Hauliers are unlikely allies for the farm organisations. There are higher fuel taxes on white diesel, used by the road freight fleet, than are imposed on green diesel for which farmers are the customers - and there are solid reasons why this should be so.

The essential reason for lower excise on green diesel is that tractors are not significant users of the road system – most never, or rarely, leave the farm.

This is not the case for saloon cars or utility vehicles used by farmers, many of which do substantial mileage. These vehicles are not entitled to use green diesel which is taxed at a discounted rate, as are fuels used in so-called stationary engines.

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Lawnmowers do not travel along the roads either, but the fuels they use face carbon tax, because all fuel combustion generates carbon emissions and it is public policy to discourage emissions.

So the price of green diesel contains an element of carbon tax, as does white diesel, but the excise charged is not designed to do the job of the carbon tax.

It is designed, along with other taxes on motoring such as the annual vehicle license fee, to help towards the very substantial costs of building, maintaining and policing the 100,000km of public roads in Ireland.

There are very few direct user charges at tolling points around the country and the annual yield from the various taxes on road users, including tolls, add up to roughly the Exchequer’s bill for the national road system. The fuel tax is a proxy for a user charge and the excise discount on green diesel reflects the reality that tractors are very infrequent users of the road system.

Different position

The hauliers are in a very different position. Almost all internal freight movement in Ireland goes by truck and these vehicles do substantial mileage. The hauliers complaining about the high cost of filling their tanks, alongside farm representatives, a couple of weeks ago, are as chalk and cheese.

The farm tractors do very little mileage while the truckers do a lot, far more than the average of 15,000km per annum of private motorists.

All road users would like to pay less for fuel, but the hauliers’ case is the weakest, both because of high mileage and also the greater weight of trucks, especially when fully laden.

In the late 1950s, the American Association of State Highway Officials (AASHO) conducted a famous on-the-road experiment, at enormous cost, to check out the impact of heavier vehicles on road damage.

They took an uncompleted section of the interstate highway system in Illinois, laid short sections with a variety of road-building materials and then sent hundreds of trucks of various designs round in a loop for months, measuring the resulting road damage. They concluded that vehicles with light axle-loadings did very little damage and that the heavier ones were the main culprits.

They estimated that the damage varies not just with the axle load, but proportionately to the fourth power of the axle load.

A large private car weighing two tonnes would have two axle loads around one tonne each, while a truck with laden weight of 40t on four axles means four axles at 10t each.

Raised to the fourth power and the damage potential of the heavier vehicles is dramatically greater. Details are on Google, just type AASHO or ‘fourth power rule’.

Refinements

There have been numerous refinements of the AASHO results over the years and improvements in road design, including use of better surfacing materials and drainage, which have reduced road damage for a given level of traffic.

Modern trucks also have more axles, which helps. Weather matters too and temperature variations cause potholes and weaken roads. But the basic finding that heavy axle loads cause most road costs has not been controverted.

One result is that many European countries, including Ireland, have truck size limits, not always enforced, on small rural roads which are not built to withstand the heaviest loads. To build minor roads to such a standard would be ruinously expensive.

Nor is it practical to differentiate the white diesel tax by axle load, so the recovery of costs from vehicles of different types needs to work through the annual vehicle registration charge or through differential tolls where these exist.

In Ireland, the largest commercial vehicles, over 12 metric tonnes, pay about €900 per annum, not much more than many lighter private cars. Given the road damage they cause, this is a bargain.

Operators of haulage fleets have been vocal, and successful, in their appeals for relief from the recent spike in auto fuel prices. They have a far less persuasive case than those operating farm tractors, who have shared the public criticism for blockades of cities, towns and oil importation facilities.