpopular) as a way to accommodate changes at the margin, with little risk of stimulating a "flight away from gas." A 10c/gallon gas tax increase applied to (e.g.) an existing 20c/gallon take at the state level is still a 25% increase in revenue, which is far larger than any recorded post-2008 changes in VMT or post-1980 changes in fleet fuel efficiency.
Put simply, if people tell you that the Prius and its ilk are a threat to the gas tax because they are so popular and so fuel-efficient, you should be challenging them to supply arithmetic proof. They won't be able to do it.
(In regard to the specific example of Washington potatoes eaten in Idaho, would you say that this is a nonsensical trade relationship, involving wasteful use of transport services, if Idaho potatoes were considered a premium product while Washington potatoes were intended for everyday eating? Remember Akerlof's rule--"Export the best because it is more likely to pay the freight.")
For the most part, yes. There are certainly premium potato products grown in Idaho and Washington, maybe more in Idaho than Washington, but the overall distribution of potato breed is very similar.
But this still isn't proof that it is wasteful to eat Washington potatoes in Idaho. The north-south corridor now known as US 95, which was built as a wagon road in the nineteenth century as a condition of unification between northern and southern Idaho, is still pretty rough and uneven. If you live in Moscow it may make more sense to eat Washington potatoes brought in from across the Snake River rather than Idaho potatoes trucked in over White Bird Hill (my Idaho geography is a bit rusty since I haven't visited since 2001, but you get the idea).